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BALANCE ABILITIES OF WORKERS IN PHYSICALLY DEMANDING JOBS: WITH
SPECIAL REFERENCE TO FIREFIGHTERS OF DIFFERENT AGES*
*Doctoral
dissertation presented on the 5th November 2004 at the Large Lecture
Hall of the Haartman Institute, Helsinki, Finland by permission
of Faculty of Medicine of the University of Kuopio, Finland.
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Department of Physiology, University of Kuopio, Department of Physiology,
Finnish Institute of Occupational Health, Kuopio, Finland
| Published
(Online) |
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01
May 2005 |
© Journal of Sports Science
and Medicine (2005) 4, Suppl.8, 1 - 47
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This
review is based on the following original publications, which will be referred
to in the text as Studies 1-5:
1.
Punakallio, A. (2003) Balance abilities of different-aged workers
in physically demanding Jobs. Journal of Occupational Rehabilitation
13, 33-43.
2.
Punakallio, A., Lusa, S. and Luukkonen, R. (2003) Protective equipment
affects balance abilities differently in younger and older firefighters.
Aviation, Space, and Environmental Medicine 74, 1151-1156.
3.
Punakallio, A., Hirvonen, M. and Grönqvist, R. Slip and fall risk among
firefighters in relation to balance, muscular capacities and age. Safety
Science (submitted).
4.
Punakallio, A., Lusa, S. and Luukkonen, R. (2004) Functional, postural
and perceived balance for predicting the work ability of firefighters.
International Archives of Occupational and Environmental Health
77, 482-490 (in press).
5. Punakallio, A. (2004) Trial-to-trial reproducibility and test-retest
stability of two dynamic balance tests among male firefighters. International
Journal of Sports Medicine 25, 163-169.
| ABSTRACT |
|
The
objectives of the present study were to investigate the associations
between balance abilities and age, occupation and the use of fire-protective
equipment (FPE) in different visual conditions, and the associations
of slip and fall risk with balance abilities among workers in physically
demanding jobs, especially among workers in fire and rescue work.
The reliability and predictive values of balance tests in respect
to perceived work ability were also studied. The professional firefighters
aged 30 to 56-years (n = 29-135), construction workers (n = 52),
home care workers (n = 66) and nursing workers (n = 51) aged 23
to 61 years participated in this study. The data were obtained with
balance tests with the use of a force platform, functional balance
tests, slipping tests and questionnaires. In one study the balance
tests were carried out with and without FPE. The slipping tests
with FPE were carried out on a straight 8-m long path that had one
area covered by water and detergent or glycerol. Perceived work
ability at baseline and after a 3-year follow-up was determined
with the use of the work ability index (WAI). In the reliability
study, the dynamic balance tests were repeated six times in two
testing periods at an interval of 2 months. The results indicated
that the balance abilities of firefighters over 49 years of age
were significantly poorer than those of firefighters in the age
groups of <40 and 40-49 years. The decline of balance abilities
among construction, home care and nursing workers was not as consistent.
Postural balance was also more harmfully affected among the older
firefighters (43-56 years) than among the younger ones (33-38 years)
by the use of FPE without visual input. Self-contained breathing
apparatus was the most significant single piece of FPE to impair
balance in both groups. Furthermore, fast and controlled performance
in the dynamic stability test based on visual feedback was related
to smaller slip and fall risk with FPE in both age groups. The older
firefighters tended to have longer and more serious slips than the
younger ones. In addition, the construction workers were significantly
faster and made fewer errors than the firefighters in the functional
balance test. Among the firefighters, poor performance on the balance
tests significantly predicted a reduced WAI after a follow-up of
3 years. The dynamic stability and functional balance tests showed
reasonable reliability, especially when the reliability was estimated
from the best of at least three repeated trials. The present results
suggest that balance abilities should be taken into account in follow-ups
of the work ability of firefighters, as well as in the organization
of work tasks and the development of the characteristics of FPE.
It is also essential to provide ample balance training opportunities
for firefighters with and without FPE. The balance assessments of
the present study can be included when prerequisites of work ability
are evaluated and followed-up for firefighters.
KEY
WORDS: Musculoskeletal equilibrium, posture, aging, occupations,
rescue work, protective devices, risk factors, occupational exposure,
comparative study, cross-sectional studies, follow-up studies, reproducibility
of results.
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| INTRODUCTION |
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The
ability to balance is a basic element of daily activity. Due to
the high balance demands, sufficient balance abilities are especially
important in physically demanding jobs, such as firefighting. Roof
work, smoke-diving or the handling of patients and heavy tools are
examples of typical tasks carried out in fire and rescue work, in
which good individual balance ability can be critical for safe and
efficient work performance. Temporary and difficult work conditions
and the use of protective equipment further increase the challenges
placed on the balance control system. Balance abilities play an
important role also in other dynamic physical occupations that include
the handling of heavy objects and locomotion in complex environments.
High balance demands of a particular type of work may also develop
balance abilities, but only a couple of studies have compared the
balance abilities of people in different jobs. For instance, construction
workers working on high buildings have been shown to sway less than
people not engaged in physical work (Gantchev and Dunev, 1978).
The workforce is aging globally. For example, the mean age of Finnish
fire and rescue, construction, home care and nursing workers is
39, 41, 45 and 41 years, respectively (Tilastokeskus, 2003).
Most studies of age-related differences in balance have shown that
older people are less stable than younger ones. In physically demanding
jobs, balance demands are, however, equally high for workers of
different ages. Moreover, age-related problems in balance control
may increase accident risk (Gauchard et al., 2001).
The risk increases when visual or proprioceptive inputs are disturbed
due to challenging work conditions. Fall victims have been shown
to have poor balance control, particularly with their eyes closed
(Vouriot et al., 2004).
Limited data are available about the age and balance abilities of
workers in specific physically demanding jobs. Thus far, only one
study, that of Pohjonen (2001a),
has reported the age-related declines in functional balance among
home care workers.
Furthermore, accidents requiring three or more days of sick leave
as a result of lose of balance because of slips, trips and falls
on level surfaces or falls and jumps from upper to lower levels
have been reported to account for considerable proportions of work-related
accidents among certain groups of workers in Finland, for example,
for 30% and 6% among firefighters, for 22% and 12% among construction
workers and for 25% and 3% among workers in health and social services,
respectively (Tilastokeskus, 1996-2001;
Tilastokeskus, 2002).
The corresponding proportions for Finnish workers in general are
clearly lower (20% and 8%) among men and higher (33% and 4%) among
women (Tilastokeskus, 2002).
Slips, trips and falls account for 20% to 40% of disabling occupational
injuries in Sweden, the United Kingdom, and the United States (Courtney
et al., 2001).
Moreover, compared with younger workers, workers over 45 years of
age have a greater number of slip- , trip- and fall-related accidents
(Kemmlert and Lundholm 2001,
Tilastokeskus, 2002).
Good balance control in relation to a specific task may also promote
health and work ability. Pohjonen (2001a)
showed that poor balance is a strong predictor of reduced work ability
among home care workers and suggested that, in addition to tests
of muscular capacities, balance tests should be included in evaluations
of the work-related fitness of home care personnel. Several field
and laboratory methods are available for evaluating balance abilities,
but their relevance and validity to evaluate balance among active
working populations with high balance demands have not been established,
and, in most cases, their reliability has not been studied according
to current recommendations.
This study aimed at investigating associations between balance abilities
and age, aspects of work demands and workers' safety in physically
demanding jobs, and it especially focused on firefighters. The reliability
and predictive value of balance tests in respect to perceived work
ability were studied also.
|
| REVIEW
OF THE LITERATURE |
Concept
of balance control and the measurement of balance
Balance is a complex motor skill that describes the dynamics of body
posture in preventing falling. Balance control can be examined from
neurophysiological, biomechanical, and functional perspectives depending
on the goals of the study. There is, however, no solid consensus regarding
the definition of balance control or globally approved "gold
standards" for measuring it (Berg, 1989;
Ekdahl et al., 1989;
Pollock et al., 2000).
Definitions of balance vary according to the scientific background
of the research team using them, and measurements depend on what information
is needed and why. The terms "equilibrium", "postural
stability" and "postural control" are used as synonyms
for balance control (Horak, 1987;
Karlsson and Frykberg, 2000;
Shumway-Cook and Woollacott, 1995).
From a neurophysiological perspective, balance studies involve the
interaction of different levels of balance control mechanisms, whereas,
biomechanically, balance can be defined as the ability (balance ability)
to maintain or return the body's center of gravity (COG) within the
limits of stability (LOS), as determined by the base of support (BOS)
(i.e., the area of the feet) (Horak, 1987;
Nashner, 1997).
Balance is related to the inertial forces acting on the body and the
inertial characteristics of body segments. Furthermore, the LOS are
the boundaries of an area of space in which the body can maintain
its position without changing the BOS (Nashner, 1997).
The ability to maintain COG within BOS is a typically used definition
of "static balance". The term "static" is, however,
imperfect, as it ignores the minor automatic adjustments that occur
continuously when a body maintains a stable position (Berg, 1989).
Furthermore, it is the same organ system that is involved in regulating
posture in static and dynamic conditions. Mechanisms and strategies
for balance control can act differently, however, in static and dynamic
tasks. For example, during quiet standing, balance is usually controlled
by the ankle strategy, whereas ankle muscle activity alone is insufficient
to maintain balance during walking (Winter, 1995; Woollacott and Tang,
1997).
This is one reason for the low correlations between static and dynamic
balance tests (Patla et al., 1990;
Shimada et al., 2003;
Tsigilis et al., 2001).
Definitions of static and dynamic balance may, however, be useful
when the character and goal of the tool measuring individual balance
ability is described.
In static balance tests, the aim is to keep the center of pressure
(COP) of the body as immobile as possible within the BOS during standing
or sitting (Woollacott and Tang, 1997).
COP can be calculated from the forces needed when maintaining balance
applied to the surface of a force platform (Hirvonen et al., 2002).
According to Hasan et al., (1996a)
COP is the position of applied force vector that is influenced by
the shear forces produced by body segment accelerations. Its displacement
is a reaction to body dynamics representing all the vertical forces
acting on the BOS. When two feet are in contact with a surface, COP
is situated between the feet and depends on the relative weight taken
by each foot. Whereas COP itself is easily quantified and directly
measured, body COG is not directly accessible (Hasan et al., 1996a).
COG (also referred as center of mass=COM) is the point at which the
vector of total body weight passes. It depends only on the displacements
of the body segments, but it is not influenced by the dynamics. Although
COG and COP occupy different roles in the balance control system,
according to Hasan et al., (1996b),
their amplitude and frequency measures are highly correlated. This
correlation supports the use of COP-based measures in quantifying
standing balance. In the present study, static standing balance tests
using a force platform are called postural balance tests.
As opposed to the goal of static tests, that of dynamic balance tests
is to actively move the COP while standing, walking or different tasks
of daily activities are performed. In this study, the term "dynamic
stability" refers to a person's ability to move the COP in a
given direction within the LOS in force platform tests. The feet of
the testee are not allowed to move. In general, LOS are not fixed
boundaries. They change according to the task, the person's biomechanics,
and environmental aspects (Nashner, 1997;
Shumway-Cook and Woollacott, 1995).
For example, in walking, the COG is kept within the BOS only during
short double-limb support periods (Woollacott and Tang, 1997).
In all dynamic movements, COG can move outside the LOS for a moment,
but BOS has to be changed to bring COG back within BOS; otherwise
a fall results. Some researchers have defined static and dynamic balance
measurements according to whether the support surface is stable or
movable. In dynamic posturography, the support surface can move in
a horizontal plane or pitch the person either forward or backward,
but the person tries to stand in place (Monsell et al., 1997).
Functionally directed balance tests are typically dynamic tests that
measure a person's ability to maintain balance as he or she walks
or performs tasks as fast as possible or reaches as far as possible
(Hertel et al., 2000;
Podsiadlo and Richardson, 1991;
Rinne et al., 2001).
Ideally, the aim of functionally oriented balance tests is to simulate
the tasks and actions of daily activities and work, because balance
is one of the baseline requirements necessary for these tasks.
Balance is an integral component of almost all daily actions. Stability
and orientation demands of balance changes with each task, and they
are higher for activities of greater force, velocity, or magnitude
(Berg, 1989).
Postural orientation is the ability to maintain an appropriate relationship
between the body segments and between the body and the environment
during a task (Shumway-Cook and Woollacott, 1995).
In addition to individual and task-related factors, environmental
factors and their interaction affect balance control (Figure
1) (Shumway-Cook and Woollacott, 1995).
Individual
aspects of balance control
Sensory components
The ability to control balance is dependent on sensory inputs from
somatosensory, visual and vestibular systems (Table
1). Information concerning the position and movement of body segments
with reference to each other and the support surface and the distension
of the respective muscles is provided through the somatosensory system,
the proprioceptors and the mechanical sensitivity of cutaneous and
subcutaneous tissue (Nashner, 1997).
Proprioceptors are located in muscles, tendons and joints, and they
include the following receptor systems: primary endings of muscle
spindles (type I), secondary endings of muscle spindles (type II),
the Golgi tendon organ and joint receptors (McComas, 1996).
Muscle stretching activates primary (type I) and secondary (type II)
endings of muscle spindles and releases a stretch reflex that monosynaptically
facilitates the agonistic muscles and inhibits the activity of the
antagonistic muscles (Noback and Demarest, 1981).
Muscle spindle type II (secondary endings) mediates the information
on the length of the muscles to the central nervous system (CNS) as
well. Increasing of the tension of the muscles also activates Golgi
tendon organs (Noback and Demarest, 1981).
The Golgi tendon reflex effect in respective muscles is opposite of
the muscle spindle reflex, namely the activation of Golgi tendon organs
causes the facilitation of antagonist muscles, whereas agonistic muscles
are inhibited (Prochazka and Wand, 1980).
These three receptor systems work in harmony by releasing segmental
reflexes and mediating information on balance changes to the CNS (Prochazka
and Hullinger, 1983).
Information is also derived from receptors located in the cutaneous
and subcutaneous tissue (Johansson and Vallbo, 1980)
of the sole of the foot. These receptors adapt either slowly or quickly
and they can detect changes in pressure, for example, postural sway,
and can react to the acceleration and magnitude of skin stretching
on the sole of the foot during standing and walking (Johansson and
Vallbo, 1980;
Magnusson et al., 1990;
Toppila and Pyykkö, 2000).
Furthermore, the visual system provides information about the body's
position and motion in relation to the environment. Vision has an
important role in balance control, but it is not essential because
it can also be compensated by other sensory inputs (Brandt et al.,
1986).
According to Brandt et al., (1986)
visual signals, which start postural corrections, seem to react to
motion as a relative image shift on the retina when visual surroundings
are stationary. Visual input is needed not only for the continuous
evaluation of head sway, but it also seems to trigger required muscle
activation in controlling postural perturbations (Brandt et al., 1986).
In general, the central area of the visual field, when compared with
that of the peripheral retina, is more important for balance control,
and the foveal region contributes powerfully, especially to lateral
postural sway (Paulus et al., 1984).
When the direction of movement is rapidly changed, it would be impossible
to maintain a stable image on the retina without some automatic control
mechanism to stabilize the direction of the gaze of the eyes. Therefore,
the purpose of the vestibulo-ocular reflex is to stabilize vision
by producing eye movements in opposite directions during the turning
of the head (Baloh et al., 1993;
Noback and Demarest, 1981).
Through this phenomenon the vestibulo-spinal reflex stabilizes the
whole body.
Input concerning the position of the head in relation to gravity,
as well as to motion through the linear and angular acceleration of
the head, is provided by the vestibular system (Noback and Demarest,
1981).
Vertical and horizontal semicircular canals sense rotational movement
of the head in the sagittal and frontal, as well as horizontal, planes,
respectively. Canals are the most sensitive to fast movements, for
example, those occurring during sudden slips and trips, and they detect
movement with a large dynamic range (frequencies 0.5-10 Hz) (Horak
and Schubert, 1994;
Toppila and Pyykkö, 2000).
Furthermore, the otoliths sense position relative to the earth's gravitation
axis and linear and slow (range 0.2-0.5 Hz) acceleration of the head
(Toppila and Pyykkö, 2000).
Saccular otoliths sense vertical linear accelerations of the head
(e.g., gravity), for example, head translations generated during deep
knee bends, whereas utricular otoliths sense horizontal linear accelerations
like head movements generated during forward walking (Horak and Schubert,
1994).
The input of otoliths and semicircular canals converges in the vestibular
nuclei in the same neuron, which also receives visual and proprioceptive
input (Toppila and Pyykkö, 2000).
Sensory strategies
Sensory strategies, which refer to the relative weight given to a
sense by the CNS, vary as a function of such individual aspects as
age, task and environment (Table
1). It has been suggested that, under normal conditions, the nervous
system weights the importance of somatosensory information for postural
control among healthy adults (aged 20-70 years) more heavily than
vision does, but, when reliable proprioceptive information is removed,
vision becomes more important to the maintenance of balance (Colledge
et al., 1994).
It has also been suggested that there may be a systematic change in
the multisensorial process, which controls balance throughout life
(Straube et al., 1988).
Young children (2-8 years) rely more on visual inputs (Straube et
al., 1988),
whereas pressoreceptor and proprioceptor systems are important for
balance control among children aged 6 to 16 years (Hytönen et al.,
1993).
Furthermore, the importance of vision in balance control increases
again among people over 60 years of age (Hytönen et al., 1993;
Straube et al., 1988),
and persons over 85 years of age become especially dependent on vision
(Pyykkö et al., 1990).
Vision, together with vestibular input, becomes especially important
with respect to compensation for continuously applied low-frequency
balance disturbances, for example, when the support surface is unstable
(Diener et al., 1986).
Furthermore, vestibular input is critical for balance control when
somatosensory and visual inputs are unavailable, as well as under
sensory conflict conditions (Allum et al., 1989)
(Table 1). According to Colledge
et al., (1994),
the vestibular system alone can only partially compensate, however,
for proprioceptive loss. Several studies concern the importance of
vestibular inputs in different aspects of balance control; for example,
Runge et al. (1998)
suggested that vestibular information is not criticalas regards selecting
and triggering hip strategy, although it may be meaningful in controlling
hip strategy in some environments. Because the environment is constantly
changing, the CNS also has to adapt according to information for multiple
sensory modalities in order for balance to be maintained. Therefore,
the most appropriate inputs have to be selected according to the requirements
of the task and environment (Shumway-Cook and Woollacott, 1995).
Integration of motor responses according to sensory input
According to sensory systems and demands of the situation, sensory
information is organized, and motor responses are chosen in the CNS
to stabilize posture or prevent a harmful change in it (Table
2). The ability to control balance emerges from a reciprocal action
of biomechanical, musculoskeletal and sensory systems and the CNS
(Nashner, 1997).
The CNS consists of the spinal cord and the brain. Several parts of
the CNS take part in posture control, and there are three motor systems
involved in balance control. The first and fastest response to a change
in posture is triggered by the myotatic stretch reflex (spinal
cord), which regulates contractile muscle forces (Noback and Demarest,
1981).
Reflexes are activated by an external stimulus and are highly stereotyped
(Nashner, 1997).
The earliest functionally effective responses to balance perturbations
are called automatic postural responses also referred to as
the long-loop reflex or the functional stretch reflex. Their postural
latencies (mean 94 ms and 120 ms for medium and long latencies, respectively)
are much longer than the spinal stretch reflex latencies (35-40 ms),
but they are shorter than voluntary reaction times (>150
ms) (Diener and Dichgans, 1986;
Nashner, 1997).
Automatic postural responses, which are mediated in the brain stem
and subcortical area, coordinate movements across joints (Nashner,
1997).
They are stereotyped, but adaptable. Like reflexes, automatic responses
are activated by external stimuli. Their responses can be thought
of as overlearned, "long-loop" reflexes that rapidly respond
by resisting disturbances (Diener and Dichgans, 1986).
Automatic responses depend on movement strategy, which in turn is
dependent on the experience of the person and on surface conditions
(Nashner, 1997).
Automatic reactions are also adaptable to specific balance demands.
Contrary to reflexes and automatic responses, voluntary postural
movements, mediated by the brainstem and cortical area, can be
initiated in response to an external stimulus, or they can be self-initiated
and generate purposeful movements and behavior (Nashner, 1997).
Postural adjustments associated with voluntary movements are organized
on the basis of internal representation (Massion, 1994).
In order to know when and how to apply restoring forces to keep the
COG within the BOS, the CNS must have an accurate picture of where
the body is in space and whether it is stationary or in motion (Shumway-Cook
and Woollacott, 1995).
Therefore, internal representations (provided by the postural body
scheme) of body geometry, body dynamics (support conditions) and body
orientation through sensory inputs are essential for the mapping of
sensation to action (Massion, 1994)
(Table 2).
Motor components
Balance control requires an ability to produce adequate muscle contractions
according to task demands. In simplifying the control demands for
the CNS, independent, although related, muscles are combined into
muscle synergies by the nervous system (Shumway-Cook and Woollacott,
1995).
Because the muscles act around the joints when balancing the body,
the role of ankle and hip strategies and their related muscle synergies
are especially important. The ankle strategy produces shifts in the
COG by rotating the body about the ankle joints. It elicits a distal-to-proximal
muscle activation of ankle, hip and trunk musculature (Horak and Nashner,
1986).
The ankle strategy uses compensatory ankle torques that are believed
to correct for small postural perturbations on firm support surfaces
(Nashner, 1997).
Therefore efficient use of the ankle strategy depends on accurate
sensations from somatosensory inputs (Horak et al., 1990).
The hip strategy controls movement of the COG primarily by flexing
and extending the hips, and it uses early proximal hip and trunk muscle
activation (Horak and Nashner, 1986).
Hip strategy occurs when the ankle is unable to exert the appropriate
torque necessary to regain balance, for example, when the support
surface is smaller than the feet or is compliant or narrow when perturbations
are large and fast or when the body COG is near the limits of stability,
as it is during walking (Nashner, 1997;
Horak and Nashner, 1986).
The third strategy used to achieve balance is the stepping strategy,
which is used when the COG is displaced outside the BOS. The stepping
strategy uses early activation of hip abductors and ankle co-contraction
(Horak and Nashner, 1986).
The muscle strategies are not as stereotyped as reflexes are. They
can also be learned with experience in new environmental contexts
(Horak and Nashner, 1986).
When conditions are intermediate, between favoring the use of the
ankle or hip strategy, and when a person must adapt to a new surface
condition, for example, to control balance on a flat support as the
magnitude of postural perturbations increases, the use of combinations
of these strategies (mixed strategies) is common (Runge et al., 1999).
Effects of age and sex on balance control
Decrease of balance abilities with aging
An age-related decline in balance abilities has been shown in several
cross-sectional studies of postural sway and functional balance among
working-aged populations and the elderly (Colledge et al., 1994;
Du Pasquier et al., 2003;
Ekdahl et al., 1989;
Era and Heikkinen, 1985;
Gill et al., 2001;
Matheson et al., 1999;
Pohjonen 2001a;
Røgind et al., 2003;
Straube et al., 1988).
Children below 10 years of age and elderly people over 60 years of
age have the most pronounced postural sway (Hytönen et al., 1993;
Pyykkö et al., 1988;
Sihvonen et al., 1998).
Some investigations have shown that differences in postural balance
abilities in a normal standing position are minor between the ages
of 17 and 54 years (Sihvonen et al., 1998)
or that no correlation exists at all between age and postural sway
(Juntunen et al., 1987)
or that body sway is the most stable among 46-to-60-year-old people
(Hytönen et al., 1993).
When the balance demands of a test are increased (i.e., the BOS is
smaller or balance is tested on a compliant surface), when the eyes
are closed, or, especially, when the eyes are closed on a compliant
surface when two sensory cues are affected, some studies have shown
detected balance declines to be greater between younger people and
the elderly (>60 years) and between different age groups of the
working population (comparisons made for people below and over 40
years of age), and others report that, among working-aged subjects,
declines can only be detected (Colledge et al., 1994;
Era and Heikkinen, 1985;
Gill et al., 2001;
Matheson et al., 1999;
Straube et al., 1988).
It should, however, be remembered that comparisons of the age-related
results reported in different studies are complicated due to differences
in the methods, the selection of the subjects, the different age ranges
of the subjects and the study design. Moreover, most studies on the
relationship between balance and age are cross-sectional in design
(Colledge et al., 1994;
Ekdahl et al., 1989;
Era and Heikkinen, 1985;
Gill et al., 2001;
Hytönen et al., 1993;
Kollegger et al., 1992;
Matheson et al., 1999;
Pohjonen, 2001a;
Røgind et al., 2003;
Sihvonen et al., 1998;
Straube et al., 1988),
and a cross-sectional design is not ideal for obtaining an understanding
of the aging phenomenon. In longitudinal studies findings of age-related
changes have, however, confirmed the results of cross-sectional studies,
and balance deterioration in people over 75 years of age has been
shown to be even more pronounced (Baloh et al., 1998;
Du Pasquier et al., 2003;
Era et al., 2002).
Despite the considerable amount of research available on age-related
differences in balance, very little is known about age-related differences
in the balance of workers in specific physical jobs.
Age-related changes in senses, reflexes and automatic muscle responses
Changes in balance control mechanisms due aging take place at different
levels of the balance control system (Woollacott et al., 1986).
There is, however, no solid consensus concerning mechanisms contributing
to these changes (Colledge et al., 1994;
Laughton et al., 2003).
In addition, most of the studies of mechanisms contributing to age-related
balance changes have dealt with elderly subjects (mean age about 70
years) and young controls of about 20 years of age. Age-related deficits
in peripheral sensory systems are important factors, as an elderly
person's balance is more altered than that of younger adults when
visual and proprioceptive or both inputs are eliminated or reduced
(Matheson et al., 1999;
Shumway-Cook and Woollacott, 2000;
Straube et al., 1988;
Woollacott et al., 1986).
For example, the following changes take place in the senses as a result
of aging: older people have higher proprioceptive thresholds (Stelmach
and Sirica, 1987)
and anatomical changes take place in the semicircular canals, the
saccule and the utricule of the inner ear (Johnson and Hawkins, 1972).
Furthermore, visual acuity declines with age, depth perception and
contrast sensitivity are poorer among the elderly, and older people
progressively lose their peripheral vision (Cohn and Lasley, 1985;
Gittings and Fozard, 1986).
Age-related changes in the aforementioned sensory inputs could decrease
the redundancy of sensory information that is normally available and
can, therefore, make a shift in the relative weighting of inputs less
effective, depending on the environmental demands (Woollacott et al.,
1986).
Colledge et al. (1994)
studied 20-to-70-year-old healthy adults and suggested that the relative
contributions of sensory inputs to balance do not, however, alter
with advancing age.
Age-related changes in the long latency automatic postural response
systems characterize reduced muscle coordination (e.g., temporal breakdown
of distal and proximal muscle activation) and increase the absolute
latency of distal muscles within a muscle response synergy (Woollacott
et al., 1986).
The variability of the relative constancy of the contraction amplitude
of distal and proximal synergies also increases with aging (Woollacott
et al., 1986).
The latency of the monosynaptic stretch reflex, which is the lowest
level of the balance control hierarchy, increases in the achilles
tendon with age (Carel et al., 1979),
whereas in the patellar tendon no significant differences have been
found between 20- and 60-year- old men (Clarkson, 1978).
Furthermore, Woollacott et al. (1986)
found impaired integration of sensory inputs among older subjects.
Changes in higher integrative mechanisms
In their study Stelmach et al. (1989)
found that elderly subjects adapt more poorly to repeated small-slow
balance perturbations, which activate integrative mechanisms through
sensory inputs, than young controls did, although the two groups responded
similarly to large-fast rotations (elicit reflexive postural responses).
The continuous control of balance is, however, highly dependent upon
the quality of the proprioceptive information, and its integration
with visual and vestibular information, and it is not based upon reflex
mechanisms (Woollacott et al., 1986;
Stelmach et al., 1989).
Therefore, older people are at some disadvantage when balance is under
the control of slower, higher level sensory integrative mechanisms
(Stelmach et al., 1989).
More recent studies have also provided additional evidence that higher
integrative levels dominate in the decline of postural control (Colledge
et al., 1994;
Rankin et al., 2000;
Shumway-Cook and Woollacott, 2000).
For example, an enriched sensory context is not necessarily related
to more stable behavior among elderly people because the reintegration
of ankle proprioceptive input causes faster postural sway both with
the eyes open and with them closed (Teasdale and Simoneau, 2001).
Therefore, in addition to decreased peripheral acuity among older
people, their sensory reweighting process is limited by the capacity
of the central integrative mechanisms that reorganize the hierarchy
among the sensory inputs (Teasdale and Simoneau, 2001).
Colledge et al. (1994)
also suggested that the increase in body sway demonstrated with normal
aging is more likely to be due to the slowing of central integrative
processes than to altered peripheral sensibility.
Furthermore, people have shown poorer balance when they perform a
simultaneous cognitive task than when they do a balance task only
(Shumway-Cook et al., 1997;
Shumway-Cook and Woollacott, 2000).
Balance during a simultaneous cognitive task is more affected among
the elderly than among younger persons when the accuracy of visual
and somatosensory inputs is reduced (Shumway-Cook et al., 1997;
Shumway-Cook and Woollacott, 2000).
A recent experiment of Rankin et al. (2000)
demonstrated that, especially among older people, muscle activity
declines significantly in the gastrocnemius and tibialis anterior
muscles if balance is measured when a math task is performed concurrently.
These findings suggest that the elderly have a lower attentional processing
capacity for balance control during a dual-task paradigm.
Age-related changes in neuromuscular synergies and muscle function
With respect to output, the slowing of peripheral nerve conduction
velocity and the decrease in the number of motor units may also be
related to balance changes among the elderly (Leonard et al., 1997).
Furthermore, elderly people have been found to have mixed hip-ankle
activation when the BOS is narrowed during standing, whereas young
subjects adapt to increased postural demands by using the ankle strategy
only (Amiridis et al., 2003).
Okada et al. (2001)
also showed that older people rely more on hip movements to control
balance while young controls rely on ankle movements. The greater
hip muscle activation in the elderly may be caused by insufficient
torque production of the ankle muscles, which is needed to counteract
the great moment of inertia in the anteroposterior direction (Amiridis
et al., 2003;
Kuo and Zajac, 1993).
According to Kuo and Zajac (1993)
the hip strategy is the most effective in controlling the COM with
minimal muscle activation.
The greater hip muscle activation in the elderly may also be caused
by greater loss of motor units in the distal than the proximal muscles
or an insufficient proprioceptive contribution (Amiridis et al., 2003;
Stelmach and Sirica, 1987).
The elderly have also been found to have a greater amount of muscle
activity during postural sway in a quiet stance than younger persons
do (Laughton et al., 2003).
The authors concluded that it is, however, unclear if high muscle
activity of the legs precedes greater postural instability or if increased
muscle activity is a compensatory response to increases in postural
sway (Laughton et al., 2003).
Effects of sex on balance control
In some studies, men have been reported to sway more than women (Ekdahl
et al., 1989;
Era et al., 1996;
Juntunen et al., 1987;
Kollegger et al., 1992;
Maki et al., 1990;
Matheson et al., 1999).
Other studies have reported contradictory findings (Panzer et al.,
1995)
or no difference between the sexes (Colledge et al., 1994;
Hageman et al., 1995;
Røgind et al., 2003).
Moreover, no results of functional balance tests (walking time, standing
on one leg) have depended on sex (Ekdahl et al., 1989).
It has been shown, however, that standardizing the balance results
by the length of the base of support or body weight removes the difference
between the sexes (Era et al., 1996;
Maki et al., 1990).
Effects of vision on balance control
It is well known that, in the eyes-closed condition, the velocity
and amplitude of postural sway is higher than in the eyes-open condition
(Colledge et al., 1994;
Matheson et al., 1999;
Stelmach et al., 1989).
For example, postural sway was shown to increase two- to threefold
with the eyes closed in all age groups of men and women aged 15-25,
45-55 and 65-75 years and also among men aged 31-35, 51-55 and 71-75
years (Era and Heikkinen, 1985;
Gill et al., 2001).
The increase in sway in a normal standing position after the eyes
are closed is clearer in the anteroposterior direction (Era and Heikkinen,
1985).
Paulus et al. (1984)
showed that especially anteroposterior sway increased gradually as
visual acuity decreased. Slower speed and smaller amplitude of COP
movement in postural balance tests are associated with better visual
acuity among elderly men and women (Era et al., 1996).
Work
task demands and balance control
Characteristics of task-related balance control demands in physical
jobs
Physically demanding work includes several complicated tasks in which
the perturbation of balance is expected. The expected threat to balance
also causes anticipatory postural adjustment (APA) (Belen'kii et al.,
1967; Cham et al., 2002; Commissaris and Toussaint, 1997a; Cordo and Nashner, 1982; Marigold and Patla, 2002). APA activates the postural muscles and actively initiates
movements, which in advance counteract the possible disturbances of
balance associated with a voluntary task and locomotion. For example,
pushing a rigid handle while standing is associated with tibialis
anterior activation, which precedes the onset of the handle force
signal (Cordo and Nashner, 1982). This APA compensates for the body displacement induced
by the voluntarily performed handle movement.
In physical jobs APA are needed in respect to work safety, but it
is as important in respect to the fluency and efficiency of work (Belen'kii
et al., 1967).
The anticipatory activation of muscles before a voluntary task is
associated with the need to maintain balance a the minimum expenditure
of energy (Belen'kii et al., 1967).
According to Zettel et al., (2002) the CNS is able to use exteroceptive
visual input to alter balance control parameters in an anticipatory
manner, even when the characteristics of the forthcoming perturbation
cannot be predicted in advance. When the conditions in which a task
is performed change, the preparation for the movement also changes
in a way that ensures balance in the new situation (Belen'kii et al.,
1967). Balance adjustments and the sequence in which postural
muscles have been activated are also task-specific (Belen'kii et al.,
1967;
Commissaris and Toussaint, 1997a;
Toussaint et al., 1997).
The work tasks that challenge balance control the most among firefighters
are associated with work with ladders, on roofs and when smoke-diving
(Gledhill and Jamnik, 1992a;
Lusa et al., 1994).
Tasks demanding good balance abilities are involved in work on scaffoldings
and roofs among construction workers (Hsiao and Simeonov, 2001).
Furthermore, in nursing and home care work, as well as in construction,
firefighting and rescue work, manual lifting of clients or construction
material and the handling of heavy tools are essential parts of the
work. For example, the clients of municipal home care workers in Finland
are frail and elderly, and they need help with daily living activities
several times a day (Pohjonen, 2001b).
Lifting a load in front of the body creates a risk of falling forward
because adding the extra mass causes the COP to shift forward in relation
to the BOS (Commissaris and Toussaint, 1997b).
Pan et al. (2003) examined postural stability in association with
four lifting methods commonly used by drywall installers and carpenters.
They found that vertical lifting of drywall sheet placed greater demands
on their subjects' balance control (i.e., higher COM accelerations
and greater postural sway) than horizontal lifting of drywall. To
minimize the effects of balance-threatening events during lifting,
preparatory actions immediately before a load, placed in front of
the toes in lifting is grasped, are characterized by a lower forward
rotational velocity, a clear increase in the backward-directed horizontal
momentum of the body COP and a backward-directed horizontal force
vector, as well as forward displacement of the COP (Commissaris and
Toussaint, 1997b;
Toussaint et al., 1997).
During lifting the electromyographic (EMG) activity of ankle plantar
flexors increases considerably before contact with the load (Commissaris
and Toussaint, 1997b).
In one study, if the weight of the lifted load was reduced unexpectedly
(16 kg to 6 kg), balance was disturbed in most of the trials (Commissaris
and Toussaint, 1997a).
In general, the APA scales the amplitude of adjustment according to
the size or amplitude of the expected perturbation (Shumway-Cook and
Woollacott, 1995).
When a worker overestimates the weight of a load to be lifted, an
unnecessarily high linear and angular momentum of the body occurs.
This momentum can lead to disturbed balance and possibly to a fall
(Commissaris and Toussaint, 1997a;
Toussaint et al., 1997a).
Moreover, Aruin et al. (1998) suggested that higher instability during
task performance can also be expected to lead to smaller APA because
the CNS makes a logical and deliberate choice to help decrease the
probability that the APA itself will produce postural instability.
Effects
of protective equipment on balance control
The use of both bulky tools and protective equipment increases the
demand for highly developed and flexible balance skills (Hsiao and
Simeonov, 2001).
A worker's balance control can be affected by specific items of protective
equipment, such as footwear, clothing, eyeglasses and respirators
(Hsiao and Simeonov, 2001).
For example, shoes act as a sensory interface between the foot and
the BOS. The properties of the shoes affect the functional limit and
the slip resistance of the BOS and the sensitivity of the foot according
to extent, friction, firmness and incline of the surface (Hsiao and
Simeonov, 2001).
The soles of a person's shoes considerably affect the frictional properties
of the shoes (Grönqvist, 1995).
It has been shown that shoes with thin hard soles provided good walking
stability, whereas shoes with thick, soft soles reduced foot position
awareness and destabilized the walking stability of men of different
ages (Robbins et al., 1997;
Waked et al., 1997).
Protective eyeglasses, masks and other face or head protectors can
form a sensory interface to the visual system. They restrict the peripheral
visual field, and therefore protective eyewear may have harmful effects
on balance (Samo et al., 2003).
Respirators and protective clothing can also influence balance control.
Firefighters have to carry out tasks with fire-protective equipment
(FPE), consisting of specialized clothing and a self- contained breathing
apparatus (SCBA), at least a few times each year (Lusa et al., 1994).
Furthermore, firefighters need to be able to work safely with FPE,
while still maintaining sufficient physical capacity for the most
demanding tasks. However, the use of FPE also has negative effects
on performance. For example, for submaximal work in a thermoneutral
environment, the use of standard European FPE (Committee of Standardization
1995)
with SCBA weighing 15 kg increases cardiorespiratory strain by 20%
(Louhevaara et al., 1984),
and it also significantly increases thermal strain (Ilmarinen and
Mäkinen, 1992).
The strain caused by SCBA is partly due to the weight of the equipment
(Louhevaara et al., 1984),
which, together with increased heat stress, may also affect postural
stability (Kincl et al., 2002).
Kincl et al. (2002) showed that standard United States FPE impaired
postural stability in terms of sway length and sway area, especially
after physical loading (sustained squatting). These findings indicate
that wearing a heavy respirator during demanding physical work may
disturb balance (Kincl et al., 2002;
Seliga et al., 1991).
Previous studies on postural sway and FPE have, however, used subjects
in a narrow age range (24-34 years) (Seliga et al., 1991)
or failed to examine age-related effects (Kincl et al., 2002).
Although the effects of European FPE (Committee of Standardization,
1995)
on the cardiorespiratory system have been well quantified (Ilmarinen
and Mäkinen, 1992;
Louhevaara et al., 1984),
no data are available on balance control.
Differences in the balance abilities of workers in various occupations
In general, only a few studies have dealt with balance control in
different occupations. According to Kohen-Ratz et al., (1994)
fighter pilots demonstrate superior and more-mature postural control
than candidates for flight training, whereas helicopter pilots show
intermediate balance values. Although helicopter pilots are also highly
selected, they represent a group not chosen as fighter pilots (Kohen-Ratz
et al., 1994).
Diard et al. (1997) also reported that pilots on active duty had significantly
better performance in dynamic posturography than former fighter pilots,
firefighters and a control group of the general working population.
Nurses showed better performance in a Flamingo balance test than a
reference group of adults with various occupations, whereas public
servants scored better than nurses (Zinzen et al., 1996)
and construction workers had better stability than workers not engaged
in physically demanding work (Gantchev and Dunev, 1978).
Reported differences in balance abilities could be caused either by
an innate ability or by training and learning (Kohen-Ratz et al.,
1994).
Furthermore, dancers are found to be able to minimize COG displacement
towards the supporting side when raising one leg laterally to an angle
of 45 degrees in response to a light (Mouchnino et al., 1992).
Compared with naive subjects, dancers reach the new COG position faster
and require only a short adjustment period. Especially under sensory
challenged conditions professional dancers were better than controls
in maintaining their balance in a one-legged stance (Crotts et al.,
1996).
These differences may be due to the better internal representation
of the biomechanical limits of the stability of dancers (Mouchnino
et al., 1992).
Era and Heikkinen (1985) reported that differences in postural sway
are minor between manual and office workers.
Environmental
factors involved in balance control
Effects of the visual and physical environmental factors on balance
control
Workers maintain their balance by means of visual and physical interaction
with the work environment, such as their interaction with the surface
on which they stand (Hsiao and Simeonov, 2001).
The work environment can also consist of such aspects as exposure
to noise, lead and organic solvents, which have detrimental effects
on balance (Juntunen et al., 1987;
Ratzon et al., 2000;
Kuo et al., 1996).
For example, men with long-term exposure to impulse noise in the military
service had poorer postural balance than controls (Juntunen et al.,
1987).
Visual input from the work environment is used in a feedback mode
(reactive) to control balance during walking and standing or in a
feed-forward (anticipatory) mode to help guide locomotion on different
surfaces and avoid obstacles (Patla, 1997).
Several factors in a visual environment, such as the distance between
the eyes and the closest object in the visual field and between the
eyes and the visual target, as well as the visual target size and
contrast, affect balance control (Paulus et al., 1984).
Furthermore, elevation (3 m and 9 m) increases postural sway, and
this effect is correlated with the absence of close visual references
and fear of falling (Hsiao and Simeonov, 2001;
Simeonov and Hsiao, 2001).
For example, during construction work at an elevation, when workers
direct their eyes to a distant target (i.e., to the ground, a tree
or a house) their visual field may not include close visual references
(Simeonov and Hsiao, 2001).
The recent report of Simeonov et al. (2003)
also showed that the destabilizing effect at heights without close
visual references is similar to that of the eyes being closed at ground
level. Furthermore, height vertigo is associated with postural instability
in conditions in which visual references are farther away than 2.5
m (Brandt et al., 1980;
Paulus et al., 1984).
The effect of height with the absence of close visual contrasts was
found to be substantially more pronounced on uneven surfaces than
on normal surfaces (Simeonov and Hsiao, 2001).
Moving visual scenes can affect postural stability harmfully; for
example, while on a roof, a worker may look at a tree moving in the
wind or at swinging objects such as materials moved by a crane (Hsiao
and Simeonov, 2001).
Fall incidents can also occur as a result of a worker miscalculating
distance and depth (Clark et al., 1996;
Hsiao and Simeonov, 2001).
The detection of obstacles and changes in the properties of surfaces
are critical for the anticipatory control of balance (Hsiao and Simeonov,
2001).
Successful anticipatory detection of potential hazards in the visual
environment depends on how distinguishable they are from their surroundings,
on subject visual attention and on the prior experience and knowledge
of the worker (Hsiao and Simeonov, 2001;
Patla, 1997).
Poor lighting detrimentally influences postural balance, especially
in more demanding reach and bending tasks, compared with the stationary
postural task (Bhattacharya et al., 2003).
Firefighters, for instance, work frequently in heavily smoky or totally
dark environments where visual input is poor. In these conditions
balance is maintained through proprioceptive and vestibular systems
(Nashner, 1997).
In firefighting and rescue work, as in other physical jobs, the surfaces
are frequently compliant, narrow, inclined or slippery, of all which
challenge the balance control system by involving different strategies
to maintain postural stability (Cham and Redfern, 2002;
Leroux et al., 2002;
Marigold and Patla, 2002).
Increase in the slope and height of a surface have been shown to increase
postural sway synergistically (Simeonov et al., 2003;
Bhattacharya et al., 2003).
Furthermore, during locomotion, a more cautious walking strategy seems
to be adopted when a potential risk of slipping exists (Cham and Redfern,
2002;
Marigold and Patla, 2002);
the foot is placed more flatly, and therefore the foot contact area
increases, and the COM is kept closer to the contralateral limb, which
is in contact with the stable surface (Marigold and Patla, 2002).
The stance duration and loading speed of the supporting foot are smaller,
and the stride length is shorter; Thus the strength requirements of
walking are decreased. According to Cham and Redfern (2002),
anticipation of slipping trials produced peak values for the required
coefficient of friction (RCOF) (shear forces divided by normal forces)
that were 16-33% lower than the corresponding values in the baseline
trials. The RCOF is believed to best reflect aspects of the ground
reaction forces in the contribution of the shoe-floor interface to
slip and fall potential.
Working on inclined surfaces also produces an increased risk for slipping
due large shear forces at the shoe-floor interface (Hsiao and Simeonov,
2001).
The RCOF increases linearly with increasing inclination (Redfern et
al., 2001).
An inclined surface affects balance control by altering sensory input
from the foot and ankle as a result of a reduced effective BOS and
modified position (Hsiao and Simeonov, 2001,
Simeonov et al., 2003).
Furthermore, anticipation during walking down an inclined surface
results in a reduced stride length and stance duration (Cham and Redfern,
2002).
According to Leroux et al., (2002), the main strategy in standing
on a slope is to maintain the COG within the BOS by orienting the
trunk and pelvis in relation to the earth's vertical across slopes.
During standing on a slope, to provide postural stability, the ankle
joints become plantar- or dorsiflexed, and a stretching or contraction
of the muscles occurs that controls the movement of the ankle joints
(Simeonov et al., 2003).
The increase in muscle activity and stiffness at the ankle joints
probably cause an increase in sway velocity (Simeonov et al., 2003).
Balance
control during slipping
The activity of the bilateral leg and thigh muscles, as well as the
coordination between the lower extremities, has been shown to be important
with respect to reactive balance control in slips occurring at heel
strike (Tang et al., 1998).
During a slipping event, ankle moment has been shown to decrease with
the severity of the slip, and knee flexor and hip extensor moments
are primarily responsible for any corrective balance reactions (Cham
and Redfern, 2001).
Key factors that distinguish between subjects who fall due to a slip
and those who maintain their balance are an increased slip distance
of the foot (Brady et al., 2000)
and a shorter double support phase on a slippery surface (You et al.,
2001).
According to different studies, the critical slip distance between
an avoidable and unavoidable fall is 5-22 cm (Brady et al., 2000;
Grönqvist et al., 1999;
Strandberg and Lanshammar, 1981).
Reactive recovery responses to unexpected slipping (first trial) consist
of a rapid onset of flexor synergy, range 150-200 ms, and this synergy
suggests that polysynaptic reflexes contribute to the regain in balance
and that proprioceptive cues are responsible for triggering the response
(Marigold and Patla, 2002).
Furthermore, a large arm elevation strategy, which helps stabilize
COM by shifting it more anteriorly, and a modified swing limb trajectory
are used as balance recovering strategies (Marigold and Patla, 2002).
In addition, grasping, arm swinging and compensatory stepping are
efficient means of restoring balance (Redfern et al., 2001).
The reweighting of the different cues controlling balance has been
found to be less efficient among fall victims than among controls
(Vouriot et al., 2004).
When a subject is exposed repeatedly to slip perturbation, the knowledge
of the slip results in a shift in the medial-lateral COM closer to
the support limb at foot contact, as well as in a flatter foot landing.
Muscle response magnitude and braking impulse are also diminished
(Marigold and Patla, 2002).
The reactive strategies to help maintain balance during slipping are
influenced by knowledge of the surface characteristics and prior experience,
and therefore the recovery strategies are modifiable (Cham and Redfern,
2001;
Marigold and Patla, 2002).
Balance assessment of workers in physical jobs
Work-related
validity
Although the importance of balance abilities in physically demanding
jobs has already been recognized (Hsiao and Simeonov, 2001;
Lusa, 1994),
there is limited knowledge of the validity of the balance tests in
use. Valid work-related balance tests are, however, needed for purposes
of screening in occupational health care and for rehabilitation so
that the effects of balance training can be evaluated. When the work-related
validity of a physical test is evaluated, it is necessary to examine
how a test result is associated with and predicts performance in an
actual work situation (criterion-related validity). The problem
in practice is the lack of quantification for gold standards (King
et al., 1998).
In general, the definition of work- or performance-related fitness
is complex, and the main problem is the lack of a conception of the
optimal level of balance tests or other fitness tests, in respect
to work (Pohjonen, 2001b).
Test drills that simulate physically demanding work tasks have been
developed and validated for evaluating firefighters' physical work
capacity, mainly in terms of aerobic capacity (Louhevaara et al.,
1994;
Williford et al., 1999).
Although some of the drills also include tasks that demand, among
other capacities, balance and agility, the outcome variable is either
performance time or heart rate without any measurement of balance
abilities specifically (Gledhill and Jamnik, 1992b;
Louhevaara et al., 1994;
Misner et al., 1989).
Performance-related balance tests that simulate important daily activities
for the elderly, such as walking, rising from a chair and turning,
have also been developed and validated (e.g., Berg, 1992;
Podsiadlo and Richardson, 1991).
Furthermore, widely used posturography tests of standing balance have
been shown to differentiate between workers with various balance abilities
(Gantchev and Dunev, 1978;
Kohen-Ratz et al., 1994).
"Standing still" tests on a force platform have, however,
been criticized for their inability to predict changes in functional
balance and gait (O'Neill et al., 1998).
Another method of validating a work-related balance test is to measure
its ability to predict future work capacity or disability (predictive
validity). Predictive validity studies of balance tests have mainly
focused on the association between balance and low-back pain among
working populations (Takala and Viikari-Juntura, 2000), between balance and health-related fitness (Suni, 2000) and between balance and risk of falling among the elderly
(Brauer et al., 2000; Lord and Clark, 1996). Other than those of a study on functional balance among
home care workers (Pohjonen, 2001a), no data are available on the predictive values of balance
tests in respect to work ability. The study of Pohjonen (2001a) showed that the perceived work ability of home care
workers with poor performance in the functional balance test was 6.5
times more likely to decrease during a 5-year follow-up than that
of workers with a good result.
Furthermore, the content validity of a test is based on theoretical
arguments in relation to a job analysis of the degree to which the
test measures the physical demands of the job (King et al., 1998). Content validity should, however, not be used as the
only basis for determining the validity of a test. With respect to
aspects of content validity, most tasks in physically demanding jobs
require balance control during movement in difficult conditions. Therefore
the work-related balance tests used should also be dynamic or otherwise
related to task and environmental demands for balance, such as the
demands of different surfaces, the BOS and visual conditions.
Reliability
In addition to validity, reliability is an essential characteristic
of a measure, but it is not so complicated to establish (King et al.,
1998). In fact, to be valid, a test must also be reliable,
because a test that measures what it is supposed to measure must consistently
provide the same results day after day (Baumgartner, 1989). It is necessary to establish reliability between test
and retests (test-retest reliability), as well as between testers
(interrater reliability). According to Atkinson and Nevill
(1998) some amount of error is always present in physical tests.
Systematic bias is a general trend for test results to differ in a
particular direction between several tests. Usually systematic bias
is associated with the effects of learning, training or fatigue. Random
error is the other type of error related to physical test results.
It is usually larger than systematic bias. Inherent biological or
mechanical variation or inconsistencies in the test protocol can be
the reasons for large random error (Atkinson and Nevill, 1998).
Suitable and multiplex statistical analyses of reliability have also
been called for (Atkinson and Nevill, 1998). Different kinds of correlation coefficients are useful
for measuring the relative reliability of showing the degree to which
subjects maintain their position within a sample (Baumgartner, 1989), but, according to Atkinson and Nevill (1998) and Lamb (1998),
they should not be employed on their own as an assessment of reliability.
Therefore, it has been recommended that both an appropriate correlation
coefficient and a method for absolute reliability (i.e., coefficient
of variation, limits of agreement (LoA) or standard error of measurement
(SEM) be used (Atkinson and Nevill, 1998; Baumgartner, 1989). Absolute reliability focuses on the amount of error
to expect in a person's score, and it makes it possible to consider
the practical significance of the reliability result as well (Atkinson
and Nevill, 1998; Baumgartner, 1989). Since some amount of error is always present in physical
test results, absolute reliability can show the amount of error that
could be acceptable in the practical use of a test (Atkinson and Nevill,
1998). Furthermore, in order to evaluate the effects of learning
and training, more than two test sessions are recommended (Atkinson
and Nevill, 1998). The process of the reliability evaluation should also
show whether or not a test needs more familiarization trials or more
time between repeated trials. The following sections present examples
of reliability studies of methods assessing postural balance, dynamic
stability and functional balance abilities mainly among working-aged
people.
Postural balance
Using the Good Balance measurement system (Metitur, 2001), Mustalampi
et al. (2003) showed intraclass correlation coefficient (ICC) values
of 0.56-0.90 for postural balance tests (anteroposterior and mediolateral
sway velocity) in a normal and a single-leg standing position with
the eyes open. Tests were performed at a 3-day interval among 35 volunteers
aged 26.6 (SD 8.9) years. The coefficient of variation of the tests
ranged from 5.4% to 8.7%. Two (normal standing) or three (single-leg
standing) trials were performed per session, and the best result was
used as an outcome variable.
Takala et al. (1997) reported that, with the use of a custom-made force platform,
sway velocity had the best day-to-day reproducibility and stability
over time (9 months) among working people aged 38.7 years (n=9), the
standard deviation (SD) being 10.9. The other sway parameters studied
were maximum lateral and anterior-posterior displacement, mean amplitude,
mean sway frequency and sway area. For most of the items the mean
differences in the parameters over time were less than 5-10% of the
measured values. Day-to-day values of the ICC for sway velocity in
a two-foot stance with the eyes open were 0.56 (mediolateral sway)
and 0.50 (anteroposterior sway), the corresponding values for the
eyes-closed condition being 0.46 and 0.54, respectively (Takala et
al., 1997). Single leg standing gave the following ICC values: 0.64
(lateral sway) and 0.72 (anteroposterior sway) for the right foot
and 0.50 and 0.46, respectively, for the left foot. The ICC values
for long-term stability were at about the same level or a little higher
for some parameters. During the one-foot tests, the subjects were
allowed to repeat the test three times if they failed the test; otherwise
one repetition was performed.
Corriveau et al. (2000)
concluded that at least four trials of the COP minus COM variable
are required to provide reliable results for postural balance among
healthy people over the age of 60 years (n=7). Their outcome variable
was the average of four trials. It has also been shown that, among
healthy navy recruits and hospital staff (20-54 years, n=60), a longer
time (>17 days) between 10 repeated postural balance tests was
less likely to produce a learning effect (Nordahl et al., 2000). The other time intervals studied averaged 11, 31 and
115 days. According to Le Clair and Riach (1996),
the best test-retest reliability is obtained for postural sway parameters
at 20- and 30-s trial durations (subjects aged 19-32 years, n=25).
They studied the reliability of five different test durations (i.e.,
10, 20, 30, 45 and 60 s) and 10 s proved to be the least reliable.
Dynamic stability
High ICC values (0.88-0.93) were reported for a dynamic stability
test (balance master) that involved weight shifting to eight targets
positioned in an ellipse (Brouwer et al., 1998). Young volunteers aged 24 (SD 3.2) years (n=33) performed
tests on three occasions at 1-week intervals. For a sample of voluntary
subjects aged 17-55 years, the results of a dynamic stability test
(Good Balance) (to move the COP actively to follow a circle shown
on a screen) improved systematically (paired T-test) in the second
trial (Hofmann, 1988). In this case, the possible effects of learning
needed to be determined by quantifying actual baseline levels and
the stability of the test over a longer period of time. Furthermore,
Hirvonen et al. (2002) found ICC values of 0.93-0.96 for a dynamic
stability test (to the move COG marker from the central target to
the eight peripheral targets) with a custom-made force platform. Healthy
subjects (aged 16-54 years, n=23) repeated the tests five times (4
practice trials and 1 test trial at each session) at different time
intervals (1-12 days). The length of the time interval between the
test occasions did not have a significant effect on test reliability,
whereas two to four parameters showed learning effects between the
first and fourth and first and fifth test sessions (Hirvonen et al.,
2002).
Functional
balance
Rinne et al. (2001) studied the ICC values and LoA of tandem walking
forwards, tandem walking backwards and standing on a bar among 25
healthy volunteers aged 36-72 years. The test-retest and interrater
reliabilities were 0.91 and 0.88, 0.85 and 0.96, and 0.92 and 0.96,
respectively. The tests were performed in three different sessions
at 1-week intervals. All the balance tests showed some learning effect;
however, the authors considered the LoA for tandem walking forwards
and tandem walking backwards to be reasonable, 2 and 3 s, respectively.
Furthermore, for a stratified sample of firefighters (aged 34-54 years,
n=50) the test-retest reliability between two functional balance tests
(walking as quickly and controlledly as possible on a wooden plank)
repeated at 1-week intervals was 0.77 (p< 0.001) according to Pearson's
correlation coefficient (Punakallio et al., 1997a). Furthermore, the reliability of a dynamic balance test
(walking on a 5-m long track while stepping only on two pads and standing
on one foot, the empty pad being retrieved from behind and placed
ahead so that the subject could proceed) at 1-week intervals was 0.90
(performance time) and 0.71 (loss of balance) for 57 healthy male
volunteers aged 38-57 years (Räty et al., 2002).
In most cases the star excursion balance test (a grid on the floor
with eight lines extending at 45 degree increments from the center
of the grid in which the subject maintains a single-leg stance while
reaching with the other leg to touch as far as possible along a line)
showed high intratester (ICC 0.78-0.96, SEM 1.8-3.4 cm) and intertester
(ICC 0.35-0.93, SEM 2.3-5.0 cm) reliability among 16 recreationally
active volunteers aged 21.3 (SD 1.3) years (Hertel et al., 2000). At two testing sessions with a 1-week interval the subjects
performed three trials (one practice trial) for both legs in each
direction. There were significant learning effects, however, for the
repetitive trials of four of the eight excursion directions, and trials
7-9 had the longest excursions in all directions.
|
|
THEORETICAL FRAMEWORK OF THE STUDY |
The
theoretical framework of this study is based on, and has been modified
from, the systems approach of Shumway-Cook and Woollacott (1995).
In the present framework balance control is considered to be an interaction
between the individual capacities of a worker, the work task with
its inherent balance demands, and the demands of the work environment
on balance (Figure 2).
The high balance demands of the work and unpredictable and rapidly
changing work conditions make balance control and maintenance challenging
for a worker in physically demanding jobs. Adequate balance control
in relation to task and environmental demands may prevent accidents
and injuries, and may also support and promote health, safety and
work ability. Therefore, in addition to the worker's individual capacities,
the aspects of the work task and the environment should be characterized
when balance abilities are studied with various tests.
|
|
AIMS OF THE STUDY AND STUDY DESIGN |
|
Aims
of the study
The
purpose of this study was to investigate the associations between
balance abilities and aspects of job
demands, work ability and safety among different aged workers in
physically demanding jobs, and, particularly, those in fire and
rescue work. The associations between individual characteristics,
such as age and occupation, with balance abilities were studied.
The task-related and environmental factors, such as the effects
of FPE, visual conditions and slippery surfaces, were also studied
in association with the balance abilities of firefighters. The methodological
aim was to evaluate the reliability of the balance tests and their
predictive value in respect to perceived work ability.
The aims of the study can be specified in the form of the following
questions:
1.
Do postural and functional balance abilities differ among different
aged workers from different physical occupations, and do balance
abilities differ between workers in different occupations? (Study
1)
2.
Does fire-protective equipment affect functional and postural balance
abilities, and do the influences differ in two visual conditions
and between younger and older firefighters? (Study 2).
3.
Are balance abilities, muscular capacities and age associated with
slip and fall risk in walking experiments with firefighters wearing
fire-protective equipment? (Study 3).
4.
Do postural, functional and perceived balance abilities predict
perceived work ability and physical work ability? (Study 4).
5.
What is the reproducibility of the dynamic stability test and functional
balance test, and what is their test-retest stability after 2 months?
(Study 5).
Study design
The study design varied depending on the specific objectives of
Studies 1-5. Cross-sectional measurements were applied in Studies
1-3, and longitudinal designs were used in Studies 4 and 5 (Table
3).
|
| METHODS |
|
Subjects
The subjects of the overall study comprised firefighters (Study
1-4), construction workers (Study 1), home care workers (Study 1)
and nursing staff (Study 1) (Table
3). The data on firefighters are based on a 3-year follow-up
of the health and physical and mental capacity of Finnish professional
firefighters (3-year study) (Punakallio et al., 1999). Most of the data in this study are from the second cross-section
in 1999 (Figure 3).
At baseline in 1996, 210 professional male firefighters were selected
from central and southern Finland by stratified sampling. Samples
were obtained from all professional, operational firefighters in
the age groups of 30-34 (n = 254), 40- 44 (n = 208) and 50-54 (n
= 120) years in 1996 and were from 28 fire departments situated
within 100 km of Helsinki (southern Finland) or 150 km of Kuopio
(central Finland). About 9 out of 10 of the selected firefighters
(89%) participated in the balance tests and responded to the questionnaire.
Study
of balance abilities among different aged workers in physically
demanding jobs (Study 1)
In addition to firefighters, Study 1 included construction workers,
nursing staff and home care workers (Table
3). The construction workers came from both private companies
and the municipal sector of the city of Lappeenranta. Their balance
was measured when they took part in their periodic health check-ups
in an occupational health care unit. Municipal home care workers,
who cared for elderly people in private or institutional homes,
from 10 entire work units in the city of Helsinki formed the initial
sample of home care workers (Pohjonen, 2001a). Their balance tests were carried out at the time of
their 5-year follow-up tests for physical fitness. Altogether 60
municipal nursing workers were selected by stratified sampling from
216 nursing workers from an old people's home in the city of Helsinki
(Pohjonen et al., 2003). The data on firefighters came from those in southern
Finland who participated in the follow-up tests (in 1999) in the
3-year study (Figure 3, Table
3).
Studies of balance and slip risk with fire-protective equipment
and the reliability of the balance tests (Study 2, 3, 5)
In Studies 2, 3 and 5, the subjects were a subgroup (n = 29) of
the 69 firefighters from southern Finland who participated in the
follow-up tests of the 3-year study (Figure
3, Table 3). A total of
36 male firefighters in two age groups (33-38 and 43-56 years) were
asked to volunteer for the studies. Six of the firefighters over
50 years of age were excluded because they had had low-back pain
during the preceding year, had acute low-back or other musculoskeletal
pain or had disorders that might have been exacerbated by the activities
in the studies. After the selection process, one young firefighter
refused to participate, and the final sample included 29 firefighters.
Study of the predictive value of the balance tests (Study 4)
In study 4 the subjects were firefighters who participated in balance
tests in 1996 and responded to the questionnaires in 1996 and 1999
in the 3-year study (Figure 3).
The study sample also included six firefighters who retired on a
disability pension during the 3-year follow-up. They were included
in the category of poor work ability in 1999. Furthermore, eight
firefighters who participated in the laboratory tests did not respond
to the questionnaire. Therefore, the final number of subjects was
135 (Table 3).
Balance measurements (Study 1-5)
Functional
balance
The balance tests and variables, from Studies 1-5, are shown in
Table 4. Functional balance
was measured by a test in which a subject walked forwards, backwards
and turned 180 degrees in the middle on a 2.5-m long, 9-cm wide
and 5-cm thick wooden plank (Figure
2, Study 5) (Punakallio et al., 1997a). He or she was instructed to walk as quickly as possible
without falling off the plank or touching the floor, as these movements
were considered errors. It was also an error if the subject turned
around before or after the mid-area of the plank or if the subject
did not step on the footprints at the ends of the plank. The performance
time and the number of errors were recorded. The sum variable was
also calculated by summing the time and the number of errors, where
one error was assigned a value of 1 s.
Postural balance
In Studies 1-3 postural balance was measured using the Good Balance
measurement device of Metitur Ltd (Metitur, 2001),
and in study 4 postural balance tests were carried out on a custom-made
force platform (Starck et al., 1993; Takala et al., 1997). The outputs of the instruments were voltage signals
from force transducers that registered vertical forces and anteroposterior
and mediolateral moments in relation to the central axis of force
platform. The force transducers were situated in each corner of
the platforms. The Good Balance measurement system consisted of
an equilateral triangular force platform, with a side length of
800 mm, connected to a computer through an electronic unit, which
included a 3-channel amplifier and a 12-bit analogue-to-digital
(A/D) converter. The analogue signals were amplified and then digitized
with a sampling frequency of 50 Hz and passed to the computer through
a serial (com) port. Then the data were filtered and processed in
digital form by the software. The data of each channel were filtered
separately using two different filters: a 7-point median filter
and lowpass infinite impulse response filter with a cut-off frequency
of 20 Hz. The median filter removed or effectively reduced impulse
noise, whereas high-frequency noise, caused by the measurement system
and A/D conversion, was reduced by lowpass filtering (Aalto, 1997).
The custom-made force platform consisted of two square metal plates,
with a side length of 400 mm, placed horizontally and separated
by force measuring elements (Starck et al., 1993; Takala et al., 1997). After amplification, 12-bit A/D conversion of the force
signals was carried out, the signals being digitized with a sampling
frequency of 40 Hz. Measurement noise was decreased by filtering
signals digitally with a three-point medianfilter and thereafter
with a 16-order finite impulse response filter with a cut-off frequency
of 10 Hz.
From the filtered data the x- and y-coordinates of the COP displacements
during the measurement time were then calculated following the procedures
reported by Hoffmann (1998)
(Good Balance) and Takala et al. (1997) (custom-made platform). In the Good Balance system on
the basis of the COP coordinates three postural balance outcome
variables were finally calculated: anteroposterior sway velocity
(mm·s-1), mediolateral sway velocity and velocity moment
(mm2·s-1) (Table
4). Sway velocities were calculated by dividing the total COP
displacements in each direction by the measurement time (s). The
velocity moment refers to first moment of velocity calculated as
the mean area covered by the movement of the COP during each second
of the test (Era et al., 1996). The calculation of the velocity moment took into account
both the distances from the geometric midpoint of the whole test
and the speed of movement during the same period (Era et al., 1996). All analyses involving the Good Balance force platform,
were carried out by centralizing the data of each test with respect
to its own midpoint. The sway parameters of the custom-made force
platform were related to the arithmetic mean point, which was calculated
by averaging the mediolateral and anteroposterior displacement of
the COP over the whole measurement period. The parameters calculated,
when the custom-made platform was used, were the mean sway velocity
(mm·s-1) and the mean sway amplitude (mm) (the anteroposterior
and mediolateral components of sway were analyzed together) with
eyes open and closed. The mean amplitude was the mean distance between
the sampling points and the arithmetic mean point.
The following measurements were carried out while the subject was
standing on the force platform: 1) normal standing for 40 s (Good
Balance) or 30 s (custom-made platform) with the the eyes open,
the hands placed on hips and the gaze fixed on a cross on the opposite
wall at eye level, 2) normal standing as before, but with the eyes
closed, 3) tandem standing for 20 s (Good Balance) with the eyes
open and the feet positioned heel-to-toe along the midline of platform,
the subject being instructed to stand as immobile as possible. Table
4 shows the variables measured with each force platform technique
in Studies 1-5.
Dynamic
stability
Dynamic stability was also assessed using the Good Balance measurement
system (Metitur, 2001).
In the test, the monitor for visual feedback was on the table in
front of the subject. Eight targets were shown in a circle on the
computer monitor (Figure 1,
Study 5). The idea of the measurement was to move the COP through
the targets. The subject was instructed to reach the targets as
quickly and as accurately as possible and to avoid unnecessary and
uneconomic movements. The performance time (s) (time used to complete
the test) and the distance (mm) (the extent of the path traveled
by the COP during the test) were measured. Dynamic stability and
postural balance and parameters were scaled according to body height
in Studies 1-3 and 5 (Metitur, 2001).
Perceived
balance abilities
Perceived balance abilities in relation to the balance demands of
work was inquired about in a questionnaire (Lusa-Moser et al., unpublished
data; Punakallio et al., 1997a) using the following question: "Balance is needed
especially in work in high and narrow places. How do you rate your
current balance abilities with respect to the balance demands of
your work?" The scale of the question ranged from 1 to 5: 1
= very poor, 2 = rather poor, 3 = moderate, 4 = rather good, 5 =
very good. Categories 1 through 2 (poor) were combined for further
analysis, as were categories 4 and 5 (good). The results were also
presented with categories 1 through 3 (poor-to-moderate) merged.
Anthropometrics, muscle capacities and cardiorespiratory capacity
(Table 4) were also measured,
and they were considered to be individual background factors for
balance. A detailed description of the measurements of balance and
the other physical capacities are given in the original studies.
Balance and slipping tests with fire-protective equipment (Study
2-3)
Fire-protective
equipment
The FPE included a two-piece multilayer fire-protective suit that
fulfilled the European standard (EN 469: 1995)
(Committee for Standardization, 1995).
The subjects also wore Nordic-type middle and under clothing, rubber
safety boots, a helmet and a tool belt, and they carried Dräger
SCBA (Germany) with one steel air bottle worn on the back and a
full-face mask. The mass of the FPE was 25.9 kg, of which the SCBA
accounted for 15.5 kg.
Balance tests with and without the fire-protective equipment
After demonstrations of the balance tests, the subjects were allowed
to practice once before the measurement. Each test and equipment
combination was measured once. First, the subjects performed the
baseline postural balance test with their eyes open and in sportswear
(t-shirt, shorts and barefoot). The test was immediately repeated
with the eyes closed. Second, the baseline functional balance test
was carried out. Third, the functional balance test was performed
1) with the full-face mask of the SCBA, 2) with the rubber safety
boots, and 3) with the SCBA. Finally, the functional and postural
balance tests were carried out with the FPE.
Slipping tests with the fire-protective equipment
Slipping tests were carried out on a straight 8-m long path. In
one area (400 x 600 mm), covered by stainless steel, water and detergent
(0.5 % by weight sodium lauryl sulfate solution) or glycerol (85
% by weight) were spread on the path. The first half of the track
was covered with a rubber carpet. The subject wore a safety harness
fastened into a rail above the track.
The length of each slip (i.e., horizontal sliding distance of the
foot in the walking direction in centimeters) was assessed with
one high-speed camera system. Video-recordings were used to estimate
the seriousness of the slipping incidents. The slips were classified
into the following four categories according to the efforts of the
walkers to restore their balance by corrective movements (Hirvonen
et al., 1994): 1) no observable slipping (the length of the slip was
less than 5 cm or the subject made no corrective movement), 2) controlled
slip (the subject swayed and made controlled corrective movements
and would probably have regained his balance even without the safety
harness), 3) vigorous slip (the subject slipped and staggered significantly;
the corrective movements were vigorous; without the harness a loss
of balance would have been possible), and 4) extremely vigorous
slip (the subject lost his balance and was suspended by the safety
harness).
Each subject performed four slipping tests under the following conditions:
1) dry path, walking speed 100 steps·min-1, 2) path spread
with water and detergent, walking speed 100 steps·min-1,
3) path spread with glycerol, walking speed 100 steps·min-1,
and 4) path spread with glycerol, walking speed 120 steps·min-1.
The walking speeds were defined by a metronome. The tests were performed
in the same order with every subject, but none of them were aware
of the slipperiness of the path in advance. The subjects were asked
to walk at the given walking speed as naturally as possible throughout
the tests.
Questionnaires (Study 1-5)
Perceived
work ability
Perceived work ability was inquired about with the work ability
index (WAI) using a questionnaire (Tuomi et al., 1991, 1998).
The WAI is a sum variable based on the subjective estimations of
work ability in relation to physical work ability (PWA) and mental
job demands. It also includes questions on current diseases, estimated
work impairment due to diseases, sick-leave days during the past
12 months, prognosis of work ability after 2 years and mental resources.
The WAI score ranges from 7 to 49 and is divided into four categories
of work ability as follows: poor (7 to 27 points), moderate (28
to 36 points), good (37 to 43 points), and excellent (44 to 49 points).
The cut points for poor and excellent work ability were chosen from
the 15th percentile of the index distribution of the
total studied population, and the moderate and good classifications
were the number of points dividing the distribution of the WAI in
half (Tuomi et al., 1985; Tuomi et al., 1998). Furthermore work experience and physical activity were
inquired about with the use of questionnaires in (Table
4).
Assessment of the predictive value of the balance tests (Study
4)
Work-related validity was assessed by examining the predictive value
of the results of the balance tests and the perceived balance abilities
in respect to the WAI and PWA. There were no classifications available
for the functional and postural balance tests. Therefore with the
continuous predictive variables, the subjects were divided into
the categories of "good", "average" and "poor"
using tertiles of the variables as the cut points. In the functional
balance test, no errors were categorized as "good", one
error was "average" and more than one error was "poor"
(Table 10).
Assessment
of reliability (Study 5)
In the evaluation of the trial-to-trial reproducibility, the functional
balance and dynamic stability tests were repeated six consecutive
times in one test period with a break of a few seconds between the
trials. The test-retest stability was studied over a longer period
by carrying out the balance tests again about 2 months later. Stability
between the test sessions was calculated using both the last trial
in each trial combination and the best values of the first to second,
first to third, first to fourth, first to fifth, first to sixth
and third to sixth consecutive trials as the outcome variables.
From these two, the outcome variable with the highest test-retest
stability was chosen as the final outcome variable.
Statistical
analysis
In
Study 1, one- and two-way general linear models were used to analyze
the effects of age and occupation on the balance tests. Furthermore,
in each occupational group, multiple comparisons with Tukey's method
were made to determine the significant differences between the age
groups. If differences were found between occupations, they were
further evaluated separately among the "male-specific"
occupations and the "female-specific" occupations. These
models were adjusted for the muscular endurance of the legs, physical
activity and work experience.
In Study 2, linear mixed models (analysis of variance with repeated
measurements) were used to study
the effect of FPE, age, and eye closure on the results of balance
abilities. The interaction of FPE and age, eye closure and age,
as well as the interaction of FPE, age, and eye closure, was added
to the models. The models were fitted using the PROC MIXED procedure
in the Statistical Analysis System, SAS Version 8.2 (SAS Institute,
1999).
In Study 3 Student's t-tests were applied to determine the statistical
significance for age, balance and muscle capacities between the
subjects as classified by sliding distance (< 5 cm versus >
5 cm).
In Study 4, logistic regression analysis with proportional odds
assumption (Peterson and Harrell, 1990) was used when the associations between the outcome (WAI
and PWA determined in 1996 and 1999) and predictive variables (functional
and postural balance and perceived balance in 1996) were examined.
The final models (outcome in 1999) were estimated by adding the
previous outcome in 1996 to the models (i.e., transition models)
(Diggle et al., 1994). In addition, the models were adjusted for age. Odds
ratios (OR) and 95% confidence intervals (CI) were calculated.
In Study 5, two-way general linear mixed models (analysis of variance
with repeated measurements) were used to assess the systematic error
between six consecutive trials and between the test sessions with
an interval of 2 months. Stability between the test sessions was
calculated using both the last trial in each trial combination and
the best values of the first to second, first to third, first to
fourth, first to fifth, first to sixth and third to sixth consecutive
trials as outcome variables.
The absolute reliability between trial-to-trial and between test-retests
in Study 5 was expressed using Bland and Altman's 95% LoA (Bland
and Altman, 1986; Atkinson and Nevill, 1998). The LoA was calculated as follows. First, the differences
(=bias) and the standard deviation between the tests were calculated,
and then the standard deviation of the differences was multiplied
by 1.96 to obtain the 95% random error component. The reproducibility
between trial-to-trial and stability between test-retests over time
was estimated by the ICC with 95% CI. The ICC values were classified
according to Fleiss (1986),
with the exception that the excellent category (> 0.75) was divided
into two categories, as follows: > 0.90 excellent, > 0.75-0.90
good, 0.40-0.75 modest and < 0.40 poor reproducibility.
Statistical analyses were performed using the Statistical Analysis
System, SAS Version 8.2 or 6.2 (SAS Institute, 1989
and 1999)
and SPSS 11.5 (SPSS Inc, 1997).
The statistical significance was defined as p < 0.05.
|
| RESULTS |
|
Relationship between balance abilities and the age and occupation
of workers in physically demanding jobs (Study 1)
In general, age and occupation had a significant effect on functional
balance (F = 15.0, p = 0.0001 and F = 69.2, p = 0.0001, respectively).
Among the firefighters, construction workers and home care workers,
the younger and middle-aged subjects were significantly (p = 0.028-0.001)
faster, and they made fewer errors than the older ones (Table
3, Study 3). About one-third of the male subjects (25% of the
construction workers, 36% of the firefighters) and 54-57% of the
female subjects made one error or more in the test. Furthermore,
the construction workers had significantly better functional balance
than the firefighters. The home care workers also performed faster
in the test than the nursing workers (Figure
4). These differences remained significant when the muscular
endurance of the legs, the intensity of physical exercise and work
experience were used as covariates.
Significant effects due to age were observed for the mean mediolateral
and anteroposterior sway velocity in normal (F = 4.5, p = 0.010,
and F = 3.9, p = 0.020, respectively) and tandem (F = 13.8, p =
0.0001, and F = 3.8, p = 0.021, respectively) standing positions.
The older subjects in each occupation tended to sway the most. The
differences in the mean velocity between the occupational groups
in the mediolateral direction in the normal standing position were
also significant (F = 5.4, p = 0.006) (Tables
5 and 6).
The differences between the different aged subjects were significant
for velocity moment as well (F = 3.8, p = 0.020, for normal standing;
F = 8.2, p = 0.0004, for tandem standing). The older subjects tended
to sway the most (Tables 5
and 6). During tandem standing,
the home care workers tended to have lower values than the other
occupational groups. The difference was significant in a comparison
with the velocity moment of the nursing workers (p = 0.040), but
the difference disappeared when the model was adjusted for muscular
endurance of the legs.
Effects of fire-protective equipment on the balance abilities
of younger and older firefighters (Study 2)
The postural balance was significantly poorer in the tests with
the FPE than in the baseline tests with the sportswear (Figure
5a-c, Table 7). Age had
a significant effect on the anteroposterior velocity, whereas eye
closure increased postural sway significantly for all the variables.
The interaction of FPE, age and eye closure was significant for
the velocity moment, and it was nearly significant for the mediolateral
velocity of postural sway (Table
7). The harmful effect of the equipment on postural balance
with the eyes closed was greater for the older firefighters (p =
0.033 for mediolateral velocity and p=0.005 for velocity moment)
than for the younger firefighters (p = 0.737 for mediolateral velocity
and p = 0.893 for velocity moment.)
The use of the FPE impaired functional balance significantly (F
= 5.5, p = 0.0002) as well. With the exception of the safety boots,
the different combinations of equipment had a significant effect
on functional balance (Figure 6).
The interaction of the FPE and age was, however, not statistically
significant (Figure 6). The
age-related differences in functional balance with and without equipment
were close to statistical significance (p = 0.065).
Balance, muscular capacities and age in association with slip
and fall risk (Study 3)
When the path was spread with water and detergent, altogether two
subjects in the older age group slipped 4 cm, and the sliding movement
and balance was well controlled. Every subject slipped in the tests
with glycerol at both walking speeds. The average values of the
sliding distances were 9.7 (SD 9.1) cm (100 steps·min-1)
and 15.6 (SD 18.2) cm (120 steps·min-1) in the 33-to-38-year
age group. The corresponding values were 10.8 (SD 15.3) cm and 18.0
(SD 18.6) cm in the 43-to-56-year age group.
Half of the subjects slipped more than 5 cm in the test. About two-thirds
(37/54) of the slip incidents were well controlled or needed no
corrective movements (Table 8).
The subjects whose sliding distance with glycerol was >
5 cm performed significantly poorer in the dynamic stability test
than those who slipped < 5 cm when the walking speed was 100
steps·min-1 (Table 9).
Predictive value of balance with respect to the perceived work
ability of firefighters (Study 4)
During the 3-year follow-up the proportion of the subjects whose
WAI category decreased was 34% (n = 45), and the proportion of those
whose WAI was poor increased from 6% to 16%. The WAI category was
increased for 21 subjects (17%) (Table
2, Study 4). Correspondingly, PWA decreased for 30% of the subjects
(n = 39) and increased for 5% (n = 7) (Table
3, Study 4).
At baseline in 1996, when the models were adjusted for age, the
poor PWA category was associated with poor results in the majority
the balance tests used (Table
10). In addition, poor results for performance time in the functional
balance test (OR 2.2, CI 0.9-5.4) and time (s) + errors (OR 2.4,
CI OR 0.9-5.4) and poor-to-moderate perceived balance (OR 9.8, CI
3.8-24.9) was associated with a lower WAI category at baseline (Table
10). The postural sway velocity and the sway amplitude with
the eyes open were excluded both from Table
10 and from the further analyses because the OR values were
not significant in any category.
The final predictive models were adjusted for age and also the baseline
WAI category. Table 11 shows
the predictors that were still statistically significant (i.e.,
poor categories in the functional balance test, sway amplitude with
the eyes closed and perceived balance in 1996 indicated 3.6-, 2.3-
and 9.5-fold risks, respectively, for a decline in the WAI during
the 3-year follow-up). The results were also calculated so that
the categories of poor and moderate were grouped together for perceived
balance. Then poor-to-moderate perceived balance showed a 2.4-fold
risk (OR 0.9-6.6) for a decline in the WAI in comparison with good
perceived balance abilities. Age and the baseline WAI category were
powerful predictors of the WAI in 1999. The final models for PWA
did not produce valid results because the PWA in 1996 and 1999 were
strongly related.
Reliability
of the dynamic stability and functional balance tests (Study 5)
Trial-to-trial reproducibility
The performance time of the dynamic stability test and the functional
balance test improved significantly (p < 0.001) after repeated
trials in both test sessions. After the third trial the improvement
between consecutive trials was no longer significant (Figure
7a-d, Table 2, Study 5),
and the average difference between the individual means was <0.5
s between the third and fourth trials (Table
3, Study 5).
In both balance tests, the LoA values were larger and the ICC values
were smaller almost without exception between the first and second
consecutive trials when compared with the results of all the other
consecutive trials (Table 12).
The differences between the LoA and ICC values of trial combinations
2-3, 3-4, 4-5, 5-6 were minor. The very lowest LoA values, as well
as the highest ICC values occurred either between the fifth and
sixth (dynamic stability) or the fourth and fifth (functional balance)
trials.
Test-retest
stability
The performance time of the dynamic stability test in two test sessions
with an interval of 2 months was significantly (p < 0.001) shorter
in the retest. The distance was shorter as well, and it was close
to statistical significance (p = 0.06) (Figure
7a-d).
The LoA and ICC values of the test-retest stability were reported
using the best values as the outcome variable because they were
smaller than the results obtained in the last trial of each trial
combination as a final score. In the dynamic stability test, the
LoA values were the smallest (Table
13), and the ICC values were the highest when the test-retest
stability was calculated using the best value of five repeated trials
as the outcome variable. In the functional balance test the most
reliable LoA and ICC values were obtained using the best of three
trials as the outcome (Table 13).
| DISCUSSION |
|
Main
findings
The main findings of this study were that the balance abilities
of the firefighters over 49 years of age were poorer than
those of the age groups of <40 and 40-49 years. The decline
of balance abilities among the construction, home care and
nursing workers was not as consistent. Postural balance was
also more detrimentally affected among the older firefighters
than among the younger ones when they used fire-protective
equipment (FPE) without visual input. Self-contained breathing
apparatus (SCBA) was the most significant single piece of
FPE to impair balance among both the younger and the older
firefighters. Furthermore, fast and controlled performance
in the dynamic stability test of the firefighters in all the
age groups was related to less slip and fall risk with FPE.
Older firefighters tended to slip longer and more seriously
than the younger ones, but the difference was not significant.
It was also found that the construction workers were faster
and made fewer errors than the firefighters in the functional
balance test. Among the firefighters, poor performance in
the balance tests was shown to be a risk factor for reduced
work ability index (WAI) category after a follow-up of 3 years.
Finally, the dynamic stability and functional balance tests
showed reasonable reliability, especially, when the reliability
was estimated from the best of at least three repeated trials.
Individual-, task- and environment-related aspects of balance
control
The results of this study agreed with previous findings of
age-related deterioration of postural and functional balance
abilities among people of working age (Choy et al., 2003; Du Pasquier et al., 2003; Gill et al., 1991; Pohjonen, 2001a; Røgind et al., 2003) and of balance being the poorest among people over 50
years of age (Ekdahl et al., 1989; Era and Heikkinen, 1985).
The present results do not agree with those of Hytönen et
al., (1993)
that postural balance was the most stable among healthy volunteers
from different occupations aged 46-60 years or with those
of Sihvonen et al. (1998),
who detected only minor decreases in the balance of the working-age
population. Although the same measurement tools and parameters
would have been used, comparisons of the age-related results
reported in different studies are difficult due to differences
in the selection of the subjects, the varying age ranges of
the subjects, and the differences in study design. As in the
previous studies of Colledge et al. (1994),
Matheson et al. (1999)
and Straube et al. (1988),
the age-related differences in the present study among home
care and nursing workers were more often significant when
the balance task was more challenging (i.e., the BOS was narrow
as in tandem standing and beam walking in the functional test).
One reason for the poorer balance among the older subjects
in this study, when compared with the results of the younger
ones, may be related to their lower muscular capacity. Colledge
et al. (1994)
suggested that the increase in body sway with aging is more
likely to be due to the slowing of central integrative processes
than to altered peripheral sensibility. Most of the studies
of mechanisms contributing age-related balance changes deal
with subjects over 60 years of age and young adults (Rankin
et al., 2000; Teasdale and Simoneau, 2001), and they do not include working-age populations.
Furthermore, the present results parallel those of Kincl et
al., (2002),
who showed that the use of FPE also impairs balance performance.
Moreover, age and visual condition affected the efficiency
of the balance control system to compensate for the inconvenience
caused by FPE. In the eyes-closed condition, FPE increased
sway clearly more among the older firefighters than among
the younger ones. These findings also show that visual afferents
are more important in balance control when FPE is used by
firefighters over 43 years of age than when it is used by
younger ones (33-38 years). The role of vision in balance
control has been shown to increase at least after the age
of 60 years (Du Pasquier et al., 2003; Perrin et al., 1997). Recently Choy et al. (2003)
reported that reliance on vision for postural stability was
clear among women aged 40-49 years in a single-limb stance,
among 50-to-59 year-olds when they were standing on foam,
and among 60-to-69-year-olds on a firm surface. The good balance
abilities and physical capacity of younger firefighters, as
well as their greater reliance on proprioceptive input rather
than visual input in balance control, may help explain their
greater ability to compensate for the harmful effects related
to the use of FPE in eyes-closed conditions.
In the present study, SCBA was the most significant single
piece of equipment to decrease balance performance. According
to Egan et al., (2001)
the United States Army's chemical protective ensemble, which
includes a M40 protective mask, a chemical overgarment, gloves,
and rubber overboots, but no respirator, does not affect postural
balance, even after simulated field tasks of 18 minutes. SCBA
affects balance control by shifting the place of body COG,
and its weight (15.5 kg) also increases strain on the balance
control system. Better balance control in firefighting and
rescue operations may be achieved by replacing the currently-used
steel air tanks with those made of composite materials, which
are at least 6 kg lighter. According to this study, balance
performance with the full-face mask of SCBA was also significantly
poorer than that in the baseline test. The mask limits visual
fields and thus may affect postural control because peripheral
vision is important in locomotion and the detection of movement
and changes of illumination (Paulus et al., 1984; Samo et al., 2003; Zelnick et al., 1994). This negative effect was not observed
in the tests with and without the M40 protective mask (Egan
et al., 2001). As in the results of Egan et al., (2001)
the use of safety boots did not affect the balance of firefighters
in this study. Therefore, safety boots do not contribute to
a decline in balance performance due to decreased tactile
cues from the feet.
SCBA is usually used in difficult, unusual and rapidly changing
work conditions, and it is associated with high physical strain.
This complex situation offers a greater challenge for a balance
control system than basic balance tests in laboratory conditions
do. According to Niinimaa and McAvoy (1983)
postural sway is greater during the aiming of an air rifle
after strenuous physical exercise than it is during the same
procedure in rest. An increase in postural sway after a 2-mile
run was also reported by Pendergrass et al. (2003).
It is possible that the negative effect of FPE on balance
control in the present study underestimated the effect in
actual fire and rescue operations. Some evidence of underestimation
has been reported by Kincl et al. (2002)
and Seliga et al. (1991), who showed that postural sway increased more consistently
after physical loading with respirators than without such
physical loading. These findings may indicate that the use
of respirators modifies the ability to maintain balance due
to a combination of work-related muscular and general fatigue
(Kincl et al., 2001; Seliga et al., 1991). Fatigue probably detrimentally affects the ability to
compensate for the respirator. In conditions of fatigue due
to physical exertion, the latency of a response to regulate
balance may be increased and may, therefore, have a degrading
effect, especially on the emergency reactive control of balance
(Hsiao and Simeonov, 2001).
The present results showing an age-related decline in balance
abilities and negative effects of FPE and eye closure on balance
control, as well as the tendency of longer slip distances
among older firefighters, are important to consider for safety
reasons in actual work situations in physical jobs. Fire and
rescue situations usually occur in heavily smoky or totally
dark environments in which visual feedback is poor, although
not absent as in the eyes-closed condition. In alarm situations,
several complicated aspects of work and the environment have
to be taken into account. Complex tasks that divide a worker's
full attention can reduce his or her ability to execute proactive
control of balance (Hsiao and Simeonov, 2001). Especially among older subjects, less attentional processing
capacity is available for balance control during a dual-task
paradigm (Rankin et al., 2000). For example, balance control has been shown to be poorer
when subjects perform a simultaneous cognitive task than when
balance performance takes place alone (Shumway-Cook et al.,
1997; Shumway-Cook and Woollacott, 2000). Furthermore, Hsiao and Simeonov (2001) and Parnianpour et al. (1989)
have suggested that the decline in postural control due to
age or inexperience may contribute to accidental falls at
work. It can be hypothesized, however, that experienced older
workers may be able to use their professional competence and
efficient work techniques to compensate for their age-related
decrease in balance control, for example, the disadvantage
related to the use of FPE. This possibility is important because
the mean age of Finnish firefighters is 39 years (Tilastokeskus,
2003),
and it will increase in the future because firefighters' retirement
age has been raised from 55 to 65 years. According to Lusa
et al., (1994)
every firefighter has to carry out tasks with FPE at least
a few times a year regardless of age. Although work in the
fire and rescue service has changed and diversified much during
the past few decades, it will contain the regular use of FPE.
Occupation-related differences in balance
In this study balance abilities were better for the workers
whose work also demanded high balance control. Compared with
the construction workers, the firefighters performed more
slowly and also made more errors in the functional balance
test. Firefighters operate on roofs or use ladders, on the
average, once in 3 months, depending on the number of alarms
(Lusa et al., 1994), whereas construction workers almost daily climb and
work for many hours on high scaffoldings. In general, the
home care and nursing staff performed more poorly than the
firefighters and construction workers did in the functional
balance test, and more poorly than the firefighters in the
postural balance test in a normal standing position. On the
other hand, the home care workers were superior in the tandem
standing position. The demands of the functional balance test
may be closer to those of dynamic physical jobs, and the differences
between the occupational groups were more consistent in the
functional balance test than in the postural balance test.
Moreover, the average results of postural balance in this
study did not differ from the reference values of the force
platform system that was used (Metitur, 2001).
The work environment may positively influence balance abilities
by providing the opportunity for intensive and specific training
and the learning of balance skills. According to Pajala et
al. (2004), individual environmental factors accounted for up to
half of the variance in postural balance among older women.
For example, fighter pilots had better balance control than
candidates for flight training (Kohen-Ratz, 1994), and construction workers showed lower body sway than
workers not engaged in manual work (Gantchev and Dunev, 1978). Unfortunately the present study did not collect postural
balance data for the included construction workers. Diard
et al. (1997)
suggested that the better performance of fighter pilots on
active duty in comparison with that of retired pilots is an
acquired skill, that requires continuous training. In addition
to learning and training in respect to environment- and task-related
balance demands of work, differences in balance abilities
between occupations may be explained by genetic effects (Kohen-Ratz
et al., 1994; Pajala et al., 2004), as well as by worker selection to a specific job. Genetic
effects accounted for one-third of the variance of the balance
factor, which consisted of data from postural sway measurements
in normal and semi-tandem positions (Pajala et al., 2004).
The aforementioned findings of occupation-related differences
in balance between male- and female-specific occupations are
preliminary, and they need further clarification because of
the possible differences between the sexes. Some studies have
shown balance differences between men and women, but the findings
are conflicting or the standardization of anthropometrics
removed the difference (Era et al., 1996; Juntunen et al., 1987; Maki et al., 1990; Matheson et al., 1999; Panzer et al., 1995). The differences in functional and postural balance between
home care and nursing staff in the present study cannot be
explained by different balance demands of the work. In the
tandem standing test, the difference between home care and
nursing staff disappeared when muscle strength and endurance
of the legs were used as covariates, but it remained significant
in the functional balance test. In this study no results were
available on the maximal rate of force development, which
may be a more important factor in balance control than maximal
strength (Izquierdo et al., 1999) or muscle endurance. Among people aged 40 and 70 years,
a decreased ability to develop force rapidly seems to be associated
with a lower capacity for neuromuscular response in controlling
postural sway (Izquierdo et al., 1999).
Slip and fall risk in association with age, balance and
muscular capacity
Although the younger and older firefighters experienced as
many slips in walking trials, there was a trend towards higher
age negatively affecting slip distances and the seriousness
of the slips, especially in the tests with a faster walking
speed. The age range of the older firefighters was also larger
(33-38 and 43-56 years) and therefore produced a high standard
deviation and reduced the differences between the age groups.
Slip distances of 2-50 cm were detected. Most of the slips
were, however, well controlled, even if their length was about
20 cm, and, in some cases, balance could be regained without
the safety harness. The recommended maximum tolerable criterion
for a slip distance ranges from 5 to 22 cm (Brady et al.,
2000; Grönqvist et al., 1999; Strandberg and Lanshammar, 1981). According to Leamon and Li (1991),
an additional load increases slip distance as well. Therefore,
the use of FPE may have produced long slip distances in the
present study. Furthermore, the muscular endurance of the
lower extremities of the firefighters in the present study
could be classified as at least good (Lusa, 1994). It is possible that the good muscular capacity of the
firefighters allowed long controlled slips. Postural activity
from bilateral leg and thigh muscles and coordination between
the two lower extremities have also been shown to be key factors
in reactive balance control in slips occurring at heel strike
(Tang et al., 1998).
The firefighters whose sliding distance with glycerol was
critical (i.e., >5 cm) tended to have poorer results
in the balance and muscular capacity tests than those who
slipped less than 5 cm at a walking speed of 100 steps·min-1.
The difference was significant only for the dynamic stability
test. The same tendency was seen with the speed of 120 steps·min-1,
but no differences were significant due to the smaller number
of studied firefighters (because of technical reasons four
firefighters' results were not available).
The dynamic stability test is based on visual feedback on
the movement of the COP through the targets shown a computer
screen. Therefore, the efficient utilization of visual input
in balance control may be an important protective factor with
respect to slip and fall risk. Although the balance task is
to move the COP actively in the dynamic stability test, it
cannot go over the BOS in a successful test. For a forward
slip during walking, a smaller movement and faster velocity
of the body's COM over the BOS plays a significant role in
slip recovery and fall termination from heel strike to contralateral
toe off (You et al., 2001). Furthermore, the dynamic stability test primarily demands
the use of ankle strategy, which is one of the protective
responses after the onset of a slip as well. The ankle moment
has been shown to decrease with the severity of the slip,
and knee flexor and hip extensor moments have been found to
be primarily responsible for corrective balance reactions
(Cham and Redfern, 2001).
Therefore, in actual work situations, ankle strategy may be
insufficient to prevent falling during a slip event.
Predictive value of balance for work ability and the reliability
of the dynamic balance tests
Poor perceived balance abilities, many errors in the functional
balance test, and a high amplitude of postural sway with the
eyes closed were the most significant predictors of decreased
WAI after the 3 years of follow-up in this study. The errors
in the functional balance test showed almost 4-fold risk for
decreased WAI when compared with an accurate performance in
the test. At baseline, good performance time in the functional
balance test was also associated with good WAI and PWA. Performance
time in the functional balance test has been shown to be a
strong predictor for WAI among home care workers (Pohjonen,
2001a) as well. In difficult environments of actual fire and
rescue work, both accurate and rapid balance adjustments are
challenged and needed. Findings of occupation-related differences
in this study also suggested that performance in the functional
balance test may be related to the balance demands of physical
work. According to the results difficulties to control balance
without visual input in the postural balance test may also
reflect problems with work ability.
The question of perceived balance may include specific aspects
of balance needed in fire and rescue work. It can be hypothesized
that the studied firefighters considered extrinsic factors,
such as difficult work environments and challenging work tasks,
when they evaluated their ability to balance in respect to
the demands of fire and rescue work. Furthermore, like perceived
balance, both WAI and PWA are based on subjective opinion,
which probably also affected the close association between
perceived balance abilities and WAI and between perceived
balance abilities and PWA. Previously, Chiou et al. (1998)
showed promising results for the Perceived Sense of Postural
Sway and Instability Scale among industrial workers in evaluating
the possible loss of balance at work. The ability to perceive
balance demands correctly was critical if the necessary postural
stabilization processes were to be triggered during work.
It was concluded that workers were able to perceive their
postural sway due to changes in peripheral vision, environmental
lighting, workload and surface firmness. In the present study
the used perceived balance abilities and the functional balance
test are simple, easy and unequivocal to perform and interpret
in occupational health services. The method using a force
platform for measuring postural sway is more complicated,
but it offers a reliable means of studying the basic balance
function with different sensory conditions.
Furthermore, six repeated trials showed improving results
in both of the dynamic balance tests. This learning effect
may be related to a person's ability to change postural strategy
to a more efficient one (Hertel et al., 2000; Hirvonen et al., 2002). According to Hansen et al. (2000)
the learning process is more pronounced for dynamic balance
tests than for static ones. In the present study, a baseline
level was attained for performance time in the dynamic stability
and functional balance tests after three repeated trials.
The results supported the concept of using more than one trial
to obtain reliable balance results (Hertel et al., 2000; Hirvonen et al., 2002). The performance time in the dynamic stability test of
the present study was also shorter in the test-retests done
after an interval of 2 months, whereas there was no significant
difference for the functional balance test. The major learning
effects associated with the dynamic stability test, when compared
with those of the functional balance test, can partly be explained
by the different balance tasks evaluated. Walking belongs
to habitual activities of daily living, which alleviates the
learning effect, whereas the firefighters moved their COP
through the targets on the computer monitor for the first
time. Kinzey and Armstrong (1998)
hypothesized that reliable balance tests should possibly involve
tasks that simulate daily activities.
Almost all of the ICC and best LoA values of the repeated
trials in the functional balance test showed good trial-to-trial
reproducibility. Although most of the ICC values of the dynamic
stability test were moderate between six repeated trials,
the LoA values between repeated trials were broad and indicated
low absolute trial-to-trial consistency and wide individual
variation. Large individual variation was also previously
reported to be a typical problem in the evaluation of balance
control within a normal population (Birmingham, 2000; Hansen et al., 2000; Takala et al., 1997). Large variation may hamper, for example, a reliable
evaluation of the changes in balance control. In general,
the individual variation in the balance results of the present
study was high. That observation and previous results strengthen
the need to use the best or average value for repeated trials
as an outcome variable for reliability, as shown previously
to improve the reliability of balance tests (Corriveau et
al., 2000; Mustalampi et al., 2003). In addition, the sample size was small, and large changes
in the results of one studied individual made the LoA values
significantly broader. The sample size of previous reliability
studies of dynamic balance tests range from 16 to 57 (Brouwer
et al., 1998; Hertel et al., 2000; Hirvonen et al., 2002; Punakallio et al., 1997a; Räty et al., 2002; Rinne et al., 2001), and some reliability studies of postural balance have
been carried out with less than 10 subjects (Corriveau et
al., 2000; Takala et al., 1997).
In
the present study, the highest test-retest reliability (moderate
level, LoA 4 s and 572 mm) was attained in the dynamic stability
test when the best results of the five first consecutive trials
were used as the outcome variables. With the exception of
the first trial combination (best of trials 1 and 2) as an
outcome variable for the functional balance test, the reliability
of different trial combinations was very similar and showed
good stability. LoA values of 2.5 s (best of three trials)
were also reasonable. The ICC values of the functional balance
test were at the same level as previously reported for functional
tests (Hertel et al., 2000; Räty et al., 2002; Rinne et al., 2001) and lower for the dynamic stability test when compared
with the findings of previous studies (Brouwer et al., 1998; Hirvonen et al., 2002). The tasks in the dynamic stability tests differed, and,
therefore, the reliability of these tests is specific for
each method.
Methodology
Subjects
The present data were based on a stratified sample, and not
only healthy volunteers were invited to participate. Therefore,
the dropout rate of 27% during the 3-year follow-up is reasonable
and describes a normal phenomenon in worklife. The reasons
for the dropout were also obvious. For example, over half
of the dropouts retired on an old-age or disability pension
or were on long-lasting sick leaves, and 15 subjects were
unwilling to participate in the follow-up tests for personal
reasons. Generalization of the results to the entire study
group is supported in that the balance test results, the WAI
and the PWA did not differ significantly at the baseline between
the study sample and the dropouts in Study 4. Furthermore,
the characteristics of the southern Finland firefighters in
Study I did not differ with respect to the entire sample measured
in 1999.
Although, the studied firefighters in Studies 2, 3 and 5 were
volunteers, and six of them were excluded from the study because
of their musculoskeletal disorders, their results at baseline
were similar with respect to balance, muscle and cardiorespiratory
endurance and the frequency of physical exercise of the entire
sample of firefighters at baseline and also when compared
with the sample of firefighters from southern Finland in 1999.
The mean age of the firefighters (41-42 years) in Studies
1-5 was at the same level as the mean age of all professional
firefighters in Finland (i.e., 39 years) (Tilastokeskus, 2003).
All the studied firefighters were men since, at the time,
there was only one female professional firefighter in Finland.
In addition to firefighters, Study 1 included construction
workers, home care workers and nursing staff. These subjects
represented ordinary workers of different ages in four occupational
groups without any specific background of motor skill training.
The subjects were either randomly chosen or all the workers
from certain work units were selected for the study, and not
only volunteers were invited. In some cases, the group sizes
were small, in most cases below 25 and in four cases below
15. Although the studied occupational groups were small, the
participation rates of the eligible subjects were high (84-88%).
Methods and study design
Balance measurements
Two physiotherapists performed the balance measurements in
Study 4 and those on firefighters and home care workers in
Study 1. Two different physiotherapists tested the nursing
staff and construction workers. In Studies 2, 3 and 5 the
same physiotherapist conducted the measurements and, in study
5, also on both occasions. All the physiotherapists were well-trained
and experienced professionals. In addition, the reliability
between two well-trained physiotherapists was shown to be
high with respect to the functional balance test (r = 0.95,
p < 0.001, n = 12) (Punakallio, unpublished data). The
present dynamic stability and functional balance tests have
previously shown some learning effects between repeated trials,
and their reliability was not defined according to current
recommendations in exercise sciences (Atkinson and Nevill,
1998; Hoffman, 1998; Punakallio et al., 1997a). Therefore the test-retest reliability of these two
dynamic balance tests was investigated in Study 5 (See section
7.4.).
In the functional balance test, in addition to short performance
time, no or few errors can be considered to show good control
of performance. When a sum variable (performance time + number
of errors) was used, a time measurement of 1 s was chosen
as one error. Then the proportion of errors (range 0-6 for
the subjects in this study) increased the mean performance
time (range 10-23 s) of the sum score, but the errors were
not emphasized in the sum score.
The postural balance tests performed in this study are widely
used, and the reliability of the used parameters was acceptable
(Mustalampi et al., 2003; Takala et al., 1997). The reliability of perceived balance abilities needs
to be established.
In Study 2, of balance abilities and FPE, the firefighters
first performed the baseline postural balance tests in two
visual conditions and the baseline functional balance test.
Thereafter, two postural balance tests and four functional
balance tests were performed with different equipment combinations.
Furthermore, in Study 5, on trial-to-trial reproducibility,
the balance tests were repeated six times. The firefighters
included in both studies were highly motivated to perform
all the repeated tests well. The tests were brief and physically
undemanding, and fatigue hardly affected performance. If there
was a learning effect from the four and five repeated trials
in Study 2, it would probably have only reduced the differences
between the baseline balance tests and the tests with FPE.
Slipping tests
So that the unexpected and surprising nature of slipping in
the tests would be preserved, the number of trials were limited
as much as possible. The firefighters received no information
on the number of trials they would perform or on the slipperiness
and slip-resistance of the path. Anticipation of slipping
could not completely be avoided, and it may have affected
the slipping responses and, therefore, possibly diminished
the differences between the firefighters. Previously, it has
been shown that a more cautious walking strategy seems to
be adopted when a potential slip risk exists (Cham and Redfern,
2002;
Marigold and Patla, 2002). Because of the anticipation and test method, the present
slipping responses obtained in the laboratory may also differ
from those in actual firefighting situations, which often
include disturbing factors such as noise, poor lighting and
psychological stress. A track with curves, uphills, and downhills,
and different kinds of obstacles would be closer to actual
work situations than the straight path used in Study 2. In
addition only one foot stepped on the slippery surface. A
larger slippery area would be nearer actual environmental
conditions.
Work
ability index
Because the concepts of job performance, work capacity and
work ability are rather confusing and therefore difficult
to define, their operationalization is also problematic, and
there is a lack of valid measures for them (Pohjonen, 2001b). Using the stress-strain concept (Rutenfrantz, 1981) Ilmarinen et al. (1991)
have defined work ability as a worker's capability to manage
his or her job demands. This group of Finnish researchers
also constructed a questionnaire-based measure, the work ability
index (WAI) (Tuomi et al., 1991; Tuomi et al., 1998), to operationalize the concept of work ability. The mean
WAI score and its classification into WAI categories have
been reported to be stable over a 4-week interval at the group
level. Repeated assessments of WAI have also provided evidence
of acceptable test-retest reliability at the individual level
(De Zvart, 2002).
In addition, the WAI is a sum variable, which also includes
many other aspects of work ability than physical work capacity.
In cross-sectional studies, prerequisites of work ability
for physical jobs (i.e., good performance in motor, musculoskeletal
and cardiorespiratory capacity tests) are associated with
a good WAI among firefighters (Pokorski et al., 2004; Punakallio et al., 1997b). Good performance on physical capacity tests (balance,
muscle strength) was strongly associated with the physical
demands and WAI of home care workers as well (Pohjonen, 2001b). Among firefighters, the higher categories of the WAI
were also significantly associated with quicker performance
through a labyrinth in a smoke chamber (Pokorski et al., 2004).
|
| CONCLUSIONS |
|
This
study was carried out among workers in physically demanding
occupations, mainly among firefighters. The demands for balance
control have been characterized as high in fire and rescue
tasks. Based on the results of this study, the following conclusions
and recommendations are justified:
1.
Balance abilities of firefighters aged 50 years and over are
poorer than those of younger firefighters. The differences
in balance abilities are not as consistent among different
aged construction, home care and nursing workers. Furthermore,
postural balance is more harmfully affected among older firefighters
than among younger ones when they use FPE with their eyes
closed. SCBA proved to be the most significant single piece
of equipment to decrease balance performance.
These aspects should be taken into consideration during the
organization of work tasks in fire departments and in the
development of the characteristics of FPE. It is also essential
to provide ample balance training opportunities for firefighters
with and without FPE. These findings also support the need
for including balance assessments when the prerequisites of
work ability are evaluated for firefighters.
2. The ability to exploit visual feedback efficiently
in balance control seems to be associated with smaller slip
risk, whereas associations between muscular capacity and the
risk of slipping are not significant. Older firefighters also
tend to slip longer and more seriously than younger ones.
In respect to safety aspects on slippery surfaces, methods
based on visual feedback may provide useful information of
balance control, and, therefore, this kind of test may be
useful in evaluations of balance ability among workers in
physically demanding jobs.
3. Many errors in the functional balance test, a high
mean sway amplitude with the eyes closed, and a poor estimate
of one's own balance abilities predicted reduced WAI category
after the follow-up.
Balance abilities proved to be related to perceived work ability,
and the aforementioned tests and parameters may therefore
be useful when prerequisites of work ability are evaluated
for firefighters.
4. The functional balance test showed high trial-to-trial
reproducibility and stability over time, and for the dynamic
stability test the result was moderate. The narrowest limits
of agreement, about ±
2
s in the functional balance test and about ±
4
s in the dynamic stability test, would be acceptable for practical
use as well.
To improve the reliability of functional balance and dynamic
stability tests, three and five trials, respectively, should
be carried out after one pretest. Then the best trial value
should be used as the final result.
5. On the whole, in respect to safety and work ability
in fire and rescue work, the present results suggest that,
when the work ability of firefighters is being followed-up
the balance abilities should be taken into account. The balance
assessments of the present study can be included when prerequisites
of work ability are evaluated for firefighters. The approach
of the present study, in which balance control is considered
an interaction between a worker's individual characteristic
and the demands of daily work tasks and the work environment,
appeared to be relevant when the balance abilities of workers
in physically demanding jobs were studied. It may be useful
to adapt the frame of reference of the present study for studies
on workers from other physically demanding occupations as
well. Additional investigations of balance abilities utilizing
the frame of reference of the present study can be recommended.
These studies should include versatile individual-, task-
and environment-related parameters in association with balance.
It would also be useful to study the occupation- and age-related
differences in balance and the associations between slip risk,
age and balance with studies of larger numbers of subjects
in various physical jobs. Methods are needed for studying
balance demands of actual or simulated work tasks, and valid
field and laboratory balance tests based on the actual work
balance demands should be developed for workers from different
kinds of physical jobs. In the laboratory balance abilities
and the risk of slipping and falling during walking should
be tested concurrently to provide more understanding of individual
balance abilities in respect to slip-related accidents and
to provide a validation of balance tests in respect to slip
and fall risk. The effect of fatigue as well as exercise on
slip and fall risk and balance control on slippery or other
difficult surfaces and the environment should also be studied.
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| ACKNOWLEDGMENTS |
This
study was carried out at the Finnish Institute of Occupational
Health in the Department of Physiology. I wish to thank Professor
Juhani Ilmarinen, the Director of the Department of Physiology,
for placing the facilities of the Institute at my disposal.
I would like to express my deepest gratitude to my supervisor,
Professor Veikko Louhevaara from the Finnish Institute of Occupational
Health and the University of Kuopio, for his valuable comments
and support throughout this study. Especially his encouragement
during the writing process and the last stages of this work
has been invaluable to me.
My most sincere thanks are also due to my supervisor, docent
Pertti Era of Metitur Ltd, for his time and professional insight
into the area of studying balance abilities. Despite the many
other demands on him, he found time to constructively criticize
my manuscripts and participate in long stimulating discussions,
both of which crucially contributed to my work.
Professor Rolf Moe-Nilssen of the University of Bergen in Norway
and Professor George Stelmach from the Motor Control Laboratory
of the Arizona State University in the United States acted as
the official reviewers of my dissertation, and I gratefully
acknowledge their significant comments on the final manuscript.
My special thanks go to my co-author Sirpa Lusa, who I want
to single out for our profound discussions, friendship and her
encouraging support. Warm thanks go also to my co-author, Ritva
Luukkonen, for her valuable advice and assistance with the statistical
analyses and also for her support. I am grateful to Raoul Grönqvist
and Mikko Hirvonen for their collaboration during the measurement
and writing process of study III. They introduced me to slip
and fall research. I also owe many thanks to Risto Toivonen
for his kind help and advice on the technical aspects of balance
measurements.
I am especially indebted to Arja Töyry, Tuija Toikka and Marja-Riitta
Suikki for their expert work with the balance measurements,
and also to the entire staff of the functional capacity laboratories
in the Unit of Work Physiology in the Finnish Institute of Occupational
Health and the Kuopio Regional Institute of Occupational Health
for their help in performing the other laboratory measurements.
I particularly wish to thank Olli Korhonen, Kaarina Eklöf and
Harri Lindholm. My thanks also go to all the staff members of
the Department of Physiology. I owe Georgianna Oja special thanks
for her detailed and flexible advice in correcting the English
language of the original articles and the summary of the thesis.
Maija Jokinen, Hanna Koskinen and Seija Reponen added their
experience in drawing the figures, and Hannele Törni in editing
the first article.
Tiina Pohjonen deserves my thanks for her supporting friendship
and enlightening discussions, which particularly directed me
to define the topic of my thesis. I am grateful also to Maritta
Österberg for her help with the references and to all my other
friends and relatives for their long-suffering support.
The approving attitude of the heads of the fire brigades, as
well as the flexible arrangements they provided, ensured the
success and high quality of the measurements. Special thanks
go to all the firefighters from Southern and Middle-Finland,
as well as to the construction, home care and nursing workers
who participated in this study. They made this thesis possible.
I appreciate the intellectual support of the executive committee
of the Association of Occupational Health Physicians in Rescue
Services. Especially, I wish to thank Saila Lindqvist-Virkamäki
for her friendship and trust in my work.
This study was financed by the Fire Protection Fund of Finland,
the Finnish Work Environment Fund and the Finnish Institute
of Occupational Health. A personal grant was obtained from the
Finnish Work Environment Fund for the writing process. The Association
of Occupational Health Physicians in Rescue Services also financially
supported the preparation of the summary of the thesis. I thank
these organizations for providing the resources necessary for
this work.
Finally, dear Jarmo, thank you for your loving support and patience
during these years. Your optimism and the smiles of our daughters
have ensured my motivation and showed me what is really meaningful
in life. My nearest and dearest Jarmo, Iina and Elli, you are
the most precious to me.
Helsinki, September 2004
Anne
Punakallio
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| AUTHOR
BIOGRAPHY |
Anne PUNAKALLIO
Employment: Researcher, Finnish Institute of Occupational
Health, Department of Physiology.
Degree: PhD
Research interests: Work ability, balance control, slipping
and tripping accidents and ageing in physically demanding occupations.
Email: anne.punakallio@ttl.fi
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