The main finding of this study was that both strength circuit training modalities (instability and conventional) induced similar effects in untrained young adults after 7 weeks of training. The 1RM variables recorded in our study are in line with those reported by others in similar studies (Sparks and Behm, 2010). Traditional resistance training is characterized by greater overload forces applied than in unstable conditions (Kibele and Behm, 2009). Results, especially the BS data, suggest that although no overloads are applied to the knee extensor muscles when using an instability device for training, strength gains could be related to increased activation of trunk muscles (Anderson and Behm, 2005b) and sympathetic transmission of motor neurons (Asanuma and Pavlides, 1997). This may promote intramuscular and intermuscular coordination in the muscle groups involved, as well as more economic activation of agonist muscles (Rutherford and Jones, 1986), increasing strength levels. These arguments acquire greater relevance if we examine the exercises performed in the two training programs examined here. Thus, greatest strength gains were produced in the lower limbs, probably because the selected exercises were mainly unilateral and standing. Indeed, as the body moves in the vertical position as an inverted pendulum, there is a tendency for the center of gravity to swing (Roberson et al., 2004), increasing the degree of instability and possibly favoring the activation of the trunk muscles (Anderson and Behm, 2005b) and inter and intra muscular coordination (Rutherford and Jones, 1986). In contrast, the lower strength gains (1RM) produced in the arms could be attributed to the fact that the exercises were conducted in a sitting position such that there is minimum displacement of the center of gravity. When we designed the protocols for the two training programs, we ensured that each exercise performed in stable conditions was matched with a similar exercise conducted in unstable conditions. The similar strength responses (1RM) produced by both modes of exercise could indicate that the body positions and degree of instability generated by the BOSU® and TRX® exercises, have a similar effect to that produced by the external load used in the conventional resistance training protocol. A further finding of our study was a significant increase in power and movement velocity in the subjects assigned to the IRT protocol. Undoubtedly, the adaptations of strength, power and velocity are determined by the intensity of established resistance (Tan, 1999). One of the main theories regarding training in unstable conditions is that it provides similar strength adaptations to training under stable conditions with the use of lighter loads (Behm et al., 2002). The responses obtained here to both training programs suggest that exercises performed using the instability devices BOSU® and TRX® at high velocity could increase power and movement velocity in similar measure to traditional resistance training. Prior studies have shown that instability training does not improve power development or movement velocity (Drinkwater et al., 2007; Kornecki and Zschorlich, 1994; Koshida et al., 2008). One of the features of instability resistance training is that exercises trigger a process of learning new motor patterns, leading to a lower execution velocity (Behm, 1995). In addition, the muscles around joints tend to favor stability over power generation. Several resistance training studies have shown that an essential factor for improving power development and movement velocity regardless of the load used is that exercises should be executed at an explosive velocity (ACSM, 2009; Häkkinen, 1989). The similar power and velocity gains produced in our two experimental groups suggest that the instability provoked and the execution velocity of movements in unstable conditions give rise to similar neuromuscular adaptations to traditional resistance training, resulting in increased power and movement velocity. As participants adapt to the degree of instability or load, it might be interesting to increase the execution velocity of muscle actions, simulating the specific motor patterns (Willardson, 2004) of other sports, provided the exercise is technically well-executed. We would argue that any improvement in power and movement velocity in conditions of instability will depend on two essential factors: that exercises are repeated and that loads are gradually increased in the mid and long term. This will likely determine a need to learn new motor patterns and adaptation towards the improved specificity of movements. In our study, we also observed a marked improvement in jumping ability despite the fact that the study participants were accustomed to sports such as basketball, volleyball and handball, in which jumping is a major specific motor action. In effect, several studies have reported significant improvements in vertical jump following lower body resistance training (Adams et al., 1992; Baker et al., 1994). Improved strength and power of the lower limbs may be a main trigger for significant gains in jumping ability (Häkkinen and Komi, 1985). The latter is in turn related to sports performance. An essential component of the design of any training program is the control of exercise intensity in the long term. Resistance training progression models in healthy adults indicate that a critical factor for power development is the gradual increase of loads (ACSM, 2009). In our study, training intensity was monitored using the indicator RPE. Several authors have shown the reliability (Day et al., 2004) of this method for strength training prescription (Lagally et al., 2004). In the subjects assigned to the TRT program, load increases were established according to self-perceived effort. Nevertheless, resistances provoked by an unstable surface whether attached to the floor like BOSU® or suspended like TRX® are autoloads. The RPE enabled us to determine the effort perceived by the subjects assigned to the instability circuit training program and to accordingly modify the degree of instability and/or body position to optimize the intensity of exercise. To date, no objective method to quantify exercise intensity for unstable surfaces has been described. This could thus be a useful tool to control exercise intensity in unstable conditions. We feel the circuit-type protocol also played an important role in controlling stimuli. By alternating upper and lower extremity exercises, muscle fatigue affecting the same muscle groups is avoided (Baechle and Early, 2008). The circuit’s stations could be designed to consider adequate recovery periods between exercises working different muscle groups. If at a given station a particular group of muscles is exercised, these muscles would not be the focus of the adjacent stations or posts until an adequate rest period were completed. The similarity of the results recorded in our tests suggests the similar perception of stimuli by the participants of both training programs. RPE was probably a key factor for the proper control of the intensity of instability training. We could interpret the results of our study as suggesting that IRT produces similar adaptations to those of TRT. Thus, exercises executed on or using unstable devices like BOSU® or TRX® could improve strength (1RM), power, movement velocity and jumping ability in young untrained adults in the same measure as weight lifting exercises performed in stable conditions. Other studies that have compared resistance training conducted in instability or stable conditions have identified two critical factors: the speed at which exercises are executed and the control of training intensity through RPE. We consider that our results cannot be extrapolated to high performance athletes or to subjects with experience in resistance training. This idea, however, prompts a new line of investigation in which the possibilities of the instability approach to training are further explored. Future studies need to establish the long term effects of instability strength training programs in other population groups. |