Static stretching (SS) and dynamic stretching (DS) are commonly performed as warm-up exercises before athletic activities (Smith, 1994; Woods et al., 2007). SS and DS improve flexibility, such as range of motion (ROM), peak passive torque (PPT), and passive stiffness (Goto et al., 2020; Iwata et al., 2019; Matsuo et al., 2019; Mizuno et al., 2013b). Low muscle flexibility (Witvrouw et al., 2003) and high stiffness (Watsford et al., 2010) are associated with a greater risk of muscle injury. Therefore, performing SS and DS as warm-up exercises may reduce muscle injury risk during sports.

Whereas SS and DS have similar positive effects on flexibility, they have distinct effects on sports and muscular performance. Many previous studies have shown that DS improves muscle power, jump height, and sprint time; SS has detrimental effects on these performance measurements (Behm et al., 2016; Behm and Chaouachi, 2011; Kay and Blazevich, 2012; Little and Williams, 2006; Perrier et al., 2011; Yamaguchi and Ishii, 2005). Therefore, DS is recommended as a component of warm-up exercises because of its beneficial effect on athletic performance.

However, SS and DS are usually performed in combination during warm-up exercises. Previous studies have investigated the effects of combined SS and DS on various aspects of sports and muscular performance (Amiri-Khorasani et al., 2016; Amiri-Khorasani et al., 2010; Amiri-Khorasani and Sotoodeh, 2013; Chaouachi et al., 2010; Faigenbaum et al., 2006; Fletcher and Anness, 2007; Hsu et al., 2020; Loughran et al., 2017; Takeda et al., 2020; Torres et al., 2008; Wong et al., 2011). Particularly regarding the effects of combined stretching (CS; i.e., DS+SS) on sports and muscular performance, the positive effects of DS on performance may counteract the negative effects of SS (Amiri-Khorasani et al., 2016; Amiri-Khorasani and Sotoodeh, 2013; Chaouachi et al., 2010; Faigenbaum et al., 2006; Fletcher and Anness, 2007; Hsu et al., 2020; Loughran et al., 2017; Wong et al., 2011). Nevertheless, the stretching method with the greatest effect on flexibility and performance remains unclear. Moreover, stretching routines are typically performed 15-60 min before competition or exercise (Woods et al., 2007). Therefore, it is important to investigate the prolonged effects of SS, DS, and CS on flexibility and performance. Matsuo et al. (2019) reported that immediate increases in ROM and PPT and immediate decreases in passive stiffness and isometric muscle force did not differ between 10 sets of 30-s SS and 10 sets of 30-sec DS. Hatano et al. (2019) reported that increases in ROM and PPT and a decrease in isometric muscle force were sustained over 30 min after one set of 300-sec SS; passive stiffness returned to baseline within 30 min. Mizuno et al. (2013a, 2013b) also reported that the effect of SS on passive stiffness diminished more rapidly than effects on ROM and PPT. Iwata et al. (2019) reported that the effects of 300-sec (10 x 30-sec) DS on ROM and passive stiffness were sustained over 90 min; PPT returned to baseline within 30 min. Accordingly, prolonged changes in flexibility and muscle force might differ between 300-sec SS and 300-sec DS, although effects immediately after SS and DS did not differ. To our knowledge, no studies have directly compared the acute and prolonged effects of SS, DS, and CS in terms of flexibility parameters (e.g., ROM, PPT, and passive stiffness) and muscle force. Additionally, there have been no analyses of these stretching methods in relation to neurophysiological activities, despite previous studies have suggested that the decrease in muscle force after SS was influenced by the changes in neurophysiological activities (Fowles et al., 2000; Kay and Blazevich, 2009; Trajano et al., 2013).

This study compared acute and prolonged effects of SS, DS, and CS (i.e., DS followed by SS or SS followed by DS) on ROM, PPT, passive stiffness, isometric and concentric muscle forces, and surface electromyographic activities. These data could guide the development of recommendations concerning the most appropriate stretching methods for warm-up exercises before athletic activity.

We conducted a randomized crossover trial, as illustrated in Figure 1. Prior to the first testing day, all participants attended a familiarization session in which they learned and practiced all stretching methods and measurements. Participants completed measurement sessions on four separate days (1 day for each stretching type). Specifically, they completed four stretching sessions comprising SS alone (SS session), DS alone (DS session), and CS (two CS protocols: 150-sec SS followed by 150-sec DS [SS-DS session] and 150-sec DS followed by 150-sec SS [DS-SS session]) of the right hamstrings for 300 sec. The order of stretching types was randomized. We recorded the ROM of passive knee extension, PPT, passive stiffness, maximum voluntary isometric and concentric knee flexion forces, and electromyographic root mean square (RMS) during force measurements before, immediately after, and 20 and 60 min after stretching (Matsuo et al., 2015). The experiment was performed in a university laboratory where the room temperature was maintained at 26°C (Matsuo et al., 2019; Matsuo et al., 2015). All measurements were performed at the same time of day (± 1 h). The mean testing interval was 17.0 ± 15.0 days (range, 7-63 days); there were no consecutive testing days.

Twenty healthy young men voluntarily participated in this study (mean ± standard deviation: age, 21.8 ± 1.4 years; height, 171.4 ± 6.3 cm; weight, 64.6 ± 10.8 kg; body mass index, 22.0 ± 3.1 kg/m²). All participants provided written informed consent to take part in the study. The study protocol was approved by the Human Research Ethics Committee of our institution. Exclusion criteria were lower extremity joint contractures, history of back or lower extremity surgery, neurological disorders, current treatment involving hormones or muscle-affecting drugs, ability to completely extend the right knee from a sitting position as described below (i.e., exceptional flexibility), engagement in competitive sports, regular resistance, aerobics, and flexibility training. Participants were asked to avoid vigorous physical activity during the experimental period.

As shown in Figure 2a, to perform SS, each participant assumed a standing upright position and placed their right heel (with the leg extended) on a 50-cm-high platform. The participant then reached forward with their arms toward the extended leg while maintaining a proper lordotic curve (Matsuo et al., 2019). SS was performed at a tolerable intensity without pain (Goto et al., 2020; Matsuo et al., 2019; Matsuo et al., 2015). At the SS session, ten 30-sec sets of SS were performed with a 20-sec rest period between each set (Matsuo et al., 2019).

As shown in Figure 2b, to perform DS, each participant assumed a standing upright position beside parallel bars and held a parallel bar with their left hand for stability. To stretch the hamstrings, the participant intentionally contracted the right hip flexors with the knee extended and flexed their right hip joint so that their right leg swung up to the anterior aspect of their body (Hough et al., 2009; Iwata et al., 2019; Matsuo et al., 2019; Yamaguchi and Ishii, 2005). The participant performed this dynamic movement every 2 sec. Each exercise was performed five times slowly for practice, then 10 times as quickly as possible without bouncing (Hough et al., 2009; Iwata et al., 2019; Matsuo et al., 2019; Yamaguchi and Ishii, 2005). At the DS session, ten 30-sec sets of DS (15 repetitions of the DS movement in each set) were performed with a 20-sec rest period between each set (Iwata et al., 2019; Matsuo et al., 2019).

At the two CS sessions, SS and DS were performed as described above. At the SS-DS session, five 30-sec sets of SS followed by five 30-sec sets of DS were performed with a 20-sec rest period between each set. At the DS-SS session, five 30-sec sets of DS followed by five 30-sec sets of SS were performed with a 20-sec rest period between each set.

The following dependent variables were used to assess the acute and prolonged effects of SS, DS, and CS. We first measured the torque–angle relationship (ROM, PPT, and passive stiffness), then recorded the isometric muscle force, concentric muscle force, and electromyographic RMS during force measurements before, immediately after, and 20 and 60 min after stretching. Considering that warm-up activities prior to SS do not enhance decreases in muscle–tendon stiffness compared with SS alone (Fujita et al., 2018), participants did not perform warm-up activities prior to stretching exercises or testing. All dependent variables, except RMS, were obtained using an isokinetic dynamometer (PrimusRS; BTE Technologies, Hanover, MD, USA); electromyographic RMS was determined by surface electromyography (Biomonitor ME6000; Mega Electronics, Kuopio, Finland). Torque and angle signals from the dynamometer and electrical signals from electromyography were subjected to analog-to-digital conversion (PowerLab 8/35; ADInstruments, Dunedin, New Zealand) and stored in a personal computer.

Measurements were performed while the participant assumed a sitting position with their hip joint flexed (Figure 3a), as previously reported (Matsuo et al., 2019; Matsuo et al., 2013; Matsuo et al., 2015). Each participant was seated on a chair with the seat maximally tilted and a wedge-shaped cushion inserted between the trunk and the backrest. The participant’s chest, pelvis, and right thigh were stabilized with Velcro straps. The knee joint was aligned with the dynamometer’s axis of rotation; the lever arm attachment was placed immediately proximal to the malleolus medialis. In this position, the mean hip and knee flexion angles were 107.1° ± 2.5° and 110.6° ± 2.3°, respectively. While the participant was seated in the chair, their knee was passively extended at 5°/s to the point of maximum knee extension immediately before the onset of pain; torque was continuously recorded during passive knee extension (Matsuo et al., 2019; Matsuo et al., 2013; Matsuo et al., 2015). ROM (in degrees) was defined as the maximum knee extension angle from the initial position (0°); PPT (in Nm) was defined as the torque immediately before the onset of pain (Goto et al., 2020; Matsuo et al., 2019; Matsuo et al., 2015). Passive stiffness (in Nm/°) was defined as the slope of the least-squares regression line calculated from the torque–angle relationship (Matsuo et al., 2019; Matsuo et al., 2013; Matsuo et al., 2015). Passive stiffness was calculated using the same knee extension angle range for all time points. The calculated knee extension angle range was defined as the angle from the 50% maximum knee extension angle to the pre-stretching maximum knee extension angle.

Isometric muscle force (in Nm) was measured in the position used to measure the torque–angle relationship (flexed position; Figure 3a). Isometric muscle force was also measured with the seat adjusted parallel to the floor (horizontal position; Figure 3b) while the participant was seated with hip and knee flexion angles of 85° and 90°, respectively (Matsuo et al., 2015). Isometric muscle force was measured in the flexed position, followed by the horizontal position. Two measurement positions were used because stretch-induced strength loss depends on muscle length (McHugh and Nesse, 2008), and the horizontal position is often used to measure isometric muscle force (e.g., Ford-Smith et al., 2001; Kollock et al., 2010). Each participant was instructed to sit with their arms crossed in front of their chest and generate maximum knee flexion force for 3 sec. They performed this exercise three times with a 45-sec rest period between trials (Matsuo et al., 2015). Verbal encouragement was provided during measurements. Peak torque was recorded for each trial; the mean of the three trials was used in subsequent analysis.

After measurement of isometric muscle force in the horizontal position, concentric muscle force (in Nm) was measured in the horizontal position described above (Figure 3b) using an isokinetic dynamometer (Matsuo et al., 2015). Three maximum voluntary concentric knee flexions were continuously performed at an angular velocity of 60°/s for the range of 85° from a knee-extended position (5°) to a knee-flexed position (90°) with the participant’s arms crossed in front of their chest. Verbal encouragement was provided during measurements. Peak torque was recorded for each trial; the mean of the three trials was used in subsequent analysis (Matsuo et al., 2015).

Surface electromyographic activities during maximal isometric and concentric contractions were recorded from medial and lateral hamstrings by a Biomonitor ME6000 (Mega Electronics) with a sampling frequency of 1 kHz. Before electrode placement, the skin under the electrodes was shaved, abraded, and cleaned. For medial hamstring assessment, a pair of Ag/AgCl sensors (Blue Sensor M-00-s; Ambu, Ballerup, Denmark) was placed halfway between the ischial tuberosity and the medial epicondyle of the tibia. For lateral hamstring assessment, another pair of sensors was placed halfway between the ischial tuberosity and the lateral epicondyle of the tibia (Matsuo et al., 2015). Surface electromyography signals were band-pass filtered (8-500 Hz). RMS values were calculated using Labchart v8 software (ADInstruments). Time windows for RMS calculation were 3 sec during isometric contraction and 1.416 sec during concentric contraction; ROM was 85° (5-90°), and angular velocity was 60°/s during concentric contraction. The mean of three trials for each contraction was used in subsequent analysis.

Test-retest reliabilities for all dependent variables were determined by calculating intra-class correlation coefficients (ICCs) and 95% confidence intervals (CIs) from pre-stretching values for all stretching sessions. The ICC_1,1 results indicated acceptable reliability (ROM [ICC_1,1: 0.845, 95% CI: 0.726-0.927], PPT [ICC_1,1: 0.923, 95% CI: 0.857-0.965], passive stiffness [ICC_1,1: 0.896, 95% CI: 0.810-0.952], isometric muscle force [ICC_1,1: 0.905, 95% CI: 0.852-0.943], concentric muscle force [ICC_1,1: 0.880, 95% CI: 0.783-0.944], and RMS [ICC_1,1: 0.748, 95% CI: 0.685-0.805]) (Koo and Li, 2016).

Sample size estimation was performed using G*Power software (v 3.0.10; Franz Faul, Kiel University, Kiel, Germany). Based on a previous study regarding the acute effects of SS and DS on passive knee extension ROM (Matsuo et al., 2019), the effect size was 0.88; α level was set at 0.05 and power was set at 0.80. Using the Bonferroni post-hoc test to identify a significant difference from the pre-stretching value, the estimated minimum number of participants was 18. Considering the potential for dropout, 20 participants were recruited.

Data normality was assessed using the Shapiro–Wilk test. The results indicated that isometric muscle force in the flexed and horizontal positions, concentric muscle force, and medial hamstring RMS values during concentric muscle contraction were normally distributed. However, the remaining data did not exhibit normal distribution. Parametric tests were applied to normally distributed data; non-parametric tests were applied to the other variables and to the relative changes (in %) for all variables. Changes in dependent variables over time were compared between stretching sessions by two-way repeated measures analysis of variance (RM-ANOVA) (stretching sessions and time) or the Friedman test. When a significant interaction effect or main effect (stretching sessions or time) was identified using two-way RM-ANOVA or the Friedman test, Bonferroni post-hoc analysis was performed to determine significant pairwise differences between sessions at each time point or to reveal differences from pre-stretching values. Analyses were performed using IBM SPSS statistics version 24.0 (IBM Corp., Armonk, NY, USA); the statistical significance threshold was set at p < 0.05. Results are expressed as means ± standard deviations.

ROM and PPT were significantly increased in all stretching sessions immediately and 20 and 60 min post-stretching, compared with pre-stretching (all p < 0.05) (Table 1). There were no significant differences in pre-stretching values, post-stretching values, or relative changes in ROM and PPT between stretching sessions.

Passive stiffness was significantly decreased in SS-DS and DS-SS sessions immediately and 20 and 60 min post-stretching, compared with pre-stretching (all p < 0.05) (Table 1). It was also significantly decreased in SS and DS sessions immediately and 20 min post-stretching, compared with pre-stretching (all p < 0.05). Passive stiffness tended to be decreased in SS and DS sessions 60 min post-stretching (SS; p = 0.09, DS; p = 0.06). There were no significant differences in pre-stretching values, post-stretching values, or relative changes in passive stiffness between stretching sessions.

Results were similar between flexed and horizontal positions. Two-way RM-ANOVA revealed no significant interaction effects but showed a significant main effect of time (p < 0.05). Isometric muscle force was significantly decreased in all stretching sessions immediately and 20 and 60 min post-stretching, compared with pre-stretching (all p < 0.05) (Table 2 and Table 3). There were no significant differences in pre-stretching values, post-stretching values, or relative changes in isometric muscle force between stretching sessions.

Two-way RM-ANOVA revealed no significant interaction effects but showed significant main effects of stretching session and time (p < 0.05). Concentric muscle force was significantly decreased in SS and SS-DS sessions immediately and 20 and 60 min post-stretching, compared with pre-stretching (all p < 0.05) (Table 4). It was also significantly decreased in DS-SS sessions 60 min post-stretching (p < 0.05), and it tended to be decreased immediately (p = 0.06) and 20 min post-stretching, compared with pre-stretching (p = 0.07). Concentric muscle force in DS sessions did not significantly change.

There were no significant differences in pre-stretching values, immediately and 60 min post-stretching values, or relative changes in concentric muscle force between stretching sessions. However, concentric muscle force was higher in DS sessions than in DS-SS sessions 20 min post-stretching (p < 0.05). Additionally, concentric muscle force tended to be higher in DS sessions than in SS-DS sessions 20 min post-stretching (p = 0.07).

Medial hamstring RMS values were significantly decreased in SS and DS sessions immediately and 20 and 60 min post-stretching, compared with pre-stretching (p < 0.05) (Table 2). They were also significantly decreased in SS-DS and DS-SS sessions immediately and 60 min post-stretching, compared with pre-stretching (all p < 0.05); they tended to be decreased 20 min post-stretching (SS-DS; p = 0.08, DS-SS; p = 0.05).

Lateral hamstring RMS values were significantly decreased in DS sessions 20 and 60 min post-stretching (both p < 0.05), and in DS-SS sessions immediately and 20 min post-stretching (both p < 0.05), compared with pre-stretching. In SS-DS sessions, they tended to be decreased 20 min post-stretching (p = 0.06). There were no significant differences in pre-stretching values, post-stretching values, or relative changes in RMS values between stretching sessions.

Medial hamstring RMS values were significantly decreased in SS-DS sessions 60 min post-stretching, compared with pre-stretching (p < 0.05) (Table 3). In DS sessions, they tended to be decreased immediately and 20 min post-stretching, compared with pre-stretching (immediately; p = 0.08, 20 min; p = 0.05). Lateral hamstring RMS values were significantly decreased in SS and DS sessions 20 min post-stretching, compared with pre-stretching (both p < 0.05). Moreover, RMS values in SS sessions 60 min post-stretching and in DS sessions immediately post-stretching tended to be decreased, compared with pre-stretching (SS; p = 0.08, DS; p = 0.08). There were no significant differences in pre-stretching values, post-stretching values, or relative changes in RMS values between stretching sessions.

Two-way RM-ANOVA revealed no significant interaction effects but showed a significant main effect of time for medial hamstring RMS values (p < 0.05). Medial hamstring RMS values were significantly decreased in DS, SS-DS, and DS-SS sessions immediately and 20 and 60 min post-stretching, compared with pre-stretching (all p < 0.05) (Table 4). They were also significantly decreased in SS sessions immediately and 60 min post-stretching (both p < 0.05); they tended to be decreased 20 min post-stretching (p = 0.07). Moreover, lateral hamstring RMS values were significantly decreased in SS sessions 60 min post-stretching (p < 0.05) and in DS sessions immediately and 20 and 60 min post-stretching (all p < 0.05), compared with pre-stretching. RMS values in SS sessions 20 min post-stretching (p = 0.07) and in DS-SS sessions immediately and 20 min post-stretching (immediately; p = 0.08, 20 min; p = 0.07) tended to be decreased. There were no significant differences in pre-stretching values, post-stretching values, or relative changes in RMS values between stretching sessions.

This study compared the acute and prolonged effects of SS, DS, and CS on ROM, PPT, passive stiffness, and isometric and concentric muscle forces. The results showed that increases in ROM and PPT and decreases in isometric muscle force after SS, DS, and CS were similar. However, CS tended to have longer effects on passive stiffness, compared with SS and DS. Additionally, concentric muscle force was u nchanged after DS; it decreased after SS and CS.

With regard to flexibility, this study showed that SS, DS, and CS enhanced ROM and PPT; they reduced passive stiffness. Moreover, the effects of all stretching methods on ROM and PPT lasted 60 min after stretching. Compared with CS, SS and DS tended to have shorter effects on passive stiffness; no significant differences were observed in pre-stretching values, post-stretching values, or relative changes between stretching methods. An increase in ROM immediately after SS and DS is attributable to an increase in PPT - caused by changes in pain threshold or stretch tolerance - and a decrease in passive stiffness (Iwata et al., 2019; Matsuo et al., 2019; Mizuno et al., 2013b). The acute and prolonged effects of SS on flexibility in our study are consistent with previous findings (Hatano et al., 2019; Mizuno et al., 2013a; Mizuno et al., 2013b). Changes in flexibility immediately after DS were also consistent with previous findings (Matsuo et al., 2019); the prolonged effects of DS on PPT and passive stiffness differed from previous work in which the effects of DS on ROM and passive stiffness were sustained over 90 min, while PPT returned to baseline within 30 min (Iwata et al., 2019). Our participants were all men, while the previous study included both men and women. Miyamoto et al. (2018) reported that, in men, ROM was associated with passive muscle stiffness and tolerance to muscle stretch; in women, it was associated with tolerance to muscle stretch but not passive muscle stiffness. Therefore, discrepancies between the studies might be related to sex differences.

The longer effects of CS on passive stiffness might arise from differences in stretched tissues. Nakamura et al. (2011) showed that 300 sec of SS decreased muscle– tendon unit stiffness and muscle stiffness; the decrease in muscle–tendon unit stiffness was caused by the decrease in muscle stiffness. Samukawa et al. (2011) observed proximal displacement of the muscle–tendon junction of the medial gastrocnemius, but no changes in pennation angle or fascicle length, after five 30-sec sets of DS involving antagonist muscle groups contraction. These authors suggested that DS primarily affects tendinous tissues. Thus, SS and DS might have distinct effects on passive muscle–tendon unit stiffness. Compared with SS and DS, CS might have greater effects on the whole muscle–tendon unit, regardless of stretching order. To determine the detailed mechanisms of the effects of stretching on flexibility, further studies are required to investigate how and which tissue is stretched using ultrasonography or shear wave elastography.

With regard to muscle force, this study showed that all stretching methods decreased isometric muscle force after 60 min. However, concentric muscle force was unchanged after DS; no significant differences were observed in pre-stretching values, post-stretching values, or relative changes between stretching methods. There is evidence that prolonged (>30-60 sec) SS has detrimental effects on muscle performance (Behm and Chaouachi, 2011; Kay and Blazevich, 2012; Simic et al., 2013). In our study, SS was performed for 300 sec in the SS session and for 150 sec in the SS-DS and DS-SS sessions. The longer duration of SS may have had detrimental effects on isometric and concentric muscle forces in sessions that included SS. The decrease in muscle force after SS might be caused by a reduction in a neural drive, such as a central drive (Trajano et al., 2013), and peripheral electromyographic activity (Fowles et al., 2000; Kay and Blazevich, 2009). The decrease in muscle force after SS might also be due to peripheral force-generating capacity, such as that caused by musculotendinous stiffness and/or associated changes in the muscle length–tension relationship (Fowles et al., 2000; Ryan et al., 2008). In our study, passive stiffness and electromyographic activity were significantly decreased after SS and CS. Therefore, decreases in muscle force after SS and CS might be caused by neurophysiological and mechanical factors.

In this study, passive stiffness and electromyographic activity after DS showed patterns similar to measurements after other stretching methods; however, concentric muscle force after DS was unchanged, while isometric muscle force was significantly decreased. Matsuo et al. (2019) reported that 300-sec DS significantly decreased isometric muscle force, in a manner similar to SS. Moreover, Yamaguchi and Ishii (2014) suggested that explosive performance might be impaired as the volume duration of DS increases. Therefore, the 150-sec or 300-sec DS used in this study might have caused isometric muscle force impairment. However, peak force and power reportedly increased when a longer duration (>90 sec) of DS was performed (Behm et al., 2016; Behm and Chaouachi, 2011). A systematic review article (Behm et al., 2016) suggested that part of the positive effect of DS might result in allowing practice at tasks similar to those in the test measurements, and also suggested that the limited evidence indicate generally inconsequential contraction type-dependent effects of DS on force production. Therefore, similar tasks might have been performed between the DS protocol and concentric muscle force measurements in the present study, although the specific components are unclear. Further studies are required to identify factors that affect isometric and concentric muscle force after stretching, along with the effects of stretching on musculoskeletal biomechanics.

This study had some limitations. First, we did not assess central factors that affect muscle performance. Therefore, future studies should assess central drive parameters, such as percent voluntary activation. Second, we used a long duration of stretching; future studies should compare the effects of our stretching method with a shorter (more common) duration of stretching. Third, we measured overall passive stiffness without considering possible effects on a single component in isolation; such isolation is difficult to perform in human volunteers. Fourth, a potentiation effect was possible during measurements performed after a 45-s rest. However, the mean peak torque from three trials was used for analysis to minimize this effect. Moreover, isometric and concentric muscle forces were measured in a consistent manner at all time points; most post-stretching muscle force values were decreased. Therefore, we presume that any potentiation effect was small. Finally, this study only included men; future studies should include both men and women to improve the generalizability of the findings.

This study showed that changes in ROM, PPT, and isometric muscle force after all stretching methods were similar. However, CS tended to have longer effects on passive stiffness, compared with SS and DS; concentric muscle force only remained unchanged after DS. These results suggest that 300 sec of SS, DS, and CS have different acute and prolonged effects on flexibility and muscle force.