DIFFERENTIAL EFFECTS OF 30- VS. 60-SECOND STATIC MUSCLE STRETCHING ON VERTICAL JUMP PERFORMANCE MATHEUS D. PINTO,1 EURICO N. WILHELM,1 VALMOR TRICOLI,2 RONEI S. PINTO,1 ANTHONY J. BLAZEVICH3
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Exercise Research Laboratory, Physical Education School, Federal University of Rio Grande do Sul, Porto Alegre, Brazil; Laboratory of Neuromuscular Adaptations to Strength Training, School of Physical Education and Sport, University of Sa˜o Paulo, Sa˜o Paulo, Brazil; and 3Center for Exercise and Sports Science Research, School of Exercise and Health Sciences, Edith Cowan University, Joondalup, Australia
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ABSTRACT Pinto, MD, Wilhelm, EN, Tricoli, V, Pinto, RS, and Blazevich, AJ. Differential effects of 30- vs. 60-second static muscle stretching on vertical jump performance. J Strength Cond Res 28(12): 3440–3446, 2014—It has been proposed that pre-exercise static stretching may reduce muscle force and power. Recent systematic and meta-analytical reviews have proposed a threshold regarding the effect of short (,45 seconds) and moderate ($60 seconds) stretching durations on subsequent performance in a multi-joint task (e.g., jump performance), although its effect on power output remains less clear. Furthermore, no single experimental study has explicitly compared the effect of short (e.g., 30 seconds) and moderate (60 seconds) durations of continuous static stretching on multi-joint performance. Therefore, the aim of the present study was determine the effect of acute short- and moderate-duration continuous stretching interventions on vertical jump performance and power output. Sixteen physically active men (21.0 6 1.9 years; 1.7 6 0.1 m; 78.4 6 12.1 kg) volunteered for the study. After familiarization, subjects attended the laboratory for 3 testing sessions. In the nonstretching (NS) condition, subjects performed a countermovement jump (CMJ) test without a preceding stretching bout. In the other 2 conditions, subjects performed 30-second (30SS; 4 minutes) or 60-second (60SS; 8 minutes) static stretching bouts in calf muscles, hamstrings, gluteus maximus, and quadriceps, respectively, followed by the CMJ test. Results were compared by repeated-measures analysis of variance. In comparison with NS, 60SS resulted in a lower CMJ height (23.4%, p # 0.05) and average (22.7%, p # 0.05) and peak power output
Address correspondence to Matheus D. Pinto, [email protected]. 28(12)/3440–3446 Journal of Strength and Conditioning Research Ó 2014 National Strength and Conditioning Association
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(22.0%, p # 0.05), but no difference was observed between 30SS and the other conditions (p . 0.05). These data suggest a dose-dependent effect of stretching on muscular performance, which is in accordance with previous studies. The present results suggest a threshold of continuous static stretching in which muscular power output in a multi-joint task may be impaired immediately following moderate-duration (60 seconds; 8 minutes) static stretching while short-duration (30 seconds; 4 minutes) stretching has a negligible influence.
KEY WORDS stretch volume, countermovement jump, muscular performance, muscle power
INTRODUCTION
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uscle power is considered a critical element for successful athletic performance (24). One way to indirectly evaluate multi-joint lower limb power production is through the use of the countermovement jump (CMJ) test. For example, Tillin et al. (29) found significant correlations between CMJ height, sprint performance, and squat rate of force development in rugby players, indicating a reasonable similarity in the factors underpinning their performances. However, an adequate level of flexibility is also a potentially important physical attribute influencing athletic performance, thus stretching exercises organized to improve flexibility are usually included in training and performance-preparation programs by individuals who require high power outputs. Indeed, a recent study demonstrated that static stretching is commonly used during training sessions and before competitions by recreational and competitive athletes (14). One reason indicated for its use is to decrease delayed onset muscle soreness (DOMS) (11), despite the prevailing evidence that stretching before an event does not reduce DOMS (8). A second, and possibly more important, reason is that stretching may increase muscle-tendon unit compliance, which was reported to be altered after short periods (e.g., 5 seconds) of
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Journal of Strength and Conditioning Research static stretching (16), may be maintained for prolonged periods when an exercise-specific warm-up is completed subsequently (26), and is associated with a reduced soft tissue injury risk (20,21,28). Given these influences, it makes sense that stretching is often still considered an important part of the warm-up procedure (8,11). It is therefore of practical importance to understand the acute effect of stretching on performance in high-power activities, possibly using exercises such as the CMJ to study these effects. Although acute stretching could have beneficial effects in some sports that require a high level of performance (20), a number of studies have reported significant decreases in maximal force production (6,19,22,30,31) and performance (3,10,15,17,20,27) following static stretching. Therefore, this balance between decrements in performance and both gains in range of motion and potential for soft tissue injury reduction should be considered. Some studies have demonstrated a threshold volume of static stretching per muscle group in multi-joint lower limb required to reduce vertical jump height (3,10,25,33). For instance, Hough et al. (10) reported a 4.2% decrease in squat jump performance after maintaining single 30second static stretches targeting the plantar flexors, hip extensors, hamstrings, hip flexors, and quadriceps femoris. Also, Bradley et al. (3) found a 4% decrease in jump height after 2 minutes of static stretching, and other studies adopting protocols where stretch durations exceeded 1 minute also realized small reductions in jump height (4,19,32). In a recent systematic review, Kallerud and Gleeson (15) suggested that static stretching for at least 2 minutes per muscle before movement tasks involving stretch-shortening cycles (SSCs) could impair performance. However, Simic et al. (27) and Kay and Blazevich (17) concluded that static stretching of less than 45 seconds in each muscle group could be used in pre-exercise routines without risk of significant decreases in strength and performance, whereas longer stretching volumes ($60 seconds) were more likely to cause a small-to-moderate reduction in performance. However, it is not clear whether these findings can be reconciled because the effect may be more pronounced when a large number of muscles involved in the complex movement activity are stretched (15). Thus, the effects of static stretching on muscle power remain unclear (27), and the existence of a threshold volume of continuous static stretching on vertical jump performance (i.e., multi-joint, high-power output) has not been explicitly tested in a single study by examining the effect of 30 vs. 60 seconds of continuous static stretching. Therefore, the purpose of this study was to specifically determine whether a volume of static stretching greater than the limit identified previously (17,27) would negatively impact on vertical jump height and power output in physically active individuals. It was hypothesized that vertical jump performance would be more affected after 60 seconds, than 30 seconds, of static stretching.
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METHODS Experimental Approach to the Problem
To examine the effects of stretching volume on the vertical jump performance, the participants visited the laboratory on 4 occasions. On the first day, the subjects were familiarized with the CMJ technique and stretching protocols, and both height and body mass were measured using a stadiometer and a scale, respectively (Filizola, Sa˜o Paulo, Brazil). The second, third, and fourth experimental sessions were randomized with the following conditions: CMJ performance (reference value) with no preceding stretching (nonstretching [NS]); 30 seconds of continuous static stretching per muscle group (30SS); or 60 seconds of continuous static stretching (60SS) per muscle group, before CMJ performance (Figure 1). The total volume under stretch was 4 and 8 minutes for the 30SS and 60SS groups, respectively. All test sessions were separated by 48 hours, and the subjects performed the static stretching exercises at the same time of the day (between 1 and 3 PM) throughout the study period. Subjects
Sixteen male university students aged between 19 and 25 years (21.0 6 1.9 years; 1.7 6 0.1 m; 78.4 6 12.1 kg) volunteered to participate in the study. All subjects were physically active (according to the International Physical Activity Questionnaire), free from musculoskeletal injuries, and able to perform maximal vertical jump and stretching protocols. All subjects abstained from vigorous physical activity 48 hours before each experimental session. This study was approved by the Institutional Research Ethics Committee, and all subjects signed an informed consent before participation. Procedures
Experimental Conditions. The subjects arrived at the laboratory and rested (seated) for 5 minutes. They then either performed the testing immediately or completed 1 of the 2 stretch conditions before testing, in a randomized order. In the no-stretch (NS) condition, the CMJ test was performed without any previous stretching. In the 2 other conditions,
Figure 1. The experimental design: subjects visited the laboratory and rested (seated) for 5 minutes. Each subject was randomly assigned to 1 of 3 conditions—no static stretching (considered as a reference value for countermovement jump), 30 or 60 seconds of static stretching. Countermovement jump height and power output were subsequently obtained.
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Effects of Volume Stretching on Jump Performance
Figure 2. Pictorial representation of the stretching exercises.
subjects completed 30 seconds (30SS) or 60 seconds (60SS) of partner-assisted static stretching including the: (a) straight-leg calf muscle stretch, (b) supine, straight-leg hamstrings stretch, (c) supine hip flexion (gluteus maximus) stretch, and (d) lying quadriceps stretch with hip extension (Figure 2) before the CMJ test. The assistant was experienced in partner-assisted stretching techniques. The subjects were instructed to remain completely relaxed during all stretches, and stretch duration was timed using a digital countdown timer. The stretching was always performed by the same researcher until the subject reported mild discomfort, thus the intensity of stretch was allowed to increase to ensure mild discomfort was maintained. The CMJ test was performed immediately after 30SS and 60SS. In the present study, stretches were performed without specific warm-up to better isolate the effects of muscle stretching from other effects (e.g., tissue cooling, motivation decline)
attempts. Jump height was assessed using an electronic contact mat (Jump System Pro; Cefise, Nova Odessa, Brazil) with the subjects instructed to perform a maximum effort and “jump as high as possible”; no information regarding the depth of countermovement was given. The subjects kept their hands on their hips and were instructed to take off with their knees and ankles fully extended and to land in a similarly extended position to ensure the validity of the test. Three basic techniques were adopted strictly: (a) correct posture (spine erect, shoulders back) and body alignment (chest over knees) throughout the jump, (b) jumping straight up with no excessive side-to-side or forward-backward movement, and (c) soft landing, including toe-to-toe heel rocking and bent knees (5). Jumps were repeated when these criteria were not met; no subject performed more than 5 jumps in a session. The highest CMJ from each condition was used for statistical analysis. Jump height was obtained using the flight time method with the Jump System Pro software. Because the subjects had a considerable variability in body mass (SD 6 12.1 kg) and the jump height was calculated by the flight time method (which disregards both the subject’s body mass and height), peak power output and average power output were calculated as described by Johnson and Bahamonde (13):
Peak power ðWÞ ¼ 78:53jump height ðcmÞ þ 60:63body mass kg 215:33height ðcmÞ21:308:
Average power ðWÞ ¼ 41:43jump height ðcmÞ þ 31:23body mass kg 213:93height ðcmÞ2431:
that may impact on performance and be imposed during a period of muscle stretching; the implications of this are discussed later. Pre-stretch (or pre-control period, referred here as NS condition) testing was also not included to minimize effects of task practice and changes in body temperature from influencing the results. Vertical Jump Test. Three maximal CMJ attempts were recorded for each subject with 10 seconds of rest between
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This allowed us to measure the maximum power output of the subjects as an indicator of their ability to produce force at fast movement speeds. Note that jump height is the mechanical consequence of the total impulse generated during the jump, according to the impulse-momentum relationship: Ft = mv, where F is the instantaneous force, t is the time of force application, m is the jumper’s mass, and v is the final velocity at take-off, and where an increased v ensures that a greater jump height is achieved. Jump
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RESULTS Countermovement Jump Height
A significant between-condition effect was observed for CMJ height (p # 0.05; ES = 0.29 for 60SS vs. NS), where height was significantly less in 60SS than NS (23.4%, p # 0.05; Figure 3). No differences were observed between 30SS and NS (21.6%, p . 0.05) or between 30SS and 60SS (z20.6%, p . 0.05) (Figure 3). Statistical power calculated post hoc was 0.88. Countermovement Jump Power Output
Figure 3. Countermovement jump height in the no-stretch (NS) condition and 30-second (30SS) and 60-second (60SS) static stretch conditions (mean 6 SD). *Significant difference between NS and 60SS (p # 0.05).
height is influenced by factors such as the subject mass, and therefore, a more global measure of SSC performance was required to allow for generalization of the results to other contexts. The calculation of peak and average power output was done in the present study to allow for this.
Peak and average CMJ power outputs were significantly different between conditions (p # 0.05). Post hoc tests indicated that 60SS resulted in a small but significant reduction in peak (22.0%, p # 0.05) and average (22.7%, p # 0.05) CMJ power output compared with NS. No differences were observed in CMJ power output between NS and 30SS (p . 0.05) or 30SS and 60SS (p . 0.05) (Table 1). The ES for differences in peak and average power were 0.129 and 0.135, respectively. Also, statistical powers calculated post hoc were 0.88 for both average and peak power output.
DISCUSSION
The aim of this study was to determine whether a greater volume of static stretching (60 vs. 30 seconds) would Statistical Analyses negatively impact vertical jump height and power output Data are presented as mean 6 SD. Data distribution was in physically active individuals. The results of the present verified by the Shapiro–Wilk test and sphericity was verified study show that 60 seconds of static stretching resulted in by Mauchly’s test. The effects of NS, 30SS, and 60SS (i.e., significant decrements in CMJ height, average power, and stretch condition) on jump height and power output were peak power, whereas no changes were observed after 30 compared by separate repeated-measures analyses of variseconds, and are thus in agreement with our hypothesis. ance. Significant between-condition differences were further These findings are also in accordance with previous preexamined using the Bonferroni post hoc test. All statistical dictions of a threshold total stretch duration in each muscle analyses were performed using SPSS software (version 20.0; group, where stretching longer than ;45 seconds causes SPSS, Inc., Chicago, IL, USA), and statistical significance a significant decrement in muscle force output and, possibly, was set at an alpha level of 0.05. Also, the effect size (ES) in athletic performance (17,20,27). for the effects of stretch condition on CMJ height and the Numerous studies have indicated that static stretching can average of power and peak power were calculated according negatively affect muscle strength and power output; howto Hedges and Olkin (7) to compare the mean change in ever, this effect seems to be volume dependent (16,19,25). a variable with the intersubject variability. Recently, systematic reviews by Simic et al. (27) and Kay and Blazevich (17) concluded that short durations of stretching per muscle group (#45 seconds) did not seem to impair TABLE 1. Peak and average countermovement jump power outputs in the 3 muscle performance, whereas experimental conditions (mean 6 SD). longer duration protocols ($60 seconds) were more likely 30-second 60-second to cause a small-to-moderate No stretching stretching stretching reduction in performance. HowPeak power (W) 4803.7 6 737.4 4755.6 6 692.5 4711.5 6 722.6* ever, Kallerud and Gleeson (15) Average power (W) 1867.9 6 368.2 1842.6 6 341.7 1819.3 6 360.1* suggested that static stretching for at least 2 minutes per muscle *Significant difference from no-stretching condition (p # 0.05). before movement tasks involving SSCs was required to impair VOLUME 28 | NUMBER 12 | DECEMBER 2014 |
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Effects of Volume Stretching on Jump Performance performance. However, the detrimental effect could be more pronounced when a large number of muscles involved in the complex movement activity are stretched, thus one could speculate that a lower volume of stretching per muscle group than the volume proposed by these authors (15) may be sufficient to impair performance. In the present study, we used the vertical jump as a model multi-joint, high-power output movement task and specifically compared 2 stretch durations (30 and 60 seconds), which are commonly used in practice and have been shown to be sufficient to reduce muscle-tendon stiffness (16), to determine a threshold volume of continuous static stretching on performance. The results indicate that static stretch routines performed with stretch times of ;60 seconds might notably and negatively affect performance in such movements, whereas stretches of shorter duration may have little effect. It is thought that acute stretching may have beneficial effects in some sports by increasing range of motion (26) and decreasing the risk of muscular strain (20,21,28). Indeed, a recent study demonstrated that many coaches insist that their athletes stretch before and after competitions (14). However, although many studies have investigated the impact of static stretching in maximal force production (1,22,30,31), it is acknowledged that power output and rapid force development might be even more important than maximal strength for many sports; thus, it is imperative to gain a better understanding of the effects of static stretching on skeletal muscle power. The results of the present study may have significant practical implications because a decrease in jump height and both peak and average jump power outputs were observed immediately after 60SS compared with the NS condition, whereas no difference was observed after 30SS. Only 2 studies have previously used stretch durations of 30 and 60 seconds (22,25); however, the effect on maximal force, rather than power, production was examined, only a single muscle was stretched (22) and intermittent static stretching was used (25). In the present study, the effect of continuous static stretching on complex motor performance was examined and all important agonist muscle groups involved in the test were stretched. It must be acknowledged, however, that the changes seen after 60 seconds of stretching were relatively small compared with the within-subject variability in jump performance (ES: 0.29, 0.129, and 0.135 for jump height, peak power, and average power, respectively), and other researchers have previously indicated that recovery of the stretch-induced force loss may occur within 15 minutes (3,30,31). Furthermore, the effects of muscle stretch were examined without specific warm-up in the present study to minimize possible differential effects of tissue cooling (2), motivational decline (2), muscle fiber conduction time (23), during the stretch bouts, so the influence of stretching as part of a complete warm-up routine is clearly required. The effect of different stretching regimes on vertical jump performance has been examined previously. Hough
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et al. (10), for example, reported a 4.2% decrease in squat jump performance, (ES = 0.29) after the performance of a single 30-second static stretch targeting the same muscles as in the present study. Also, Bradley et al. (3) found a 4% decrease in jump height (ES = 0.19) after 2 minutes of static stretching (4 sets 3 30 seconds; quadriceps, hamstrings, and plantar flexors), and other studies adopting protocols where stretch durations exceeded 1 minute also reported small reductions in jump height and power output measured during vertical jumps (4,18,25,32). These results are not in agreement with those of the present study, even though the stretch protocol used, the order of stretches, and vertical jump technique adopted were similar to those in the present study. The reasons for these disparate findings are not known, but could be related to differences in the total stretch volume [i.e., 4 and 8 minutes for 30SS and 60SS, respectively, in the present study vs. 10 minutes in the study by Bradley et al. (3) and ;7 minutes in the study by Hough et al. (10)], the method of stretching used [i.e., continuous stretch in the present study versus intermittent stretching in Bradley et al. (3), which has been shown to cause a greater magnitude of reduction in force production (30)], or the type of jump test performed [e.g., squat jump in the study by Hough et al. (3) but CMJ in the present study]. These factors also could affect the magnitude of the effects and, indeed, our results are in agreement with the overall ES proposed by Simic et al. (27). It is important to note that Jeffrey et al. (12) did not find a statistical change in jump height following a single set of static stretching lasting 20 seconds, and Holt et al. (9) found no change in vertical jump performance after 15 seconds (3 3 5 seconds) of static stretching. These findings are in agreement with our results and suggest that shorter durations of stretching may not negatively affect jump performance. Nevertheless, the overall results of the present study (reductions of jump height and muscle power of 1–3%) are in accordance with values showed by Kay and Blazevich (17) (;4% after 60 seconds of stretching) and Simic et al. (27). To the best of our knowledge, the present study is the first to directly compare the effects of short (e.g., 30 seconds) vs. moderate durations (60 seconds) of continuous static stretching on CMJ performance (i.e., height, average and peak power output) under identical experimental conditions. In conclusion, the present results show that 60 seconds of passive stretching resulted in small but statistically (and practically) significant decrements in CMJ height (23.4%), average power (22.7%), and peak power (22.0%) on physically active men; it is of future interest to determine whether similar effects are found in clinical, elderly, or elite athlete populations. No effect was observed after 30 seconds of stretching, which suggests that a threshold of stretching volume must be attained to impair muscle performance in these types of activities. In future studies, different subject populations, durations of effect, and the effect of various warm-up routines (i.e., of different volumes
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PRACTICAL APPLICATIONS This study highlights that different stretch volumes may have distinct effects on muscle power production during SSC activities performed over a large range of motion (i.e., similar to the CMJ). According to the present results, shortduration (e.g., 30 seconds per stretch; 4 minutes total volume) stretching does not cause detrimental effects on power performance and may be suitable before such activities when acute increases in range of motion are required. More importantly, a moderate stretching volume protocol (e.g., 60 seconds per stretch; 8 minutes total volume) resulted in a significant, although small (23.4, 22.7, and 22.0% for jump performance, average and peak power, respectively), decrement in lower limb power output. It is important to note that these performance changes are specific to the protocols used presently and different results can be expected if the total stretch volume (4 and 8 minutes for the 2 stretch conditions) is apportioned differently between the muscles. For example, imposing longer stretches on fewer, functionally important muscles may have a different (likely greater) effect on performance. These results may have important practical applications for athletic populations because many perform multiple-muscle stretching routines before athletic events (which include SSC activities the CMJ). Therefore, it is suggested that, if stretching is required, a stretch duration of ;30 seconds may be used, whereas durations of 60 seconds should be avoided when possible. The effects of these different stretch durations should hence be examined when used within a complete sport-specific warm-up program.
ACKNOWLEDGMENTS The authors would like to thank Camila Sanzi who made the pictorial representations of the stretching exercises with no costs and CNPq and CAPES for their researcher funding support. The research was conducted at Exercise Research Laboratory, Physical Education School, Federal University of Rio Grande do Sul, Porto Alegre, RS, Brazil. No funding was received to conduct this research.
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Exercise Research Laboratory, Physical Education School, Federal University of Rio Grande do Sul, Porto Alegre, Brazil; Laboratory of Neuromuscular Adaptations to Strength Training, School of Physical Education and Sport, University of Sa˜o Paulo, Sa˜o Paulo, Brazil; and 3Center for Exercise and Sports Science Research, School of Exercise and Health Sciences, Edith Cowan University, Joondalup, Australia
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ABSTRACT Pinto, MD, Wilhelm, EN, Tricoli, V, Pinto, RS, and Blazevich, AJ. Differential effects of 30- vs. 60-second static muscle stretching on vertical jump performance. J Strength Cond Res 28(12): 3440–3446, 2014—It has been proposed that pre-exercise static stretching may reduce muscle force and power. Recent systematic and meta-analytical reviews have proposed a threshold regarding the effect of short (,45 seconds) and moderate ($60 seconds) stretching durations on subsequent performance in a multi-joint task (e.g., jump performance), although its effect on power output remains less clear. Furthermore, no single experimental study has explicitly compared the effect of short (e.g., 30 seconds) and moderate (60 seconds) durations of continuous static stretching on multi-joint performance. Therefore, the aim of the present study was determine the effect of acute short- and moderate-duration continuous stretching interventions on vertical jump performance and power output. Sixteen physically active men (21.0 6 1.9 years; 1.7 6 0.1 m; 78.4 6 12.1 kg) volunteered for the study. After familiarization, subjects attended the laboratory for 3 testing sessions. In the nonstretching (NS) condition, subjects performed a countermovement jump (CMJ) test without a preceding stretching bout. In the other 2 conditions, subjects performed 30-second (30SS; 4 minutes) or 60-second (60SS; 8 minutes) static stretching bouts in calf muscles, hamstrings, gluteus maximus, and quadriceps, respectively, followed by the CMJ test. Results were compared by repeated-measures analysis of variance. In comparison with NS, 60SS resulted in a lower CMJ height (23.4%, p # 0.05) and average (22.7%, p # 0.05) and peak power output
Address correspondence to Matheus D. Pinto, [email protected]. 28(12)/3440–3446 Journal of Strength and Conditioning Research Ó 2014 National Strength and Conditioning Association
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(22.0%, p # 0.05), but no difference was observed between 30SS and the other conditions (p . 0.05). These data suggest a dose-dependent effect of stretching on muscular performance, which is in accordance with previous studies. The present results suggest a threshold of continuous static stretching in which muscular power output in a multi-joint task may be impaired immediately following moderate-duration (60 seconds; 8 minutes) static stretching while short-duration (30 seconds; 4 minutes) stretching has a negligible influence.
KEY WORDS stretch volume, countermovement jump, muscular performance, muscle power
INTRODUCTION
M
uscle power is considered a critical element for successful athletic performance (24). One way to indirectly evaluate multi-joint lower limb power production is through the use of the countermovement jump (CMJ) test. For example, Tillin et al. (29) found significant correlations between CMJ height, sprint performance, and squat rate of force development in rugby players, indicating a reasonable similarity in the factors underpinning their performances. However, an adequate level of flexibility is also a potentially important physical attribute influencing athletic performance, thus stretching exercises organized to improve flexibility are usually included in training and performance-preparation programs by individuals who require high power outputs. Indeed, a recent study demonstrated that static stretching is commonly used during training sessions and before competitions by recreational and competitive athletes (14). One reason indicated for its use is to decrease delayed onset muscle soreness (DOMS) (11), despite the prevailing evidence that stretching before an event does not reduce DOMS (8). A second, and possibly more important, reason is that stretching may increase muscle-tendon unit compliance, which was reported to be altered after short periods (e.g., 5 seconds) of
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Journal of Strength and Conditioning Research static stretching (16), may be maintained for prolonged periods when an exercise-specific warm-up is completed subsequently (26), and is associated with a reduced soft tissue injury risk (20,21,28). Given these influences, it makes sense that stretching is often still considered an important part of the warm-up procedure (8,11). It is therefore of practical importance to understand the acute effect of stretching on performance in high-power activities, possibly using exercises such as the CMJ to study these effects. Although acute stretching could have beneficial effects in some sports that require a high level of performance (20), a number of studies have reported significant decreases in maximal force production (6,19,22,30,31) and performance (3,10,15,17,20,27) following static stretching. Therefore, this balance between decrements in performance and both gains in range of motion and potential for soft tissue injury reduction should be considered. Some studies have demonstrated a threshold volume of static stretching per muscle group in multi-joint lower limb required to reduce vertical jump height (3,10,25,33). For instance, Hough et al. (10) reported a 4.2% decrease in squat jump performance after maintaining single 30second static stretches targeting the plantar flexors, hip extensors, hamstrings, hip flexors, and quadriceps femoris. Also, Bradley et al. (3) found a 4% decrease in jump height after 2 minutes of static stretching, and other studies adopting protocols where stretch durations exceeded 1 minute also realized small reductions in jump height (4,19,32). In a recent systematic review, Kallerud and Gleeson (15) suggested that static stretching for at least 2 minutes per muscle before movement tasks involving stretch-shortening cycles (SSCs) could impair performance. However, Simic et al. (27) and Kay and Blazevich (17) concluded that static stretching of less than 45 seconds in each muscle group could be used in pre-exercise routines without risk of significant decreases in strength and performance, whereas longer stretching volumes ($60 seconds) were more likely to cause a small-to-moderate reduction in performance. However, it is not clear whether these findings can be reconciled because the effect may be more pronounced when a large number of muscles involved in the complex movement activity are stretched (15). Thus, the effects of static stretching on muscle power remain unclear (27), and the existence of a threshold volume of continuous static stretching on vertical jump performance (i.e., multi-joint, high-power output) has not been explicitly tested in a single study by examining the effect of 30 vs. 60 seconds of continuous static stretching. Therefore, the purpose of this study was to specifically determine whether a volume of static stretching greater than the limit identified previously (17,27) would negatively impact on vertical jump height and power output in physically active individuals. It was hypothesized that vertical jump performance would be more affected after 60 seconds, than 30 seconds, of static stretching.
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METHODS Experimental Approach to the Problem
To examine the effects of stretching volume on the vertical jump performance, the participants visited the laboratory on 4 occasions. On the first day, the subjects were familiarized with the CMJ technique and stretching protocols, and both height and body mass were measured using a stadiometer and a scale, respectively (Filizola, Sa˜o Paulo, Brazil). The second, third, and fourth experimental sessions were randomized with the following conditions: CMJ performance (reference value) with no preceding stretching (nonstretching [NS]); 30 seconds of continuous static stretching per muscle group (30SS); or 60 seconds of continuous static stretching (60SS) per muscle group, before CMJ performance (Figure 1). The total volume under stretch was 4 and 8 minutes for the 30SS and 60SS groups, respectively. All test sessions were separated by 48 hours, and the subjects performed the static stretching exercises at the same time of the day (between 1 and 3 PM) throughout the study period. Subjects
Sixteen male university students aged between 19 and 25 years (21.0 6 1.9 years; 1.7 6 0.1 m; 78.4 6 12.1 kg) volunteered to participate in the study. All subjects were physically active (according to the International Physical Activity Questionnaire), free from musculoskeletal injuries, and able to perform maximal vertical jump and stretching protocols. All subjects abstained from vigorous physical activity 48 hours before each experimental session. This study was approved by the Institutional Research Ethics Committee, and all subjects signed an informed consent before participation. Procedures
Experimental Conditions. The subjects arrived at the laboratory and rested (seated) for 5 minutes. They then either performed the testing immediately or completed 1 of the 2 stretch conditions before testing, in a randomized order. In the no-stretch (NS) condition, the CMJ test was performed without any previous stretching. In the 2 other conditions,
Figure 1. The experimental design: subjects visited the laboratory and rested (seated) for 5 minutes. Each subject was randomly assigned to 1 of 3 conditions—no static stretching (considered as a reference value for countermovement jump), 30 or 60 seconds of static stretching. Countermovement jump height and power output were subsequently obtained.
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Effects of Volume Stretching on Jump Performance
Figure 2. Pictorial representation of the stretching exercises.
subjects completed 30 seconds (30SS) or 60 seconds (60SS) of partner-assisted static stretching including the: (a) straight-leg calf muscle stretch, (b) supine, straight-leg hamstrings stretch, (c) supine hip flexion (gluteus maximus) stretch, and (d) lying quadriceps stretch with hip extension (Figure 2) before the CMJ test. The assistant was experienced in partner-assisted stretching techniques. The subjects were instructed to remain completely relaxed during all stretches, and stretch duration was timed using a digital countdown timer. The stretching was always performed by the same researcher until the subject reported mild discomfort, thus the intensity of stretch was allowed to increase to ensure mild discomfort was maintained. The CMJ test was performed immediately after 30SS and 60SS. In the present study, stretches were performed without specific warm-up to better isolate the effects of muscle stretching from other effects (e.g., tissue cooling, motivation decline)
attempts. Jump height was assessed using an electronic contact mat (Jump System Pro; Cefise, Nova Odessa, Brazil) with the subjects instructed to perform a maximum effort and “jump as high as possible”; no information regarding the depth of countermovement was given. The subjects kept their hands on their hips and were instructed to take off with their knees and ankles fully extended and to land in a similarly extended position to ensure the validity of the test. Three basic techniques were adopted strictly: (a) correct posture (spine erect, shoulders back) and body alignment (chest over knees) throughout the jump, (b) jumping straight up with no excessive side-to-side or forward-backward movement, and (c) soft landing, including toe-to-toe heel rocking and bent knees (5). Jumps were repeated when these criteria were not met; no subject performed more than 5 jumps in a session. The highest CMJ from each condition was used for statistical analysis. Jump height was obtained using the flight time method with the Jump System Pro software. Because the subjects had a considerable variability in body mass (SD 6 12.1 kg) and the jump height was calculated by the flight time method (which disregards both the subject’s body mass and height), peak power output and average power output were calculated as described by Johnson and Bahamonde (13):
Peak power ðWÞ ¼ 78:53jump height ðcmÞ þ 60:63body mass kg 215:33height ðcmÞ21:308:
Average power ðWÞ ¼ 41:43jump height ðcmÞ þ 31:23body mass kg 213:93height ðcmÞ2431:
that may impact on performance and be imposed during a period of muscle stretching; the implications of this are discussed later. Pre-stretch (or pre-control period, referred here as NS condition) testing was also not included to minimize effects of task practice and changes in body temperature from influencing the results. Vertical Jump Test. Three maximal CMJ attempts were recorded for each subject with 10 seconds of rest between
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This allowed us to measure the maximum power output of the subjects as an indicator of their ability to produce force at fast movement speeds. Note that jump height is the mechanical consequence of the total impulse generated during the jump, according to the impulse-momentum relationship: Ft = mv, where F is the instantaneous force, t is the time of force application, m is the jumper’s mass, and v is the final velocity at take-off, and where an increased v ensures that a greater jump height is achieved. Jump
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RESULTS Countermovement Jump Height
A significant between-condition effect was observed for CMJ height (p # 0.05; ES = 0.29 for 60SS vs. NS), where height was significantly less in 60SS than NS (23.4%, p # 0.05; Figure 3). No differences were observed between 30SS and NS (21.6%, p . 0.05) or between 30SS and 60SS (z20.6%, p . 0.05) (Figure 3). Statistical power calculated post hoc was 0.88. Countermovement Jump Power Output
Figure 3. Countermovement jump height in the no-stretch (NS) condition and 30-second (30SS) and 60-second (60SS) static stretch conditions (mean 6 SD). *Significant difference between NS and 60SS (p # 0.05).
height is influenced by factors such as the subject mass, and therefore, a more global measure of SSC performance was required to allow for generalization of the results to other contexts. The calculation of peak and average power output was done in the present study to allow for this.
Peak and average CMJ power outputs were significantly different between conditions (p # 0.05). Post hoc tests indicated that 60SS resulted in a small but significant reduction in peak (22.0%, p # 0.05) and average (22.7%, p # 0.05) CMJ power output compared with NS. No differences were observed in CMJ power output between NS and 30SS (p . 0.05) or 30SS and 60SS (p . 0.05) (Table 1). The ES for differences in peak and average power were 0.129 and 0.135, respectively. Also, statistical powers calculated post hoc were 0.88 for both average and peak power output.
DISCUSSION
The aim of this study was to determine whether a greater volume of static stretching (60 vs. 30 seconds) would Statistical Analyses negatively impact vertical jump height and power output Data are presented as mean 6 SD. Data distribution was in physically active individuals. The results of the present verified by the Shapiro–Wilk test and sphericity was verified study show that 60 seconds of static stretching resulted in by Mauchly’s test. The effects of NS, 30SS, and 60SS (i.e., significant decrements in CMJ height, average power, and stretch condition) on jump height and power output were peak power, whereas no changes were observed after 30 compared by separate repeated-measures analyses of variseconds, and are thus in agreement with our hypothesis. ance. Significant between-condition differences were further These findings are also in accordance with previous preexamined using the Bonferroni post hoc test. All statistical dictions of a threshold total stretch duration in each muscle analyses were performed using SPSS software (version 20.0; group, where stretching longer than ;45 seconds causes SPSS, Inc., Chicago, IL, USA), and statistical significance a significant decrement in muscle force output and, possibly, was set at an alpha level of 0.05. Also, the effect size (ES) in athletic performance (17,20,27). for the effects of stretch condition on CMJ height and the Numerous studies have indicated that static stretching can average of power and peak power were calculated according negatively affect muscle strength and power output; howto Hedges and Olkin (7) to compare the mean change in ever, this effect seems to be volume dependent (16,19,25). a variable with the intersubject variability. Recently, systematic reviews by Simic et al. (27) and Kay and Blazevich (17) concluded that short durations of stretching per muscle group (#45 seconds) did not seem to impair TABLE 1. Peak and average countermovement jump power outputs in the 3 muscle performance, whereas experimental conditions (mean 6 SD). longer duration protocols ($60 seconds) were more likely 30-second 60-second to cause a small-to-moderate No stretching stretching stretching reduction in performance. HowPeak power (W) 4803.7 6 737.4 4755.6 6 692.5 4711.5 6 722.6* ever, Kallerud and Gleeson (15) Average power (W) 1867.9 6 368.2 1842.6 6 341.7 1819.3 6 360.1* suggested that static stretching for at least 2 minutes per muscle *Significant difference from no-stretching condition (p # 0.05). before movement tasks involving SSCs was required to impair VOLUME 28 | NUMBER 12 | DECEMBER 2014 |
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Effects of Volume Stretching on Jump Performance performance. However, the detrimental effect could be more pronounced when a large number of muscles involved in the complex movement activity are stretched, thus one could speculate that a lower volume of stretching per muscle group than the volume proposed by these authors (15) may be sufficient to impair performance. In the present study, we used the vertical jump as a model multi-joint, high-power output movement task and specifically compared 2 stretch durations (30 and 60 seconds), which are commonly used in practice and have been shown to be sufficient to reduce muscle-tendon stiffness (16), to determine a threshold volume of continuous static stretching on performance. The results indicate that static stretch routines performed with stretch times of ;60 seconds might notably and negatively affect performance in such movements, whereas stretches of shorter duration may have little effect. It is thought that acute stretching may have beneficial effects in some sports by increasing range of motion (26) and decreasing the risk of muscular strain (20,21,28). Indeed, a recent study demonstrated that many coaches insist that their athletes stretch before and after competitions (14). However, although many studies have investigated the impact of static stretching in maximal force production (1,22,30,31), it is acknowledged that power output and rapid force development might be even more important than maximal strength for many sports; thus, it is imperative to gain a better understanding of the effects of static stretching on skeletal muscle power. The results of the present study may have significant practical implications because a decrease in jump height and both peak and average jump power outputs were observed immediately after 60SS compared with the NS condition, whereas no difference was observed after 30SS. Only 2 studies have previously used stretch durations of 30 and 60 seconds (22,25); however, the effect on maximal force, rather than power, production was examined, only a single muscle was stretched (22) and intermittent static stretching was used (25). In the present study, the effect of continuous static stretching on complex motor performance was examined and all important agonist muscle groups involved in the test were stretched. It must be acknowledged, however, that the changes seen after 60 seconds of stretching were relatively small compared with the within-subject variability in jump performance (ES: 0.29, 0.129, and 0.135 for jump height, peak power, and average power, respectively), and other researchers have previously indicated that recovery of the stretch-induced force loss may occur within 15 minutes (3,30,31). Furthermore, the effects of muscle stretch were examined without specific warm-up in the present study to minimize possible differential effects of tissue cooling (2), motivational decline (2), muscle fiber conduction time (23), during the stretch bouts, so the influence of stretching as part of a complete warm-up routine is clearly required. The effect of different stretching regimes on vertical jump performance has been examined previously. Hough
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et al. (10), for example, reported a 4.2% decrease in squat jump performance, (ES = 0.29) after the performance of a single 30-second static stretch targeting the same muscles as in the present study. Also, Bradley et al. (3) found a 4% decrease in jump height (ES = 0.19) after 2 minutes of static stretching (4 sets 3 30 seconds; quadriceps, hamstrings, and plantar flexors), and other studies adopting protocols where stretch durations exceeded 1 minute also reported small reductions in jump height and power output measured during vertical jumps (4,18,25,32). These results are not in agreement with those of the present study, even though the stretch protocol used, the order of stretches, and vertical jump technique adopted were similar to those in the present study. The reasons for these disparate findings are not known, but could be related to differences in the total stretch volume [i.e., 4 and 8 minutes for 30SS and 60SS, respectively, in the present study vs. 10 minutes in the study by Bradley et al. (3) and ;7 minutes in the study by Hough et al. (10)], the method of stretching used [i.e., continuous stretch in the present study versus intermittent stretching in Bradley et al. (3), which has been shown to cause a greater magnitude of reduction in force production (30)], or the type of jump test performed [e.g., squat jump in the study by Hough et al. (3) but CMJ in the present study]. These factors also could affect the magnitude of the effects and, indeed, our results are in agreement with the overall ES proposed by Simic et al. (27). It is important to note that Jeffrey et al. (12) did not find a statistical change in jump height following a single set of static stretching lasting 20 seconds, and Holt et al. (9) found no change in vertical jump performance after 15 seconds (3 3 5 seconds) of static stretching. These findings are in agreement with our results and suggest that shorter durations of stretching may not negatively affect jump performance. Nevertheless, the overall results of the present study (reductions of jump height and muscle power of 1–3%) are in accordance with values showed by Kay and Blazevich (17) (;4% after 60 seconds of stretching) and Simic et al. (27). To the best of our knowledge, the present study is the first to directly compare the effects of short (e.g., 30 seconds) vs. moderate durations (60 seconds) of continuous static stretching on CMJ performance (i.e., height, average and peak power output) under identical experimental conditions. In conclusion, the present results show that 60 seconds of passive stretching resulted in small but statistically (and practically) significant decrements in CMJ height (23.4%), average power (22.7%), and peak power (22.0%) on physically active men; it is of future interest to determine whether similar effects are found in clinical, elderly, or elite athlete populations. No effect was observed after 30 seconds of stretching, which suggests that a threshold of stretching volume must be attained to impair muscle performance in these types of activities. In future studies, different subject populations, durations of effect, and the effect of various warm-up routines (i.e., of different volumes
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PRACTICAL APPLICATIONS This study highlights that different stretch volumes may have distinct effects on muscle power production during SSC activities performed over a large range of motion (i.e., similar to the CMJ). According to the present results, shortduration (e.g., 30 seconds per stretch; 4 minutes total volume) stretching does not cause detrimental effects on power performance and may be suitable before such activities when acute increases in range of motion are required. More importantly, a moderate stretching volume protocol (e.g., 60 seconds per stretch; 8 minutes total volume) resulted in a significant, although small (23.4, 22.7, and 22.0% for jump performance, average and peak power, respectively), decrement in lower limb power output. It is important to note that these performance changes are specific to the protocols used presently and different results can be expected if the total stretch volume (4 and 8 minutes for the 2 stretch conditions) is apportioned differently between the muscles. For example, imposing longer stretches on fewer, functionally important muscles may have a different (likely greater) effect on performance. These results may have important practical applications for athletic populations because many perform multiple-muscle stretching routines before athletic events (which include SSC activities the CMJ). Therefore, it is suggested that, if stretching is required, a stretch duration of ;30 seconds may be used, whereas durations of 60 seconds should be avoided when possible. The effects of these different stretch durations should hence be examined when used within a complete sport-specific warm-up program.
ACKNOWLEDGMENTS The authors would like to thank Camila Sanzi who made the pictorial representations of the stretching exercises with no costs and CNPq and CAPES for their researcher funding support. The research was conducted at Exercise Research Laboratory, Physical Education School, Federal University of Rio Grande do Sul, Porto Alegre, RS, Brazil. No funding was received to conduct this research.
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