Static stretching of skeletal muscle is commonly performed before exercise to increase flexibility (1). To increase flexibility, the American College of Sports Medicine recommends that the duration of static stretching should be 15–30 seconds (1), whereas the National Strength and Conditioning Association recommends static stretching for 30 seconds (36). However, the evidence used to generate these recommendations is relatively limited, and in some cases, discrepant.
Although many previous studies have examined the effects of static stretching, they have shown that different durations of static stretching are associated with different responses in the various dependent variables assessed. For example, 20 seconds of static stretching was recommended as the most effective duration because static passive torque (SPT), assessed during static stretching, decreased immediately after the onset of stretching (26). Dynamic passive torque (DPT), assessed during isokinetic passive joint movement, and stiffness, which was derived from DPT, did not change after 90-second stretching, but decreased significantly after 300-second stretching or after a total of 300-second stretching (4,28). Previous studies have reported that muscle force did not change after 10- and 20-second stretching, but decreased significantly with more than 30-second stretching (30,34,37). In addition, a previous systematic review examining the effects of stretching on hamstrings reported that 30-second stretching was most effective for increasing mobility (7). Therefore, it seems that the recommendations for the optimal duration of static stretching, as proposed by the American College of Sports Medicine (1) and the National Strength and Conditioning Association (36), need to be reconsidered.
Based on the findings of the previous studies, it was hypothesized in this study that longer static stretching durations would have a greater effect on skeletal muscle function compared with what shorter durations have. Furthermore, very few previous studies have examined the acute effects of different stretching durations on multiple skeletal muscle function parameters. Therefore, this study was designed to investigate the acute effects of different stretching durations on functional outcomes of relevance to physical activity and flexibility, including passive torque, mobility, and isometric muscle force. These resulting data should not only resolve current discrepancies among prior studies but also help to guide the development of future recommendations for static stretching before physical activity.
Experimental Approach to the Problem
A randomized crossover trial was conducted. The stretching protocol required the subjects to stretch their right hamstrings for a specific duration on 4 separate days. The stretches were performed for 20, 60, 180, or 300 seconds and the order of the duration of stretch performed was determined randomly. The experiment was done between 09:00 and 19:00 hours each day. Each stretch was performed at almost the same time of the day in individual subjects. Parameters included resistance to stretch (SPT and DPT), stiffness, straight leg raise (SLR), and isometric muscle force were assessed twice (prestretching and poststretching), and compared to investigate the influence of stretching durations.
Twenty-four healthy college students (17 men and 7 women) voluntarily participated in the experiment. The mean ± SD for the age, height, body mass, and body mass index was 20 ± 0.8 years, 168.7 ± 8.4 cm, 60.2 ± 8.7 kg, and 21.1 ± 2.0 kg·m−2, respectively. The participants were excluded if they had lower-extremity contracture, an operation performed on their back or lower extremity, neurological findings, took hormone or muscle-affecting drugs, could completely extend their right knee from a sitting position (described below), or if they were engaged in competitive sports. The subjects were asked to refrain from alcohol for at least 1 day before the visit; no other nutrition and hydration requirements were implemented. The most active subject included in this study participated in recreational activities 3 times per week. The subjects continued their level of activity throughout the study, although they were asked to refrain from or limit their activity on the day before and just before the experiment.
The study received ethical approval from the Research Ethics Committee of the Nagoya University School of Health Sciences. After being informed of the purpose and protocol of the study, the subjects provided written informed consent before undertaking any experiments. The study was conducted between June 16, 2010, and December 3, 2010. The mean ± SD time from the first to the last visit was 23.6 ± 10.7 days (range, 7–37 days).
Based on the findings of previous studies (21–24), we used a sitting position that has been shown to efficiently stretch the hamstrings (Figure 1). The subjects were seated in an isokinetic dynamometer (Primus RS; BTE Technologies, Hanover, MD, USA) with the seat raised maximally and a wedge-shaped cushion inserted between their trunk and the backrest. The angle between the seat face and back was approximately 60°. Each subject's chest, pelvis, and right thigh were stabilized with Velcro straps. The knee joint was aligned with the axis of rotation of the isokinetic dynamometer. The lever arm attachment was placed just proximal to the malleolus medialis and stabilized against the limb with Velcro straps. Stretching and assessment of passive torque and isometric muscle force were performed in this position, with the mean angles of hip and knee flexion recorded as 109.6 ± 2.9° and 112.3 ± 2.5°, respectively.
The right hamstrings were stretched by the isokinetic dynamometer. The range of motion of the dynamometer was set so that the stretch continued to a point just before pain was felt (15,29), and lasted 20, 60, 180, and 300 seconds (Figure 2).
The parameters assessed included SPT and DPT, SLR, and isometric muscle force. Static passive torque and DPT were measured as markers of static and dynamic forces required for passive movements during the stretch. Stiffness and SLR were measured as markers of flexibility. Isometric muscle force was measured as a marker of static force generated by the hamstring.
Static Passive Torque
Static passive torque produced by the hamstrings during stretching was measured continuously throughout the stretch as torque (newton meter) using the isokinetic dynamometer. The recorded value for analysis was determined from the difference between the value measured at the onset of stretching and the value measured at the end of stretching.
Dynamic Passive Torque
Dynamic passive torque produced by the hamstrings during passive knee extension was measured as torque (newton meter) using the isokinetic dynamometer. With the subject in the seated position the knee was extended passively at 5°·s−1 to a point of maximum knee extension just before the onset of pain with torque being measured continuously. Stretching at 5°·s−1 was used (21–24) because previous studies examining hamstring stretching have shown that reflexive muscle contraction does not occur at this velocity (11,20). The value recorded for analysis was determined from a comparison of the prestretching and after stretching values. The DPT values before and after stretching were compared taking into account the maximum knee extension angle before stretching. However, if the maximum knee extension angle after stretching was smaller than that before stretching, DPT before and after stretching were compared on the basis of the maximum knee extension angle after stretching.
In previous studies, stiffness was defined as the slope of the torque-angle curve recorded during measurement of DPT (21–24). Based on previous studies (16,18), this study defined stiffness (newton meter per degree) as the slope of the regression line that was calculated from the torque-angle curve using the least-squares method. Stiffness was calculated from the same knee extension angle range before and after stretching, and the prestretching value was compared with that after stretching. The calculated knee extension angle range was defined as the angle from the 50% maximum knee extension angle to the prestretching maximum knee extension angle. However, if the poststretching maximum knee extension angle was smaller than before stretching, stiffness before and after stretching was calculated from the 50% maximum knee extension angle to the poststretching maximum knee extension angle.
Straight Leg Raise
The SLR for the right lower limb was measured manually by 2 examiners. The subjects lay supine on an examination table and their pelvis and left knee were stabilized with Velcro straps. The stationary arm of a universal goniometer (KG-5; Kansai Ika Kogyou, Shiga, Japan) was aligned with the trunk, and the movable arm was aligned with the femur. With the subject's right leg maintained straight passively, one examiner lifted the subject's right leg to a point just before the subject felt pain, and the other examiner measured the SLR angle.
Isometric Muscle Force
With their arms crossed in front of their chest, the subjects performed a 6-second maximum isometric knee flexion contraction using the isokinetic dynamometer. Peak isometric torque (newton meter) was recorded for analysis. The subjects received encouragement from the examiner during the isometric contraction.
This experiment was performed in the university's Exercise Laboratory, which was maintained at a temperature of 26° C throughout the study. All the subjects were familiarized with the experimental procedure before the first testing day. On the testing day, DPT and isometric muscle force were measured first. The SLR was measured after a 5-minute rest. The subjects rested for 60 minutes to exclude the influence of the prestretching measurements. After the 60-minute rest, the subjects stretched their right hamstrings for 20, 60, 180, or 300 seconds, and SPT was measured as they stretched. After stretching, DPT, isometric muscle force, and SLR were measured in the same way as before stretching. The typical interval between measurement of isometric force and SLR was 5 minutes, although the interval between the other measurements was much shorter. The SPT was measured for the same duration as the stretch. Measurements of the DPT and SLR took about 1 minute, and isometric force measurement took 6 seconds. Stretching was performed once between the prestretch and poststretch measurements.
Intraexaminer reliability was determined using the test-retest method before the study started. Based on intraclass correlation coefficients (1), intraexaminer reliability was found to be high for all parameters assessed (SPT: 0.892; DPT: 0.833; stiffness: 0.794; muscle force: 0.844; SLR: 0.968).
Measured absolute values and pre-post ratios (percentage) were analyzed. All results are expressed as mean ± SD. Normality of the values was assessed using the Shapiro-Wilk test, and all data were normally distributed. Interaction effects were then assessed using a 2-way repeated measures analysis of variance (ANOVA; durations of stretching × prestretching or poststretching), and the difference between the durations of stretching at prestretching and poststretching were assessed using Bonferroni post hoc testing. For comparisons in which the interaction was not significant, the ANOVA was repeated without the interactive term to confirm the original findings. Paired Student's t-tests were used to compare the prestretching and poststretching values for each duration of stretching. The Friedman test was used to compare the pre-post ratios between the durations of stretching, and the Wilcoxon signed-rank test (with Bonferroni adjustment) was used as the post hoc test. In addition, Spearman's rank-order correlation analysis was conducted between the durations of stretching and the amount of change (percentage) for all parameters assessed. Analyses were performed using SPSS software (version 16.0J; SPSS Inc., Chicago, IL, USA). Differences were considered significant at an α level of p < 0.05. The 95% confidence intervals comparing the outcomes between each duration of stretch are listed in Table 1. The 95% confidence intervals for the pre-post change in each outcome are presented in Table 2.
Post hoc power calculations confirmed that the statistical power (1 − β) was generally high, being 1.00 for SPT and SLR at each of 20, 60, 180, and 300 seconds. For DPT, the power was 0.06, 0.90, 1.00, and 1.00 for 20, 60, 180, and 300 seconds, respectively. For stiffness, the power was 0.12, 0.77, 1.00, and 1.00, respectively. For muscle force, the power was 0.99, 0.99, 1.00, and 0.99, respectively.
Static Passive Torque
There was an interaction effect between the stretching durations and prestretching or poststretching SPT values (p < 0.01; Figure 3A). Static passive torque decreased significantly after stretching for all durations of stretching (p < 0.05). The SPT after 60-, 180-, and 300-second stretching was significantly lower than after 20-second stretching (p < 0.05). There were no differences between the prestretching SPT values for all durations of stretching. The pre-post ratio of SPT for 180-second stretching was significantly lower than for 20-second stretching (p < 0.05), and the pre-post ratio of SPT for 300-second stretching was significantly lower than for the other stretching durations (p < 0.05).
Dynamic Passive Torque
There was no interaction effect between the durations of stretching and prestretching or poststretching DPT values. Dynamic passive torque decreased significantly after 180- and 300-second stretching (p < 0.05; Figure 3B). The DPT after 60- and 300-second stretching was significantly lower than after 20-second stretching (p < 0.05). There were no differences between the prestretching DPT values for all durations of stretching. There were no differences between the pre-post ratios of DPT for all durations of stretching. Similar results were obtained when we repeated the analysis excluding the interactive term (duration of stretch, p < 0.05; pre/post, p < 0.05; data not shown).
There was no interaction effect between the durations of stretching and prestretching or poststretching stiffness values. Stiffness decreased significantly after 180- and 300-second stretching (p < 0.05; Figure 3C). Stiffness after 300-second stretching was significantly lower than after 20-second stretching (p < 0.05). There were no differences between the prestretching stiffness values for all durations of stretching. There were no differences between the pre-post ratios of stiffness for all durations of stretching. Similar results were obtained when we repeated the analysis excluding the interactive term (duration of stretch, p < 0.05; pre/post, p < 0.05; data not shown).
Straight Leg Raise
There was an interaction effect between the durations of stretching and prestretching or poststretching SLR values (p < 0.01; Figure 3D). Straight leg raise increased significantly after stretching for all stretching durations (p < 0.05). The SLR after 180-second stretching was significantly greater than after 20-second stretching (p < 0.05). The SLR after 300-second stretching was significantly greater than after 20- and 60-second stretching (p < 0.05). There were no differences between the prestretching SLR values for all durations of stretching. The pre-post ratio of SLR for 180-second stretching was significantly greater than for 20-second stretching (p < 0.05). The pre-post ratio of SLR of 300-second stretching was significantly greater than for the other durations (p < 0.05). Straight leg raise measured at baseline was not significantly correlated with the changes in other parameters.
Isometric Muscle Force
There was no interaction effect between the durations of stretching and prestretching or poststretching isometric muscle force values. Isometric muscle force decreased significantly after stretching for all durations of stretching (p < 0.05; Figure 3E). There were no differences between the prestretchings and poststretching isometric muscle force values for all durations of stretching. There were also no differences between the pre-post ratios of isometric muscle force for all durations of stretching. When we repeated the analysis excluding the interactive term, the duration of stretch was not statistically significant (p > 0.05) whereas the pre-post effect was significant (p < 0.05; data not shown).
Correlation Coefficients Between the Durations of Stretching and the Amount of Change (Percentage) in Each Parameter
There was a strong negative correlation between the durations of stretching and the amount of change (%) for SPT (ρ = −0.742, p < 0.01), a moderate positive correlation with SLR (ρ = 0.577, p < 0.01), and a fair negative correlation with DPT (ρ = −0.255, p < 0.05) and stiffness (ρ = −0.235, p < 0.05). However, there was no significant correlation between the durations of stretching and the amount of change (percentage) for isometric muscle force (ρ = −0.08, p > 0.05).
This study investigated the acute effects of different static stretching durations (20, 60, 180, and 300 seconds) on SPT, DPT, stiffness, SLR, and isometric muscle force.
Static passive torque declined significantly after stretching for all durations of stretching. This result was consistent with that of previous studies (21–23,25,26). Research demonstrated that H-wave amplitude, the excitatory index of the anterior horn cell, declined during triceps surae stretching (10,12). This suggested that SPT decline in response to static stretching may be because of a neurophysiological mechanism. In addition, the present study found a strong negative correlation between the durations of stretching and the amount of change (percentage) in SPT. Duong et al. (8) reported that SPT decreased by approximately 20% after 5 minutes of triceps surae stretching and by approximately 42% after 42 minutes of triceps surae stretching. Their results and the present results suggest that the longer the duration of stretching, the greater the decline in SPT after stretching. Future research should simultaneously assess SPT and the H-reflex for the triceps surae to determine the relationship between neurophysiological mechanisms and changes in SPT in response to static stretching.
Dynamic passive torque and stiffness decreased significantly after 180- and 300-second stretching. In addition, DPT after 60- and 300-second stretching were significantly lower than after 20-second stretching, whereas stiffness after 300-second stretching was significantly lower than after 20-second stretching. These results were similar to previous studies where DPT and stiffness did not change after 90-second stretching (21) and then decreased significantly when stretching exceeded 120 seconds or totaled 300 seconds (4,28,33). Although SPT decreased after all durations of stretching, DPT and stiffness did not significantly change after stretching for 20 and 60 seconds. In addition, there were fair negative correlations between the durations of stretching and the amount of change (percentage) for DPT and stiffness. Dynamic passive torque and stiffness, calculated from torque-angle curve, are thought to reflect the viscoelasticity of the muscle-tendon unit (23,24). Although the muscle-tendon unit viscoelasticity did not change after 20 and 60 seconds of stretching, the decline in DPT and stiffness after 180 and 300 seconds of stretching possibly represented a significant improvement in the viscoelasticity of the muscle-tendon unit. In contrast, Halbertsma et al. (13) reported that stiffness did not change after 600-second stretching. In recent years, ultrasonography has been used to calculate the elastic (stiffness) and viscous (hysteresis) indexes for tendons (17,28). Future research should use these ultrasonography derived indexes to examine the relationship between the durations of stretching and DPT or stiffness. It was previously reported that the increment of tendon stiffness in long muscles might contribute to increased torque output after isometric training (19). Therefore, studies should also examine the direct impact of static stretching on the contribution of tendon stiffness, independent of muscle stiffness, to power output and flexibility of the muscle-tendon unit.
These observed results for SPT and DPT indicate that less force is required to maintain a static joint position or provide dynamic movement, whereas the reduction in stiffness indicates greater muscle and joint flexibility. These factors are associated with the risk of muscle injury during stretching and physical activity. Therefore, reducing SPT, DPT, and stiffness may reduce the risk of muscle damage.
Straight leg raise increased significantly after all durations of stretching. This result was consistent with those of previous studies (3,13,21,22,30). In addition, this study found a moderate positive correlation between the durations of stretching and the amount of change (percentage) for SLR. We suggest that the factors responsible for the increase in SLR were changes in muscle-tendon unit extensibility and viscoelasticity, and pain threshold. These factors may have been responsible for the SPT results, but do not explain the results for DPT and stiffness. Several studies reported that mobility was increased but that DPT and stiffness did not change after stretching (12,21,24). These studies suggested that the result was because an increase in stretch tolerance. Stretch tolerance was assessed as the joint angle at which the subjects felt a pain or discomfort while undergoing passive stretching (12,24). In support of the role of stretch tolerance, Magnusson et al. (24) reported that the knee extension angle, at which subjects felt pain in the hamstring, was increased despite DPT not changing after static hamstring stretching. These authors proposed that the knee extension angle increased because of an increase in stretch tolerance and not an improvement in hamstring viscoelasticity. Although they did not discuss the mechanism for the increase in stretch tolerance in detail, it was proposed that one of the responsible factors was an elevation in pain threshold. Fukunaga et al. (9) reported that there was a tendency for muscle blood flow to increase after stretching. Mense and Stahnke (27) reported that the activity of C-fiber during muscle contraction without muscular ischemia was significantly less than with muscular ischemia. Therefore, a relationship between muscle blood flow and pain threshold was suggested. Based on this previous research it is possible that the pain threshold for muscle stretching was elevated in this study. This may have been related to an increase in muscle blood flow caused by the stretching. Therefore, the increase in SLR after 20- and 60-second stretching was because of the increase in hamstring extensibility that was regulated by neurophysiological properties and an increased elevation in the pain threshold that was related to an increase in blood flow. This proposed mechanism is further supported by the finding that DPT and stiffness did not change significantly after 20- and 60-second stretching. In addition, it is suggested that the increase in SLR after 180- and 300-second stretching was because of an improvement in the viscoelasticity of the muscle-tendon unit in addition to the 2 factors discussed above, because DPT and stiffness declined significantly after 180- and 300-second stretching. From this discussion, it can be seen that an increase in muscle blood flow may play an important role in the alteration in muscle function with static stretching. Future research should examine the relationship between blood flow and mobility in response to static stretching.
Isometric muscle force decreased significantly after all durations of stretching. This result was consistent with previous studies, showing that muscle function deteriorates somewhat after static stretching in training programs (2,3,34). This reduction in isometric muscle force after static stretching, particularly with longer stretch durations, may be detrimental to performance. A recent systematic review of 106 studies of acute static stretching (14) revealed a dose-dependent relationship between duration of acute static stretching and muscle performance. The authors of that study reported that the detrimental effects of acute static stretching were mainly limited to stretches lasting ≥60 seconds, as shorter stretch durations had no detrimental (<30 seconds; pooled estimate = −1.1%) or no significant (30–45 seconds; pooled estimate = −1.9%) effects on performance. The effects of static stretch on performance were independent of performance task, contraction mode, or muscle group.
It is possible that the factors responsible for the decrease in isometric muscle force were changes in muscle-tendon unit extensibility and viscoelasticity, and neurophysiological properties. Siatras et al. (34) reported that isometric muscle force did not change after 10- and 20-second stretching, but decreased significantly after 30- and 60-second stretching. In contrast, we found that isometric muscle force decreased after 20 seconds of stretching. In support of this finding, McHugh et al. (25) reported that isometric knee flexion force decreased when hamstrings were in a shortened position after stretching, but not when they were in a lengthened position after stretching. It was argued that the decrease in isometric muscle force was because of a stretching induced increase in muscle extensibility. It was further argued that the decrease in isometric muscle force was because of a stretching induced alteration in the muscle length-tension relationship. In this study, it is possible that isometric knee flexion force decreased after stretching because muscle force was assessed with the hamstrings in a shortened position. Another factor responsible for the decrease in isometric force may have been related to the stretching induced alteration in SLR. This study found that SLR increased significantly after all durations of stretching. This increase in the muscle extensibility may have changed the muscle length-tension relationship, thereby decreasing isometric muscle force production. Another factor responsible for the decrease in muscle force after stretching may have been a change in muscle-tendon unit viscoelasticity (34,39). However, the DPT and stiffness results after 20- and 60-second stretching were not associated with the decrease in isometric force. Finally, we suggest that the decrease in isometric muscle force after stretching may have been because of neurophysiological alterations (34,39). Previous studies have found that electromyography (EMG) amplitude during maximum voluntary contraction decreased after stretching (2,5). Future research examining the relationship between stretching and isometric force should include the use of EMG to examine the neurophysiological determinants of alterations in performance. Studies should also examine whether the reduction in isometric force has any significant consequences on the performance of physical activities, and hence evaluate whether this factor outweighs the benefits of longer durations of stretch on muscle/joint flexibility. Considering that different stretch procedures may have differing effects on specific activities (32,35,38), this reduction in isometric force at longer static stretch durations may mean it should be reserved for specific activities requiring flexibility rather than force generation. However, as described by Kay and Blazevich (14), the decrease in performance may be small or unnoticed at shorter stretch durations, and that maintaining a static stretch for ≥60 seconds is uncommon in preexercise routines, so the relative impact of this physiological phenomenon on performance is likely to be small.
This reduction in muscle force (performance) may be a trade-off for achieving greater joint flexibility/reduced muscle stiffness with longer durations of static stretches. Several studies have demonstrated that static stretching training programs achieve meaningful improvements in joint flexibility (6,32). Because there appeared to be no marked differences between dynamic and static stretching in the improvements in flexibility, the use of dynamic stretching may be preferred for training and warm-up programs in individuals requiring muscle power or a balance between muscle power and flexibility. Intriguingly, however, a short-term study examining the effects of warm-up stretching on hamstring flexibility in individuals with prior injury suggested that static stretching, but not dynamic stretching, increased hamstring flexibility (31). This effect was apparent, but not significant, in individuals with reduced flexibility postinjury. This raises the question of whether static stretching, as in this study, could be more beneficial in facilitating the recovery from injury, and whether longer durations of stretch may allow quicker recovery of muscle flexibility.
This study found that the longer the duration of stretching, the more SPT and SLR changed after stretching. In addition, to improve DPT and stiffness, it is suggested that longer durations of static stretching may be required. Thus, it is possible that static stretching should be performed longer than it is performed generally to improve flexibility and reduce the risk of muscle/joint injury. However, there may be a threshold level for optimum performance, as the longest duration of stretching tested in this study resulted in a decrease in isometric force. Further studies are necessary to investigate the effects of stretching intensity, duration, and frequency to not only resolve the current discrepancies among prior studies, but also help to guide the development of future recommendations for static stretching before physical activity. It is also possible that static stretching programs would facilitate the recovery from injury in terms of improving muscle flexibility. This is an interesting concept that would need to be carefully examined by comparing the effects of stretch duration and type of stretch on the restoration of flexibility. Additionally, future studies should examine the implications of the reduced isometric force observed in this study with longer durations of stretching, and whether this limitation outweighs the potential benefits of longer stretching on flexibility, for example.
This work was supported in part by a grant from A-kit Co., Ltd. and the Public Advertisement Research Project of Nihon Fukushi University. All the authors declare that there is no potential conflict of interest regarding this article. This study does not indicate endorsement by the National Strength and Conditioning Association.
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