The study abided by the principles of the Declaration of Helsinki, and written informed consent was obtained from each participant prior to participation in the study. The participants were instructed to maintain their normal dietary habits and to refrain from vigorous physical activity for two days before the experiment. At least one day before the testing day, the participants reconfirmed their health history and familiarized themselves with the equipment and experimental procedure.
Measurements were obtained at the same time points but without any DS in the control group. Each subject performed DS of their right leg only to keep experimental conditions consistent between subjects. The right leg was dominant in all 24 subjects; this was determined by asking the subjects which leg they use to kick a soccer ball. No warm-up was performed in any subjects to eliminate any potential interaction between warm-up and stretching.
The subjects stood upright with their feet parallel and facing forward while holding onto parallel bars with both hands original standing position; Figure 2A. The subjects were then instructed to contract their hip flexors intentionally once every 2 s so that their hip joints flexed while keeping their knees extended so that their right leg swung up to the anterior aspect of their body and their right hamstring muscles were stretched Figure 2B.
A 5-min period of DS was selected because our previous studies showed that 5 min of static stretching at a tolerable intensity and without pain significantly increased ROM and PT, and decreased passive stiffness after stretching Matsuo et al. The procedure for dynamic stretching of the hamstring muscles. A The subjects started in a standing position with their feet parallel and facing forward and their hands holding the parallel bars original standing position.
Measurements were made with the participant seated on a chair Figure 1A with an attached isokinetic dynamometer. We confirmed with electromyography that the muscles did not contract during this passive knee extension. With the isokinetic dynamometer programmed in passive mode, the torque and angle signals were continuously measured and recorded. Gravity correction was not performed when the torque-angle curve was measured, per our previous protocols Kataura et al.
All output signals underwent analog-to-digital conversion and were recorded using LabChart 8. Passive stiffness was calculated from the same knee extension angle range before and after stretching, and the pre-stretching value was compared with that after stretching.
The lower pre-stretching and post-stretching maximum knee extension angles were used for passive stiffness calculation. The coefficient of variation for the measures also showed acceptable reliability ROM: 6. To allow for possible dropout, 12 participants were recruited for each group. Sigma Plot 13 software Systat Software Inc. Nonparametric tests were performed on the passive stiffness data, which was shown to be non-normally distributed using the Shapiro-Wilk normality test Shapiro and Wilk, The Friedman repeated-measures analysis of variance on ranks was used to calculate temporal changes in outcome measures ROM, PT at the onset of pain, and passive stiffness within each group.
When a significant time effect was found, post-hoc analysis with Wilcoxon signed-rank tests was conducted with a Bonferroni correction applied to detect significant differences from pre-DS values. Between-group differences in pre-DS values for all outcome measures were assessed using the Mann-Whitney U test.
Between-group differences in post-DS value for all outcome measures were then assessed for each time point using the Mann-Whitney U test. Owing to the non-normal data distribution, all data are presented as medians and interquartile ranges unless otherwise noted.
There were no significant between-group differences in the average angles of hip flexion There were also no significant between-group differences in the pre-DS values of any of the outcome measures Table 1. Changes in range of motion, passive torque at the onset of pain, and passive stiffness before and after dynamic stretching.
Values are expressed as median interquartile range. No significant temporal changes in any of the outcome measures in the control group were found, but significant changes in all outcome measures were observed over time in the experimental group Table 1 , Figure 3.
In the experimental group, knee ROM increased significantly by a median of 8. The significant increase in knee ROM was sustained over 90 min 15 min: 9.
Boxplot figures showing percent changes in A range of motion, B passive torque at the onset of pain, C passive stiffness before and after dynamic stretching. The line within the box marks the median, crosses represent the mean, the boundary of the box indicates the 25th and 75th percentiles, the error bars indicate the 10th and 90th percentiles, and the dots above and below the error bars represent outliers.
Similarly, PT at the onset of pain increased significantly by 9. Passive stiffness decreased significantly by 7. Normalized ROM values were significantly higher in the experimental group at all time points than in the control group higher by 9. Similarly, normalized PT values at the onset of pain were significantly higher by 8. Normalized passive stiffness values were significantly lower in the experimental group at all time points after DS lower by 6. We investigated whether DS would influence the following parameters of hamstring flexibility: ROM, PT at the onset of pain, and passive stiffness of the hamstring muscles.
In addition, we studied the long-term effects of DS by evaluating the temporal changes in each flexibility parameter after DS.
We found that DS produced an immediate increase in passive knee extension ROM, a decrease in passive stiffness of the hamstrings, and an increase in PT at the onset of pain. In contrast, reductions in passive stiffness have been reported to be less prolonged after SS. For example, SS reduced the passive stiffness of the triceps surae for only 15—20 min Mizuno et al. Furthermore, in our study the increase in PT at the onset of pain lasted for a shorter duration 15—30 min than the passive stiffness reduction following DS of the hamstrings.
In contrast, the effects of SS of the triceps surae on PT at the onset of pain and on ROM lasted longer than its effects on passive stiffness Mizuno et al. Our results also differed from those of previous studies that investigated the effects of DS on the triceps surae Mizuno, ; Mizuno and Umemura, We speculate that these differences in the effects of DS could be due to differences in the target muscles hamstrings vs.
The DS of the triceps surae was performed by adding a light resistance to the antagonistic muscles of the triceps and using a lower number of sets 1—7 than we used in the current study Mizuno, The prolonged reduction in passive stiffness following our DS protocol might have resulted in the more sustained increase 90 min in ROM in our study.
Passive stiffness, which is calculated from the torque—angle curve, is thought to reflect the viscoelasticity of the muscle-tendon unit Magnusson et al. In contrast, stretch tolerance is thought to be the tolerance to the tensile strength produced in muscles that are subjected to passive extension. The index used to reflect this in previous studies has been PT at the onset of pain Kataura et al.
Thus, the immediate increase in knee ROM that occurs following DS likely results from both a decrease in passive stiffness i.
Since the passive stiffness changed greatly following DS, it can be inferred that a physiological response that causes elasticity change in the muscle-tendon complex occurred. Specifically, there is a possibility that some elastic elements in the muscle-tendon complex were changed, such as fascia, elastic proteins such as titin also known as connectin , and tendons.
However, it is important to note that the changes in passive stiffness of the hamstring muscles after DS persisted for a longer period than did the changes in PT at the onset of pain. These results suggest that the sustained increase in knee ROM following DS is primarily attributable to the reduced passive stiffness of the hamstring muscle—tendon unit Opplert and Babault, and not to stretch tolerance.
Our finding that ten s sets of 15 repetitions of DS increased knee extension ROM is consistent with the results of previous studies Chen et al. Both the duration and intensity of stretching have been shown to influence passive stiffness of the muscle-tendon unit Opplert and Babault, For example, while relatively short periods of SS did not affect passive stiffness of the hamstrings Magnusson et al.
In addition, the intensity of SS has been reported to decrease passive stiffness in a dose-dependent manner Kataura et al. Similarly, a higher number of repetitions and addition of resistance to the antagonistic muscles have both been shown to increase ROM following DS of the triceps surae Mizuno, Taken together, these data suggest that the relatively high intensity of DS in our study resulted in a greater dose-dependent change in hamstring flexibility, which in turn resulted in a more prolonged effect on passive stiffness.
Thus, the reduction in passive stiffness was sustained for a longer time than the effect on PT at the onset of pain. This study had several limitations. First, we used a nonrandomized, sequential study design where the control group was selected to match the baseline characteristics of the experimental group.
The experimental group was unavailable for a follow-up visit; this prevented us from investigating outcomes with and without DS in the same subjects. Such paired analysis could presumably have allowed more robust investigations of the effect of DS. Finally, we did not measure hamstring tension or extension during DS. As DS relies on a subject consciously and actively contracting antagonist muscles to stretch the targeted muscle Samukawa et al.
Newer techniques using videotaping and electrogoniometry to measure angular velocity and ROM could potentially allow real-time quantification of the loaded tension in the hamstrings.
We investigated the acute and sustained effects of DS on flexibility parameters in healthy volunteers who performed 10 sets of DS 15 repetitions per set of the hamstring muscles. We found that DS caused a sustained reduction in passive stiffness of the hamstrings and increase in knee ROM, as well as a less lasting increase in PT at the onset of pain. As increased passive stiffness of the hamstrings and decreased knee ROM are both risk factors for hamstring injury during sports Goldman and Jones, ; Woods et al.
The funding sources had no role in the study design, in the collection, analysis and interpretation of data, in the writing of the report, or in the decision to submit the article for publication.
The experiments comply with the current laws of the country in which they were performed. The authors have no conflicts of interests to declare. E-mail: pj. E-mail: moc. Muscle weakness and pain with work-related musculoskeletal disorders.
National Center for Biotechnology Information , U. J Sports Sci Med. Author information Article notes Copyright and License information Disclaimer. Received Oct 18; Accepted Nov This article has been cited by other articles in PMC. Abstract Dynamic stretching DS is often performed during warm-up to help avoid hamstring muscle injuries, increase joint flexibility, and optimize performance.
Key points. Key words: Flexibility, muscle stretching, retention time, muscle-tendon unit, stretch tolerance, exercise. Introduction Hamstring strain injuries are one of the most common types of noncontact injuries experienced during sports activities and exercise Opar et al. Open in a separate window. Figure 1. Dynamic stretching protocol Each subject performed DS of their right leg only to keep experimental conditions consistent between subjects. Figure 2. Measurement procedure and outcomes Measurements were made with the participant seated on a chair Figure 1A with an attached isokinetic dynamometer.
Results Baseline measures There were no significant between-group differences in the average angles of hip flexion Table 1. Intragroup analysis of changes in outcome measures after DS No significant temporal changes in any of the outcome measures in the control group were found, but significant changes in all outcome measures were observed over time in the experimental group Table 1 , Figure 3.
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