| Summary: | Background: The demands of modern day sport require athletes to reach their optimal sporting performance. Flexibility is an important component of exercise performance. The high incidence of hamstring strain injuries in various sporting codes has been linked to reduced hamstring flexibility. Stretching has been used as the primary method to improve or maintain flexibility as a prophylactic prevention of muscle strains in many sporting codes. While a variety of stretching techniques exist, contract-relax-agonist-contract (CRAC) stretching, a type of proprioceptive neuromuscular facilitation stretching, appears to induce greater flexibility improvements than other forms of stretching. However, the effectiveness of this stretch as a method of enhancing agility and sprint performance, as functional measures of athletic performance, has yet to be determined. Objective: To determine the effect of hamstring contract-relax-agonist-contract stretch on flexibility, agility and sprint performance as functional measures of muscle performance in moderately active adult males. Methods: Forty healthy male volunteers between the ages of 21 and 35 years, who performed between three and five hours of physical activity per week were recruited for this study, which had a true experimental design. Participants provided written informed consent, and completed medical- and exercise-related questionnaires. Body mass, stature and body mass index were measured. Participants were randomly assigned to either an experimental group, which received the CRAC intervention, or the control group, which did not receive CRAC intervention. Participants attended a total of three testing sessions. During the first session, hamstring flexibility and sprint and agility times were measured. In the second session, pre- and post-CRAC hamstring flexibility was measured and the best of twotimed trials was recorded for the sprint and agility tests. During the final testing session, pre-CRAC hamstring flexibility was recorded and following a standardised warm-up, post-CRAC hamstring flexibility was measured at specifically timed intervals (0, 2, 4, 6, 8, 15, and 20 min) on a randomly selected leg (referred to as the "thixotropy" leg). The hamstring flexibility of the opposite leg (the "control" leg) was measured at 0 and 20 min only. A standardised warm-up was performed prior to the hamstring CRAC stretch in all testing sessions. During testing sessions, participants in the control group were asked to rest in supine lying for 6 min, which was equivalent to the time taken to perform the CRAC stretch for participants in the experimental group following the warm-up. Results: There was a significant difference between groups in body mass (p = 0.02), with participants in the experimental group (n = 20) having a significantly higher body mass, compared to participants in the control group (n = 20). There were no significant differences between groups for any other descriptive variables. There was a significant increased percentage change in hamstring flexibility of the experimental group, compared to the control group (p <.001). No significant differences were found in the percentage of change of agility, best 10 m or best 25 m sprint times between groups. There was a significant difference between groups with repeated flexibility measurements conducted over regularly timed intervals (F(7, 266) = 38.95; p <.001). Hamstring flexibility remained significantly increased for the duration of 8 min in the experimental group post-CRAC stretch, compared to the control group (p <.001). There were no significant differences between the knee extension angles of the "thixotropic" and "control" leg in the experimental and control groups at the 20 minute interval when compared to baseline knee extension angles within each group. Conclusion: Hamstring flexibility was significantly increased for up to 8 min following the CRAC stretch. However, the CRAC stretch was ineffective in enhancing agility and sprint performance. The need for further research into the use of CRAC stretching as a method of functional performance enhancement was highlighted. There should be a standardised protocol of CRAC application, and future studies should determine the effects of chronic stretch adaptations following regular, longterm hamstring CRAC application on measures of exercise performance. This study showed that CRAC is an effective, time-efficient method of stretching that does not have a detrimental effect on exercise performance.
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