The Effects of Eccentric Loading on Hamstring Flexibility

Presentation given at Physical Therapy Association-Combined Sections Meeting. Purpose/Hypothesis: Hamstring injury is a common issue in athletic populations. This results from adaptively shortened hamstrings that are expected to rapidly lengthen with activities such as sprinting. Therefore, the need to improve hamstring flexibility is of clinical importance. The purpose of this study was to assess hamstring flexibility during a 6-week training study utilizing either eccentric exercise or static stretching interventions. An increase in hamstring flexibility is expected with both the eccentric loading group and static stretching group, however, we predicted a greater increase with the eccentric loading group. Number of Subjects: 44 Materials and Methods: Inclusion criteria consisted of asymptomatic participants with hamstring muscle tightness defined by greater than 20 degrees of knee flexion measured using the passive knee extension test. Individuals were excluded if they had a history of hamstring or lower extremity injury requiring medical care within the past 12 months. Three groups were compared across the 6-week training study: control, eccentric, and static stretching groups. The eccentric portion of the Nordic Hamstring Exercise (NHE) was used for the eccentric group and a seated hamstring stretch for the stretching group. Training groups attended 2 sessions per week for 6 weeks, and the control group was instructed to maintain their current exercise level. Between and within group differences in hamstrings flexibility was assessed using a mixed model analysis of variance. The sample size was estimated using power-based calculations resulting in a n=63 (alpha level = 0.5, beta level = 0.2, power level of 80%). Results: The results demonstrated that both eccentric exercise and static stretching resulted in statistically significant increases in hamstring flexibility at 3 and 6 weeks. Although no statistical significance was found between groups from 0 to 6 weeks, the stretching group increased 11.4 degrees and the eccentric 12.8 degrees, both of which are higher than the 5-degree MCID value, while the control group only had a difference of 4.4 degrees. Conclusions: This study adds to the body of research demonstrating the positive effect of eccentric training on muscle length. It also revealed the NHE results in similar increases in hamstring length when compared to static hamstring stretching. Both groups had greater changes in hamstring length compared to the control group. As a result, either stretching or eccentric training can effectively be used to address hamstring flexibility impairments. Clinical Relevance: Both static stretching and eccentric exercise can improve hamstring flexibility over a 6-week time frame. Our study demonstrates that eccentric exercise is as effective as static stretching in increasing hamstring flexibility. As both are equally effective, to improve efficiency in the clinic, we recommend utilizing eccentric exercise to improve flexibility and also receive the benefit of strength gains. An additional benefit of the NHE resides in its injury reduction effect in athletics