Passive force, angle, and stiffness changes after stretching of hamstring muscles

Periodic or long-term Stretching programs have demonstrated changes in joint range of motion. The suggested mechanisms for these increases in range of motion are changes in the tissue properties of the muscle and more recently stretch tolerance. However, few studies have examined changes in passive resistive forces and related these changes to increases in range of motion. Purpose: The purpose of this study was to investigate the influence of a 6-wk hamstring-stretching program on knee extension range of motion, passive resistive forces, and muscle stiffness. Methods: A randomized control trial with repeated measures was undertaken with 43 school-age subjects. Hamstring extensibility was assessed by a passive knee extension test using a Kincom(R) isokinetic dynamometer. The intervention group participated in a 6-wk hamstring-stretching program. Stretches were performed 5 d(.)wk(-1), once per day, held for 30 s, and for 3 repetitions. The control group did not stretch over the 6-wk intervention period. Measurements of hamstring extensibility were repeated at the end of the 6-wk intervention. Results: After the intervention period, significant (P < 0.05) increases in knee extension range of motion, passive resistive force, and stiffness were observed in the experimental group. No significant differences were observed in the control group's findings for the same variables. Conclusions: The findings of this study are consistent with other literature that shows periodic stretching programs over a 6-wk time frame produce significant changes in knee extension range of motion. The finding of increased stiffness in the new range of motion, gained as a result of the stretching program, provides some evidence that structural changes had occurred in the stretched muscles.