The effects of acute static and dynamic stretching on spring-mass leg stiffness.

Objective: This study investigated the effect of brief static and dynamic stretching on spring-mass leg stiffness in a vertical bilateral hopping task. Method: 38 men and 18 women were randomly assigned to either a natural (NAT; n = 27), or maximal (MAX; n = 29) hopping group. NAT bounced at their self-selected stiffness and MAX were instructed to bounce as stiffly as possible. Hopping was performed at 2.2 Hz on a force plate. After each of four treatment conditions (no stretch, 30 s stretch, multiple (4 ) 30 s stretch, and dynamic stretch), subjects completed three  30 s bouts (2 minute rest periods) of hopping, in a counterbalanced crossover design. Stretches were performed on: gluteals, hamstrings, quadriceps and calves. Spring-mass leg stiffness was calculated as the ratio of peak vertical force to vertical displacement during ground contact. Results: The results revealed that men displayed greater leg stiffness than women (mean difference: 6.04 kN·m-1; 95% likely range: 1.94 – 10.13 kN·m-1), and that MAX produced higher stiffness values than NAT (mean difference: 10.93 kN·m-1; 95% likely range: 6.84 – 15.03 kN·m-1). Although there were no significant effects of treatment (p = 0.85) or time (p = 0.54) on leg stiffness, there was a significant treatment × time interaction (p = 0.015). Nevertheless, post hoc analyses were unable to identify where those differences were. Conclusion: Relative to control, the results of this study showed that brief static stretching or non task-specific dynamic stretching does not affect spring-mass leg stiffness during vertical bilateral hopping.