The effect of menstrual-cycle phase on hamstring extensibility and muscle stiffness.

CONTEXT Preventing noncontact ACL injuries has been a major focus of athletic trainers and researchers. One factor that may influence female noncontact ACL injury is the fluctuating concentrations of hormones in the body. OBJECTIVE To determine whether muscle properties change across the menstrual cycle. DESIGN Repeated measures. Testing was performed within 3 d after the onset of menses and ovulation. Repeated-measures ANOVAs were used to determine changes in variables across the menstrual cycle, and Pearson correlations were used to determine relationships between variables. PARTICIPANTS 8 women with normal menstrual cycles. MAIN OUTCOME MEASURES Active hamstring stiffness and hamstring extensibility. RESULTS Hamstring extensibility (P = .003) increased at the ovulation testing session but hamstring muscle stiffness (P = .66) did not. CONCLUSIONS The results indicate that hamstring muscle stiffness did not change across the menstrual cycle and hamstring extensibility increased at ovulation, when estrogen concentration increases.