Background: This study examined whether passive hamstring tissue stiffness and/or stretch tolerance explain the relationship between sex and hamstring extensibility.
Methods: Ninety healthy participants, 45 men and 45 women (mean ± SD; age 24.6 ± 5.9 years, height 1.72 ± 0.09 m, weight 74.6 ± 14.1 kg) volunteered for this study. The instrumented straight leg raise was used to determine hamstring extensibility and allow measurement of stiffness and stretch tolerance (visual analog pain score, VAS).
Results: Hamstring extensibility was 9.9° greater in women compared to men (p = 0.003). VAS scores were 16 mm lower in women (p = 0.001). Maximal stiffness (maximal applied torque) was not different between men and women (p = 0.42). Passive stiffness (slope from 20-50° hip flexion) was 0.09 Nm.°(-1) lower in women (p = 0.025). For women, linear and stepwise regression showed that no predictor variables were associated with hamstring extensibility (adjusted r(2) = -0.03, p = 0.61). For men, 44% of the variance in hamstring extensibility was explained by VAS and maximal applied torque (adjusted r(2) = 0.44, p < 0.001), with 41% of the model accounted for by the relationship between higher VAS scores and lower extensibility (standardized β coefficient = -0.64, p < 0.001).
Conclusions: The results of this study suggest that stretch tolerance and not passive stiffness explains hamstring extensibility, but this relationship is only manifest in men.