Musculotendinous stiffness provides an estimate of resistance to joint perturbation, thus contributing to joint stability. Females demonstrate lesser hamstring stiffness than males, potentially contributing to the sex discrepancy in anterior cruciate ligament injury risk. However, it is unclear if the sex difference in hamstring stiffness is due to differences in muscle size or to inherent/material properties of the musculotendinous unit. It was hypothesized that hamstring stiffness, stress, strain, and elastic modulus would be greater in males than in females, and that hamstring stiffness would be positively correlated with muscle size.
Stiffness was assessed in 20 males and 20 females from the damping effect imposed by the hamstrings on oscillatory knee flexion/extension following joint perturbation. Hamstring length and change in length were estimated via motion capture, and hamstring cross-sectional area was estimated using ultrasound imaging. These characteristics were used to calculate hamstring material properties (i.e., stress, strain, and elastic modulus).
Stiffness was significantly greater in males than in females (P<0.001). However, stress, strain, and elastic modulus did not differ across sex (P>0.05). Stiffness was significantly correlated with cross-sectional area (r=0.395, P=0.039) and the linear combination of cross-sectional area and resting length (R(2)=0.156, P=0.043).
Male's hamstrings possess a greater capacity for resisting changes in length imposed via joint perturbation from a structural perspective, but this property is similar across sex from a material perspective. Females demonstrate lesser hamstring stiffness compared to males in response to standardized loading conditions, indicating a compromised ability to resist changes in length associated with joint perturbation, and potentially contributing to the higher female ACL injury risk. However, the difference in hamstring stiffness is attributable in large part to differences in muscle size.
"This finding  is confounded by a small, mixed sample of healthy controls and LBP patients, and absence of a direct stretch tolerance measure. Other evidence using a prone knee flexion/extension perturbation model reported lower active and passive hamstring stiffness in women [18,19]. Thus it is reasonable to believe that passive stiffness will be lower in women, although it is not clear whether this will explain differences in extensibility during the iSLR. "
[Show abstract][Hide abstract] ABSTRACT: Background
This study examined whether passive hamstring tissue stiffness and/or stretch tolerance explain the relationship between sex and hamstring extensibility.
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).
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 r2 = -0.03, p = 0.61). For men, 44% of the variance in hamstring extensibility was explained by VAS and maximal applied torque (adjusted r2 = 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).
The results of this study suggest that stretch tolerance and not passive stiffness explains hamstring extensibility, but this relationship is only manifest in men.
"In the present study, the angle between each TI arm and the aponeuroses were greater in males than females (Fig. 6). This difference could be expected, as it may be related to a greater muscle mass and thickness in males compared with females (Blackburn et al., 2009). In addition, the results showed relatively longer TI arm normalized lengths in females compared with males. "
"MTS, tendon stiffness, and strength), however, only strength and tendon stiffness differed across sex following normalization to body mass. The values we obtained are consistent with previous research regarding cross-sectional area (Blackburn et al., 2009), pennation angle (Kellis et al., 2009; Wickiewicz et al., 1983; Wickiewicz et al., 1984; Woodley and Mercer, 2005), hamstring MTS (Blackburn et al., 2004a, 2004b), and posterior thigh fat thickness (Doxey, 1987). However, fascicle length was slightly greater in our sample than in previous studies (Kellis et al., 2009). "
[Show abstract][Hide abstract] ABSTRACT: Greater hamstring musculotendinous stiffness is associated with lesser anterior cruciate ligament loading mechanisms during both controlled joint perturbations and dynamic tasks, suggesting a potential protective mechanism. Additionally, lesser hamstring stiffness has been reported in females, potentially contributing to their greater risk of anterior cruciate ligament injury. However, the factors which contribute to high vs. low stiffness are unclear. Muscle geometry and architecture influence force production and may, therefore, influence stiffness. The purpose of this investigation was to evaluate the contributions of geometric and architectural muscle characteristics to hamstring stiffness.
Thirty healthy individuals (15 males, 15 females) volunteered for participation. Biceps femoris long head cross-sectional area, pennation angle, fiber length, tendon stiffness, and posterior thigh fat thickness were assessed via ultrasound imaging, and strength was measured via isometric contraction. Stiffness was assessed via the damped oscillatory technique.
Following normalization to anthropometric factors, only strength (r=0.535) and posterior thigh fat thickness (Spearman ρ=-0.305) were correlated with stiffness. Normalized tendon stiffness (0.06 vs. 0.10N/m·kg(-1)) and strength (7.1 vs. 10.0N·kg(-1)) were greater in males, while posterior thigh fat thickness (10.4 vs. 5.0mm) was greater in females.
Greater posterior thigh fat thickness may influence stiffness by contributing to greater intramuscular fat and shank segment mass, and lesser muscle per unit mass in the thigh segment. These findings suggest that training designed to increase hamstring strength and decrease fat mass may be beneficial for anterior cruciate ligament injury prevention.
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