Sex Comparison of Hamstring Stiffness and Elastic Modulus (2008)

Undergraduate: Megan Kimsey

Faculty Advisor: Troy Blackburn
Department: Exercise & Sport Science

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Anterior cruciate ligament (ACL) injury occurs at a higher rate in females than in males. The hamstrings muscles limit ACL loading and contribute to knee joint stability. Musculotendinous stiffness refers to the ratio of change in force to change in length of a muscle in response to joint perturbation (?force/?length), and provides an estimate of a muscle’s ability to provide dynamic joint stability. Hamstring stiffness is lesser in females compared to males, potentially contributing to the sex discrepancy in ACL injury rates. However, stiffness is dependent on muscle size, and it is unclear if this sex difference is attributable to differences in hamstring size or material properties. Stiffness was measured in forty subjects (20 males, 20 females). Elastic modulus was estimated based on the stiffness values, and represents stiffness after accounting for the influence of muscle size. Hamstring length and change in length were measured via motion capture techniques, and hamstring cross-sectional area (CSA) was measured using ultrasound imaging. These characteristics were used to calculate the following hamstring material properties: stress (?force/CSA), strain (?length/pre-perturbation length), and elastic modulus (stress/strain). Stiffness was significantly greater in males, but stress, strain, and elastic modulus did not differ significantly across sex. These results suggest that males’ hamstrings possess a heightened ability to resist changes in length associated with joint perturbation, but that this difference is primarily attributable to larger muscle size. Further research is necessary to evaluate the role that hamstring stiffness plays in knee joint stability and ACL injury risk.