Comparison of triceps surae structural stiffness and material modulus across sex.

Background: Dynamic joint stability is derived, in part, from active muscle stiffness. Previous research has identified greater structural musculotendinous stiffness in males than in females, suggesting potential sex differences in joint stability. However, structural stiffness is influenced by anthropometrics, and it is currently unclear if sex differences in musculotendinous stiffness are purely functions of anthropometrics or related to additional factors. The purpose of this investigation was to compare structural stiffness and material modulus of the triceps surae between sexes to determine the relative influence of anthropometrics. It was hypothesized that males would demonstrate greater structural stiffness and material modulus.
Methods: Twenty male and 20 female individuals volunteered for participation. Active triceps surae structural stiffness was estimated from the damped frequency of oscillation of the shank about the ankle. Material modulus was calculated as the ratio of stress to strain, derived from estimates of triceps surae length and physiological cross-sectional area.
Findings: Structural stiffness was significantly greater in males [mean (SD)] [137.41 (26.99) N/cm] than females [91.06 (20.10) N/cm]. Similarly, material modulus was significantly greater in males [2778.51 (549.95) Pa] than females [1968.58 (439.61) Pa].
Interpretation: Greater structural stiffness in males identified in previous literature appears to be a true phenomenon. Identification of greater material modulus in males suggests that these differences are likely attributable to sex differences in tendon stiffness and muscle architecture. These results indicate that male musculature is more effective at resisting changes in its length, a finding which may have implications for joint stability.