Angular Velocity Dependence of Passive Ankle Joint Torque Correlates with Viscosity Coefficient of the Medial Gastrocnemius in Males: A Step Toward Quantifying Muscle Viscosity Using Shear Wave Elastography.

OBJECTIVE: To investigate whether shear wave elastography (SWE)-derived elasticity and viscosity indices of the medial gastrocnemius are associated with the velocity-dependent characteristics of passive ankle joint torque during muscle elongation at different angular velocities. METHODS: SWE measured the shear wave velocity, elastic modulus, viscosity coefficient, and shear wave dispersion at multiple static joint positions (at plantarflexion angles of 30°, 15°, and 0°) in 15 healthy young men. Passive dorsiflexion from 30° plantarflexion to 20° dorsiflexion was applied at multiple angular velocities using a dynamometer. The structural damping coefficient in a lumped-parameter model of the ankle and associated muscles was defined as the slope of the regression relating peak torque to angular velocity. RESULTS: No correlation was observed between the SWE-derived viscosity coefficient and the SWE-derived dispersion slope, indices of viscous properties (r = -0.013, p = 0.930). The shear wave velocity, elastic modulus, and viscosity coefficient increased with dorsiflexion, but their change patterns differed. The shear wave dispersion was unaffected by the ankle angle. Significant correlations with the structural damping coefficient were observed only for the viscosity coefficient at 15° (r = 0.652, p = 0.008) and 0° (r = 0.537, p = 0.039) plantarflexion. CONCLUSIONS: The SWE-derived viscosity coefficient, unlike the shear wave dispersion slope, demonstrated a moderate correlation with structural damping in this small study cohort when SWE was measured during moderate stretch of the muscle (plantarflexion). These preliminary findings suggest that the SWE-derived viscosity coefficient may serve as a superior quantitative index for evaluating intrinsic muscle viscosity.