Simultaneous in vivo Estimation of Muscle, Tendon, and Ankle Impedance

Appropriate regulation of joint impedance is required to successfully navigate our environment. Joint impedance is strongly dependent upon the mechanical properties of the muscles and tendons spanning it. While the impedance of the joint has been well characterized, methods to determine the individual contribution from the muscles and tendons are limited. This is a crucial gap as muscle and tendon impedance can be selectively altered by aging, pathology, or injury. Therefore, we developed an innovative in vivo method that allows for the simultaneous quantification of joint, muscle, and tendon impedance. Stochastic perturbations of ankle angle were applied while a B-mode ultrasound was used to image the displacement of the medial gastrocnemius muscle-tendon junction. Non-parametric system identification was used to quantify ankle impedance and the frequency response function between ankle rotations and muscle-tendon junction displacements. The latter represents, when scaled by Achilles tendon moment arm, the ratio between the net musculotendon impedance and the impedance of the muscle, a relationship we refer to as the impedance ratio. Muscle and tendon impedance can be calculated from these experimental estimates. The ability to simultaneously quantify joint, muscle, and tendon impedance will provide a clearer understanding their respective roles in our ability to navigate our environment, and how changes in those roles may contribute to functional impairments.