Ankle mechanical impedance under muscle fatigue

Abstract

This study reports the effects of ankle muscle fatigue on ankle mechanical impedance. It suggests that decreasing ankle impedance with muscle fatigue may contribute to an increased probability of ankle injury. If confirmed, this observation may have important athletic, military and clinical implications. The experiment was designed to induce fatigue in the tibialis anterior and triceps surae muscle groups by instructing subjects to perform isometric contractions against a constant ankle torque generated by a backdrivable robot, Anklebot, which interacts with the ankle in two degrees of freedom. Median frequencies of surface electromyographic (EMG) signals collected from tibialis and triceps surae muscle groups were evaluated to assess muscle fatigue. Using a standard multi-input and multi-output stochastic impedance identification method, multivariable ankle mechanical impedance was measured in two degrees of freedom under muscle fatigue. Results indicate that ankle mechanical impedance decreases in both the dorsi-plantarflexion and inversion-eversion directions under tibialis muscle fatigue. However, the effect of triceps surae on ankle mechanical impedance is uncertain since the current experimental protocol could not effectively induce fatigue in triceps surae.