Invariance of ankle dynamic stiffness during fatiguing muscle contractions.

Our objective was to determine the effect of muscle fatigue on the dynamic stiffness of the human ankle. Four subjects were required to maintain constant-force contractions of tibialis anterior until the required force could no longer be maintained. Repeated pseudo-random displacements of ankle angular position were applied throughout each contraction. The dynamic relation between ankle angular position and ankle torque was identified by determining non-parametric compliance impulse response functions (CIRFs). The CIRFs were redetermined every 2.55S throughout the sustained contractions to provide a quantitative measure of changes in ankle stiffness dynamics. Inspection of these CIRFS revealed little change in shape or magnitude throughout the contractions, despite large increases in tibialis anterior EMG. The dynamics were further quantified by estimating the equivalent joint inertia, viscosity and elasticity associated with each CIRF. As each contraction progressed, the inertial and elastic terms remained constant whereas the viscous term decreased slightly. These findings demonstrate that fatigue of tibialis anterior during sustained constant mean force contractions results in little change in the mechanical dynamics of the human ankle.