Muscle fatigue may be defined as any exercise-related reduction in the force-generating capacity of a muscle. Its effect on joint stiffness is not well understood. The purpose of this study was to determine the effect of triceps surae muscle fatigue on the dynamic stiffness of the ankle joint. Pseudorandom binary sequence (PRBS) displacements were applied before, during, and after fatiguing contractions of the triceps surae. Position and torque were recorded and analyzed to identify the intrinsic and reflex contributions to ankle stiffness. Fatigue caused significant changes in both intrinsic and reflex properties. For intrinsic properties, there was a decrease in the elasticity, and an increase in the viscosity of the joint. For reflex properties, there was an increase in the gain and a change in the reflex dynamics. Thus, it appears that, during fatigue, reflex stiffness increases to compensate, at least in part, for decreased intrinsic stiffness.
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