Triceps surae fatigue, and its effects on intrinsic and reflex contributions of human ankle dynamics

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: (1) to characterize triceps surae muscle fatigue, and (2) to determine its effect on the dynamic stiffness of the ankle joint. To address the first question, subjects were asked to maintain constant, submaximal, isometric contractions at levels ranging from 10--50% of maximum voluntary contraction (MVC) for as long as they could. Position, torque, and electromyograph (EMG) signals were recorded. EMGs from ankle extensors---triceps surae group---and flexors---tibialis anterior---measured the progression of fatigue. Generally, EMG RMS, tracking error and tremor increased with fatigue. The synergistic triceps surae muscles exhibited alternate activity between two states: active and silent. Moreover, as contraction level was increased, both alternation onset time and endurance time decreased. To address the second question, 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