Sites of failure in muscle fatigue

The sites of failure in muscle fatigue were investigated by applying controlled tapping to a muscle tendon before and after fatigue of the muscle. The resulting reflex responses were evaluated to assess muscle activation (using the EMG signal for activation failure) and joint torque (for contractile failure). An instrumented hammer was used to tap the triceps muscle tendon and record the tapping force, while the triceps EMG signal and reflex joint torque were recorded to provide measures of the reflex responses. Elbow extensor muscle fatigue was induced through repeated voluntary isometric contraction. The subject generated elbow extension torque in a 6 sec on and 4 sec off pattern for 15 minutes. A rest period of ten minutes was used to let the acute fatiguing effects diminish. Identical tendon tapping tests were done before and after fatigue. Tendon reflex gain (calculated from the tapping force input to the reflex torque output) and tapping-induced EMG gain (calculated from the tapping force input to the reflex-mediated EMG output) were used to characterize tendon reflexes. Following fatigue, we recorded substantial reductions in maximal voluntary elbow extension torque, which was more severe in some subjects than in others. It was found that less severe muscle fatigue was associated only with contractile failure, as indicated by reduction in elbow extension torque but not in EMG response to the controlled tendon tapping. More severe fatigue was associated with both activation and contractile failures, as indicated by reductions in both EMG and joint torque responses to the controlled tendon tapping. The controlled tendon tapping minimized the variations in central drive to motor neurons and neural strategy associated with voluntary contractions and evaluation of the induced reflex EMG and joint torque helps us better understand the underlying mechanisms and sites associated with muscle fatigue.