Human muscle function and fatigue.

Fatigue is defined as a failure to maintain the required or expected force. The force of a voluntary contraction is graded according to both the tension generated in each muscle fibre and the number of fibres recruited. The same is true of fatigue. Percutaneous electrical stimulation of a muscle via its motor nerve allows the contractile function to be measured independently of volition. Studies have been made of the forces generated isometrically at different stimulation frequencies (frequency: force curve), and of fatiguability (tendency to lose force in a given time at specified stimulation frequencies), in the quadriceps and adductor pollicis muscles. Electrical stimulation recordings of the programmed stimulation myograms distinguish forms of muscle fatigue. Low frequency fatigue which implies impaired excitation-contraction coupling is long-lasting, whereas high frequency fatigue which represents impaired muscle membrane excitation recovers rapidly. Electromyographic (EMG) indicators of fatigue are well recognized but their use is limited because they cannot alone indicate whether alterations in excitation-contraction coupling underlie fatigue. Alterations in the power spectrum of the EMG precede (force) fatigue in sustained maximum voluntary contractions. Fatigue may ultimately be due to a failure of the rate of energy supply to meet demand, but the precise expression of this defect may vary, such that failure of excitation or of activation may predominate over failure of the energy supply.