EMG and fatigue of human voluntary and stimulated contractions

During a 60s maximal voluntary isometric contraction (MVC) of the adductor pollicis muscle the loss of force is accompanied by a parallel decline in both the integrated surface electromyogram (EMG) and the single muscle fibre spike counts recorded intramuscularly. This decline is not due to neuromuscular block since the muscle mass action potential (M wave) evoked by single maximal shocks to the nerve is well maintained; nor does the size of the single fibre spike change. It must, therefore, reflect a decline in the firing pattern of the motor neuron pool. The force of a sustained MVC continues to match that from maximal tetanic nerve stimulation; thus, all motor units remain active. Continuous nerve stimulation at the frequency required to match the voluntary force of unfatigued muscle leads to a progressive failure of the M wave, and a more rapid force loss than in an MVC. Both are largely restored by reducing the stimulus frequency. The decline in neural firing rate correlates well with the rate of muscle contractile slowing. It thus optimizes force by maintaining a relatively constant degree of tetanic fusion, while avoiding peripheral failure of electrical propagation.