Excitation frequency and muscle fatigue: electrical responses during human voluntary and stimulated contractions.

Changes in the electrical activity of the human adductor pollicis muscle during fatiguing maximal voluntary contractions (MVCs) were compared to those resulting from equal periods of maximal ulnar nerve stimulation at different frequencies. In each case the force and smoothed, rectified EMG (SRE) were monitored continuously, and the area of the evoked surface action potential (SAP) was measured at intervals. During high-frequency stimulation (50 and 80 Hz), both the SRE and SAP area increased in the first 10 to 20 s, thereafter declining to very low values. With low-frequency stimulation (20 Hz), both increased gradually throughout the contraction. The increases in SAP area were related to a slowing of conduction velocity. In all experiments in which the frequency of stimulation was constant, changes in the SRE and SAP area mirrored one another. In sustained MVCs, the rate of force loss was less than during high-frequency stimulation. SAPs evoked by periodic single maximal shocks to the nerve increased initially in area but then remained relatively constant. The SRE no longer paralleled the SAP; it generally increased initially, but then declined roughly in proportion to the force. When the nerve was maximally stimulated at a progressively reduced frequency (80 to 20 Hz), force loss and SAP area were similar to those recorded during an MVC. The SRE was also similar in form. It is concluded that (a) during continuous high-frequency stimulation, much of the fatigue is due to failure of electrical propagation, probably largely at the muscle fiber membrane; and (b) in voluntary contractions where no similar failure was observed, muscle fatigue is minimized by a progressive reduction in motor unit activation.