Motor unit activity in the voluntary contraction of human muscle

Abstract

Human muscles were stimulated with square pulses applied locally and via the motor nerve at different frequencies. The tensions were recorded with a strain gauge. Maximum tension produced by tetanic stimulation equalled that developed in a maximum voluntary contraction. Recordings were made from single motor units with fine wire electrodes during voluntary contractions of various strengths, and the frequency of firing was measured. The relation between strength of contraction and rate of firing is an S-shaped curve. Experiments were conducted which indicate that gradation of contraction in the muscles investigated is brought about mainly by motor unit recruitment, except at very low and high contraction strengths.