The Electrical Activity of Single Motor Units Under Slight Contraction in Human Muscle

Abstract

A simple modification of a commercial electrode (Beckman "Biopotential Skin Electrode") made it possible to record from single motor units of biceps brachialis without penetrating the skin. Such recordings were subject to specific experimental conditions including the use of lean subjects, slight contraction and careful placement of electrodes. The results were reproducible in each subject and had the great advantage that irritation from needle electrodes was completely eliminated.

Using this electrode system several aspects of the control of single motor units were investigated. The variation in frequency of single-unit discharge was found to be constant at constant tension with a coefficient of variation in the order of 10%. This variation contains information on the stability of the feedback system.

The tension was increased in two ways: (a) in steps of 100 grams and (b) continuously. The discharge of the single motor unti was quite different under these two conditions and the analysis suggests that in (a) there may be a rearrangement of tension among the motor units while in (b) a single motor unit adapts continuously.

This analysis may be useful in designing myoelectric control systems for prosthetics using surface electrodes.