Isometric force production by motor units of extensor digitorum communis muscle in man

Abstract

The firing rates and sizes of motor units of the human middle finger extensor (digitorum communis) muscle were recorded during steady voluntary isometric contractions. The results displayed a systematic organization of the motor unit population in terms of unit recruitment thresholds relative to unit size and of unit firing rates relative to total muscle force and unit size. Pairs of simultaneously recorded homonymous units maintained their firing rate relationships over time when voluntary isometric contractions were slowly varied in strength. Larger units were recruited at higher force levels than smaller units and larger units increased their firing rates more rapidly than smaller units, once recruited. This systematic unit behavior provided for an indirect means of estimating the motor unit composition of the whole muscle, on the basis of observations on a relatively small selected number of units. Data on unit sizes, firing rates, composition and the contractile response of units completed a functional model of the voluntary force production process. This model was suitable for a wide variety of comparative studies.