An electromyographic analysis of functional differentiation in human pectoralis major muscle

The aim of this investigation was to determine whether functional differentiation was present within the radiate muscle, pectoralis major, by determining whether motor units within individual muscular segments were independently controlled by the central nervous system (CNS) during a series of isometric muscle actions. Six miniature surface electrode pairs were placed along the origin of the pectoralis major of eight male subjects. Electromyographic (EMG) potentials were then recorded during isometric shoulder actions in various planes and at two levels of contraction intensity. The EMG waveforms were integrated (IEMG) and then normalized. The normalized IEMG waveforms (nIEMG) were then statistically compared to determine whether motor units within the detection area of each electrode pair had varied their contribution to the total activity of the muscle from one isometric action to another. An alteration in the contribution of each electrode site (muscular segment) to the total activity of the pectoralis major was taken to be indicative of functional differentiation. The results of this study indicated that the muscular segments of pectoralis major were controlled independently during performance of the isometric tasks. Isometric shoulder adduction saw moderate to high levels of activation expand from the inferior segments of the muscle to all segments, excluding the clavicular head, as the position of the upper limb changed from 90° to 0° of abduction. The most inferior muscular segments of pectoralis major were utilized preferentially when the subjects performed shoulder extension motions from a flexed shoulder position. Conversely, shoulder flexion motions utilized the superior segments of the muscle. Finally, horizontal flexion of the shoulder joint elicited activity from the middle segments of the muscle regardless of the degree of shoulder flexion. The results of this study support the notion that the CNS 'fine tunes' the activation patterns of muscular segments within pectoralis major to maximize the muscle's potential to contribute efficiently to isometric actions of the shoulder.