Acoustic myography is the recording of sounds produced by contracting muscle. These sounds become louder with increasing force of contraction. We have compared muscle sounds with surface EMG to monitor the dissociation of electrical from mechanical events (presumably, the loss of excitation-contraction coupling) which occur with motor unit fatigue. Acoustic signals were amplified using a standard phonocardiograph, recorded on FM magnetic tape, and digitally analyzed. Muscles were examined at rest, with intermittent contractions, and with sustained contractions. We found that with fatigue, the acoustic amplitude decayed, but the surface EMG amplitude did not. With decreased effort, however, the acoustic and the surface EMG amplitudes declined simultaneously. By simultaneously recording acoustic signals and needle EMG, individual motor units were resolved acoustically in two muscles with decreased numbers of motor units and increased motor unit size. Fasciculations also produced acoustic signals, although no acoustic signal has yet been found that correlates with fibrillations. Analysis of acoustic signals from muscle provides a noninvasive method for monitoring motor unit fatigue in vivo. It may also be useful in distinguishing muscle fatigue from decreased volition.

A case report of an isolated musculocutaneous nerve injury distal to the branch to the coracobrachialis muscle of the non-dominant arm is described. The injury occurred in the context of an industrial setting, specifically, an assembly plant. The non-dominant limb was injured; in contrast, most reports indicate the dominant limb was affected during strenuous activity. Nerve function was monitored with serial electromyography. Management of the injury was conservative, with eventual full functional recovery.