Pattern of electrical activity in normal and diseased muscle

Examination of the number of spikes (turns) per 5 sec and the mean amplitude of electrical activity, measured both at a force adjusted relative to maximum force (Po) and at a standard force, made it possible to distinguish between a decrease in force arising from loss or inactivity of motor units, and loss of muscle fibers or diminution in their cross-sectional area. The initial decrease in force of disused quadriceps muscle (following immobilization of the knee joint) resulted mainly from inactivity of motor units. In patients with motoneuron disease or a lesion of the brachial plexus, the number of turns at a force adjusted relative to Po was diminished because of increased potential duration and loss of motor units. Turns at a standard force of 2 kg were normal. In myopathy, the ratio of turns to amplitude was increased in two-thirds of the patients.