Electrical muscle activity during a gradual increase in force in patients with neuromuscular diseases

The electrical activity of the brachial biceps muscle from 31 patients with neuromuscular diseases were quantified during a gradual increase in force from zero to maximum within 10 sec. In patients with myopathy the ratio of potential reversals per 100 msec (turns) to mean amplitude was increased more often at 10% and 20% of maximum force than at greater force (i.e., in four-fifths of the patients). Similarly, the mean amplitude as a function of turns was more diagnostic at low than at high force, possibly due to an affection of low threshold motor units. In 14 patients with neurogenic disorders turns was decreased in nearly two-thirds of the patients at a force of 20% and 30% of maximum. In muscles where the force is easily determined it is suggested to quantitate the electrical activity during constant forces of 10% and 30% of maximum. In muscles where the force is difficult to determine the analysis of the ratio of turns to mean amplitude should be performed when motor unit potentials begin to interfere.