Motor-Unit Action-Potential Counts

Their Significance in Isometric and Isotonic Contractions

¹ Highland-Alameda County Hospital, Oakland

Muscle action-potential counts may be coveniently recorded by electronic counters. Total counts for a chosen time interval may be recorded, or a continuous count (analogue display) may be recorded. Total counts and analogue displays

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for muscle contractions make possible a numerical representation of action potentials produced by a given volume of muscle. The integrated (averaged) curve of total electrical activity may thus be replaced by the action-potential count of the electromyogram having a total, or continuous, numerical expression. For muscle-function analysis and locomotion studies, the continuous action-potential count with established threshold values, adds significantly to the information gained from the conventional electromyogram.

The time constant of conventional electrical integrators, due to dependence upon capacitors, is overcome. A definite delay in the rise of tension following the appearance of the rising action-potential count is observed (Figs. 5 and 6). The electromyogram, its count rate and averaged form, decline well in advance of tension.

Total counts for ten-second periods of activity increase in linear fashion as the strength of the isometric contractions increases. A skeletal-pin technique has been used for accurate measurements of muscle tension. The range of muscle length studied is limited by physiological joint ranges of motion. Thus, only the central portion of the theoretical length-tension cuve, the physiological potion, has been studied.

The continuous action-potential count is related directly to tension during isometric or isotonic contractions as long as the muscle is loaded.

The electronic counter appears to have great possibilities of application in the study of neuromuscular disorders. Since different adjustments of both the oscilloscope preamplifier and the counter are necessary, results are not directly comparable with recordings from normal muscle. However, normal, dystrophic, and poliomyelitic muscle have definite characteristics of action potential, frequency, and amplitude.

Thus, a motor-unit action-potential classification for types of muscle is possible:

Normal

moderate amplitude

high frequency

Dystrophic

low amplitude

high frequency

Poliomyelitic

high amplitude

low frequency

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