Discharge frequency and discharge pattern of human motor units during voluntary contraction of muscle

Abstract

M.rectus femoris motor unit potentials were picked up by a bipolar needle electrode during voluntary isometric contraction. The tension varied from 0 to 4–14 kg (17–47% of maximal strength). When the contraction was increased, not only more motoneurones were recruited but their frequency was also increased. Low threshold motoneurones reached the highest frequency (up to 18–21/sec). The majority of motoneurones discharged with frequencies exceeding 11/sec. Fluctuations of strength were accompanied by corresponding fluctuations of frequency. During a long contraction at constant strength the frequency decreased during the first 1–2 min to the level of not more than 10–13/sec. A voluntary increase of tension was accompanied by a new increase in frequency. During a long contraction at constant strength new motoneurones were gradually recruited. Thus, the change of frequency is an important mechanism of contraction gradation, especially of its dynamic component. It is a mechanism of smooth and precise control of contraction. The frequency decrease during a long contraction is probably caused by adaptation. The stationary segments of spike trains were analysed statistically. Two patterns of activity were found: (1) in the range of frequencies below 8–10/sec the inter-spike interval histograms were skewed to the right. The standard deviation was rather high and depended on the mean interval. No correlation between adjacent intervals was found; (2) at frequencies above 10–13/sec the histograms had a normal distribution. The standard deviation was low and independent of the mean interval. A negative correlation between adjacent intervals was found. Thus during a short contraction, usual for m.rectus femoris (this muscle being rather a fast one), the second pattern of activity was typical, while during an unusual, prolonged contraction the first pattern was found. The transition from the first pattern to the second took place when, with increased frequency, impulses began to arise during after-hyperpolarization. The hypothesis is advanced that after-hyperpolarization is the mechanism of diminution of inter-spike interval variability, the duration of after-hyperpolarization being correlated with the lower limit of motoneurone working frequency. Thus a motoneurone seems to ensure moderation of random fluctuations of synaptic input and to code the information as mean frequency; it may, therefore, be regarded not only as a summating unit but also as a “damping” one.