Discharge behaviour of single motor units during maximal voluntary contractions of a human toe extensor

Abstract

1. While it is known that the average firing rate of a population of motoneurones declines with time during a maximal voluntary contraction, at least for many muscles, it is not known how the firing patterns of individual motoneurones adapt with fatigue. To address this issue we used tungsten microelectrodes to record spike trains (mean +/- s.e.m., 183 +/- 27 spikes per train; range, 100-782 spikes) from 26 single motor units in extensor hallucis longus during sustained (60-180 s) maximal dorsiflexions of the big toe in seven human subjects. 2. Long spike trains were recorded from 13 units during the first 30 s of a maximal voluntary contraction (mean train duration, 9.6 +/- 1.2 s; range, 3.6-21.9 s) and from 13 units after 30 s (mean train duration, 16.6 +/- 3.7 s; range, 7.1-58.1 s). Maximal isometric force generated by the big toe declined to 78.3 +/- 6.3 % of its control level by 60-90 s and to 39.5 +/- 1.4 % of control by 120-150 s. Despite this substantial fatigue, mean firing rates did not change significantly over time, declining only slightly from 15.8 +/- 0.7 Hz in the first 30 s to 14.0 +/- 0.5 Hz by 60-90 s and 13.6 +/- 0.3 Hz by 120-150 s. 3. To assess fatigue-related adaptation in discharge frequency and variability of individual motor units, each spike train was divided into 2-15 equal segments containing at least 50 interspike intervals. Discharge variability was measured from the coefficient of variation (s.d. /mean) in the interspike intervals, with the s.d. being calculated using a floating mean of 19 consecutive intervals. Adaptation was computed as the average change in firing rate or variability that would occur for each 1 s of activity. There were no systematic changes in either firing rate or variability with time. 4. We conclude that single motoneurones supplying the extensor hallucis longus, a muscle comprised primarily of slow twitch muscle units, show little adaptation in firing with fatigue, suggesting that a progressive reduction in firing rate is not an invariable consequence of the fatigue associated with sustained maximal voluntary contractions.

Figure 1

The analyses of motor unit activity were restricted to discharge trains of >100 spikes (26 units). Each train was divided into segments of equal numbers of spikes but with no less than 50 interspike intervals per segment. The upper panel shows a sample plot of interspike intervals (ISIs) versus time. The mean firing rate (FR) for each segment was determined as the inverse of the mean ISI. Discharge variability was calculated as the coefficient of variation (CV, s.d./mean) of the ISIs. To minimize inflation of variability estimates due to long-term trends in the interval data, the s.d. was calculated over a floating mean of 19 consecutive intervals (lower panel).

Figure 2

Train of action potentials recorded from a single motor unit in EHL during a sustained maximum voluntary contraction. This unit was first detected 91 s after the start of the contraction. The inset to the right shows 50 consecutive action potential waveforms superimposed.

Figure 3

Mean firing rates of the 13 motor units recorded during the first 30 s of a maximal voluntary contraction. Each line is defined by the mean data points from each segment of 50 spikes, such that those trains composed of just 100 spikes would be represented by a line starting with the mean frequency calculated over data segment 1 and finishing with the mean frequency calculated over segment 2. Inflections in the longer trains represent the mean values calculated over each 50 spike segment.