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

1. While it is known that the average firing rate of it population of moton eurones declines with time during a maximal voluntary contraction, at least for many muscles, it is not known how the firing patterns of individual mo toneurones adapt with fatigue. To address this issue we used tungsten micro electrodes to record spike trains (mean +/- S.E.M., 183 +/- 27 spikes per t rain; range, 100-782 spikes) from 26 single motor units in extensor halluci s longus during sustained (80-180 s) maximal dorsiflexions of the big toe i n 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 decl ined 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 rate s 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 variabil ity of individual motor units, each spike train was divided into 2-15 equal segments containing at least 50 interspike intervals. Discharge variabilit y was measured from the coefficient of variation (S.D./mean) in the intersp ike intervals, with the S.D. being calculated using a floating mean of 19 c onsecutive intervals. Adaptation was computed as the average change in firi ng rate or variability that would occur for each 1 s of activity. There wer e no systematic changes in either firing rate or variability with time. 4. We conclude that single motoneurones supplying the extensor hallucis lon gus, 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 invarial,le consequence of the fatigue associated with sustained maximal voluntary contractions.