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Department of Medicine, Charing Cross and Westminster Medical School, London.

1. Motor unit firing has been studied in human sternocleidomastoid muscle. 2. Two needle electrodes were inserted into the muscle and the activity of pairs of motor units recorded during (a) reflex hypercapnic obstructed breathing, (b) eucapnic voluntary copying of (a) against the same inspiratory resistance and (c) voluntary copying of (a) without any resistance, accompanied by isometric neck rotation. 3. Cross-correlation histograms of the firing of unit pairs showed a clear central peak, indicative of synchronization. The mean duration of the peak during voluntary breathing was 25 ms (range 9-40 ms). There was no difference in duration of synchronization during the different tasks. 4. For the duration of the synchronization peak, the mean strength of synchronization expressed as the number of concomitant discharges of the two units as a proportion of the total number of discharges was 0.026 (range 0.011-0.058) for reflex hypercapnic obstructed breathing. For the same unit pairs the strength of synchronization for isometric neck rotation was the same as that during reflex hypercapnic breathing but for voluntary obstructed breathing it was, on average, threefold greater. 5. In three out of twenty-two motor units studied, 'discharge' occurred with an interval of less than 10 ms ('doublet' firing) at the onset of each inspiration during both types of obstructed breathing; this was rarely observed during neck rotation. 6. The results are interpreted in terms of different synaptic drives to the motor units during the three different tasks.

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