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Department of Clinical Neurophysiology, Prince Henry Hospital and Prince of Wales Medical Research Institute, University of New South Wales, Sydney, Australia.

1. The study was designed to determine the degree to which normal subjects can control motoneurones innervating a leg muscle when acutely deprived of muscle afferent feedback. Microneurographic recordings were made from eighteen motor fascicles in the common peroneal nerve, of which thirteen innervated tibialis anterior and five toe dorsiflexor muscles. The nerve was then blocked completely at a distal site near the fibular head with local anaesthetic. A sequence of tests was performed with each fascicle to determine the degree to which the subject could control the motoneuronal drive to the paralysed muscle. 2. During a complete distal block of the common peroneal nerve, motoneurones innervating tibialis anterior were frequently activated during weak attempted contraction of the synergist toe extensors and vice versa. 3. When subjects attempted contractions of the paralysed muscles at a constant effort, pressure applied to the dorsum of the foot caused relatively small changes in the level of neural output, producing a small increase in motoneuronal drive to tibialis anterior, but no consistent change in the drive to toe extensor fascicles. 4. Subjects were able to increase the motoneuronal drive to the paralysed tibialis anterior in five steps of effort each lasting 10 s. The level of motor output increased linearly with step number, but declined as the step was maintained, more so when auditory feedback was withdrawn. 5. There was hysteresis in the relationship between motoneuronal output and force (measured on the contralateral side) during attempts to make slowly increasing then decreasing ramps of effort on both sides over 20-120 s. Motor drive to the paralysed muscle increased disproportionately rapidly compared with contralateral force when subjects attempted bilaterally symmetrical increasing efforts. 6. Subjects attempted to activate the paralysed muscle group maximally for 20-30 s with auditory feedback of the neurogram and verbal encouragement. There was a small statistically significant reduction in the motoneuronal output 5-10 s into the 30 s effort but, with further encouragement, it recovered towards the end of the effort. 7. When compared directly in the same recording sequences, attempts to make rapid brief maximal efforts (2-3 s duration) produced the same motoneuronal output as attempts to make sustained efforts. 8. Similar results occurred when the motoneuronal output to tibialis anterior was recorded during a selective distal block of tibialis anterior sparing toe dorsiflexors.(ABSTRACT TRUNCATED AT 400 WORDS)

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