MOTONEURONAL OUTPUT AND GRADATION OF EFFORT IN ATTEMPTS TO CONTRACT ACUTELY PARALYZED LEG MUSCLES IN MAN

1. The study was designed to determine the degree to which normal subj ects can control motoneurones innervating a leg muscle when acutely de prived of muscle afferent feedback. Microneurographic recordings were made from eighteen motor fascicles in the common peroneal nerve, of wh ich thirteen innervated tibialis anterior and five toe dorsiflexor mus cles. 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 c omplete distal block of the common peroneal nerve, motoneurones innerv ating tibialis anterior were frequently activated during weak attempte d contraction of the synergist toe extensors and vice versa. 3. When s ubjects attempted contractions of the paralysed muscles at a constant effort, pressure applied to the dorsum of the foot caused relatively s mall 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 incre ase 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increased linearly with step number, but declined as the step was maintained, m ore so when auditory feedback was withdrawn. 5. There was hysteresis i n 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 t o the paralysed muscle increased disproportionately rapidly compared w ith contralateral force when subjects attempted bilaterally symmetrica l increasing efforts. 6. Subjects attempted to activate the paralysed muscle group maximally for 20-30 s with auditory feedback of the neuro gram and verbal encouragement. There was a small statistically signifi cant reduction in the motoneuronal output 5-10 s into the 30 s effort but, with further encouragement, it recovered towards the end of the e ffort. 7. When compared directly in the same recording sequences, atte mpts to make rapid brief maximal efforts (2-3 s duration) produced the same motoneuronal output as attempts to make sustained efforts. 8. Si milar results occurred when the motoneuronal output to tibialis anteri or was recorded during a selective distal block of tibialis anterior s paring toe dorsiflexors. 9. These results establish that subjects can recruit and grade the motor drive to a muscle in the absence of feedba ck from that muscle (and its synergists) and even in the absence of au diovisual feedback. The findings indicate that knowledge of the motor command can be adequate for some contractions and help define the role of afferent feedback in motor control. However, when compared with da ta for intrinsic muscles of the hand studied under similar circumstanc es, the present findings suggest a lesser ability to activate the ankl e dorsiflexors selectively. In addition, the ability to activate ankle dorsiflexor motoneurones voluntarily appeared less dependent on cutan eous input than is the case for hand motoneurones.