Facility of motor unit control during tasks defined directly in terms of unit behaviors

Abstract

The phenomenon of recruitment order reversal among pairs of motor units during “single motor unit training” was studied in 11 normal subjects in abductor pollicis brevis, first dorsal interosseus, and extensor digitorum muscles of the hand. Feedback provided by visual and auditory cues allowed all subjects to control single-unit behavior directly, such as the production of single spikes, doublets, and steady firing at various frequencies. However, verified recruitment order reversal among motor unit pairs, recorded from the same fine-wire bipolar intramuscular electrode was obtained only during contractions of markedly different orientation in a multifunctional muscle. These results suggest that limited control of motor unit recruitment order is possible only in anatomically defined muscles that have multiple physiologic functions. Single motor unit frequency modulation was tested during an eight-target tracking task. The calculated information outflow rate achieved with motor unit frequency modulation performance was comparable to that obtained by modulation of the full-wave rectified surface electromyogram.