RANK-ORDERED REGULATION OF MOTOR UNITS

Myoelectric signals were detected from the tibialis anterior muscle of 5 subjects with a quadrifilar needle electrode while the subjects gen erated isometric forces that increased linearly with time (10% of maxi mal voluntary contraction/s) up to maximal voluntary level. Motor unit firing rates were studied as a function of force throughout the full range of muscle force output. The relationship between force and firin g rate was found to contain three distinct regions. At recruitment and near maximal force levels, firing rates increased more rapidly with f orce than in the intermediate region. Furthermore, in the regions with rapid increases, the rate of change of firing rate was correlated to the recruitment threshold, with higher recruitment threshold motor uni ts displaying greater rates of change. In the intermediate region, all motor units had similar rates of change of firing rate, A weak positi ve correlation was found between initial firing rate and recruitment t hreshold, Firing rates of motor units at any instant were found to be ordered according to the recruitment order: at any given time in the c ontraction motor units with lower recruitment thresholds had higher fi ring rates than units with higher recruitment thresholds. Firing rates of all motor units were observed to converge to the same value at max imal forces. Mechanisms underlying motor unit recruitment and firing r ate modulation are discussed in the context of a conceptual model. (C) 1996 John Wiley & Sons, Inc.