The purpose of this study was to investigate the occurrence of motor unit doublet discharges in young and older individuals at different rates of increasing force. Participants included eight young (21.9 ± 3.56 yr) and eight older (74.1 ± 8.79 yr) individuals, with equal numbers of males and females in each group. Motor unit activity was recorded from the tibialis anterior during isometric dorsiflexion using a four-wire needle electrode. Subjects performed three ramp contractions from zero to 50% maximal voluntary contraction (MVC) force at each of three rates: 10, 30, and 50% MVC/s. Overall, the occurrence of doublets was significantly higher in the young than in the older individuals. However, neither group showed differences in the occurrence of doublets across the three rates of force production. Doublet firings were observed in 45.6 (young) and 35.1% (old) of motor units at 10% MVC/s; 48.6 (young) and 22.5% (old) of motor units at 30% MVC/s; and 48.4 (young) and 31.4% (old) at 50% MVC/s. The maximal firing rate was significantly higher and the force at which the motor units were recruited was significantly lower for those units that fired doublets than those that did not. The force at which doublets occurred ranged from 3.42 to 50% MVC in the young subjects and from 0 (force onset) to 50% MVC in the older subjects. The results of this study suggest that the occurrence of doublets is dependent on both motor unit firing rate and force level. The lower incidence of doublets in older individuals may be attributable to changes in the intrinsic properties of the motoneurons with aging, which appear to play a role in doublet discharges.
Changes in firing rate of human motor units during linearly changing voluntary contractions
