Motor unit recruitment in human biceps brachii during sustained voluntary contractions

Abstract

The purpose of the study was to examine the influence of the difference between the recruitment threshold of a motor unit and the target force of the sustained contraction on the discharge of the motor unit at recruitment. The discharge characteristics of 53 motor units in biceps brachii were recorded after being recruited during a sustained contraction. Some motor units (n = 22) discharged action potentials tonically after being recruited, whereas others (n = 31) discharged intermittent trains of action potentials. The two groups of motor units were distinguished by the difference between the recruitment threshold of the motor unit and the target force for the sustained contraction: tonic, 5.9 ± 2.5%; intermittent, 10.7 ± 2.9%. Discharge rate for the tonic units decreased progressively (13.9 ± 2.7 to 11.7 ± 2.6 pulses s −1 ; P = 0.04) during the 99 ± 111 s contraction. Train rate, train duration and average discharge rate for the intermittent motor units did not change across 211 ± 153 s of intermittent discharge. The initial discharge rate at recruitment during the sustained contraction was lower for the intermittent motor units (11.0 ± 3.3 pulses s −1 ) than the tonic motor units (13.7 ± 3.3 pulses s −1 ; P = 0.005), and the coefficient of variation for interspike interval was higher for the intermittent motor units (34.6 ± 12.3%) than the tonic motor units (21.2 ± 9.4%) at recruitment (P = 0.001) and remained elevated for discharge duration (34.6 ± 9.2% versus 19.1 ± 11.7%, P < 0.001). In an additional experiment, 12 motor units were recorded at two different target forces below recruitment threshold (5.7 ± 1.9% and 10.5 ± 2.4%). Each motor unit exhibited the two discharge patterns (tonic and intermittent) as observed for the 53 motor units. The results suggest that newly recruited motor units with recruitment thresholds closer to the target force experienced less synaptic noise at the time of recruitment that resulted in them discharging action potentials at more regular and greater rates than motor units with recruitment thresholds further from the target force.

Motor unit activity when young and old adults perform steady contractions while supporting an inertial load

“…The experimental protocol replicated previous studies with young (Riley et al 2008a) and old ) adults but also involved an inertial load hung from the wrist. The following six tasks were performed with the elbow flexor muscles of the left arm of the volunteers in each experimental session: 1) assessment of the MVC force; 2) identification of a single motor unit; 3) measurement of the recruitment and derecruitment thresholds of the motor unit; 4) performance of steady contractions while supporting a mass that was less than the recruitment threshold force of the motor unit; 5) evaluation of the recruitment and derecruitment thresholds of the motor unit immediately after the final steady contraction; and 6) a final MVC with the elbow flexor muscles.…”

“…The dependent variables were also compared with data from previous studies on young (Riley et al 2008a) and old adults in which the participants pulled against a rigid restraint (Fig. 1A).…”

“…The motor units of young adults discharged action potentials either repetitively or intermittently depending on the difference between the target force and recruitment threshold force (Riley et al 2008a), whereas the motor units of old adults always discharged action potentials repetitively at recruitment irrespective of the difference between the two forces . Additionally, the mean and variability of the discharge times for the first five interspike intervals (ISIs) at recruitment did not differ between the two target force conditions for old adults, whereas it did for young adults.…”

“…It has recently been shown that the discharge characteristics of the same motor unit in the biceps brachii that was recruited during a sustained isometric contraction when pulling up against a rigid restraint were influenced by the relative target force in young adults (Riley et al 2008a) but not in old adults . The motor units of young adults discharged action potentials either repetitively or intermittently depending on the difference between the target force and recruitment threshold force (Riley et al 2008a), whereas the motor units of old adults always discharged action potentials repetitively at recruitment irrespective of the difference between the two forces .…”

Reflex Inhibition in Human Biceps Brachii Decreases With Practice of a Fatiguing Contraction

“…tractions sustained at a submaximal intensity are characterized by a progressive increase in the amplitude of the EMG activity and the appearance of bursts of activity due to the transient recruitment of higher-threshold motor units (Carpentier et al 2001;Hunter et al 2002;Kouzaki and Shinohara 2006;Riley et al 2008a). The adaptations achieved by the subjects differed in the two practice studies, presumably due to clustering of groups based on time to failure for the participants in the study by Hunter and Enoka (2003).…”