Motor unit activation patterns during concentric wrist flexion in humans with different muscle fibre composition

Abstract

Muscle activity was recorded from the flexor carpi radialis muscle during static and dynamic-concentric wrist flexion in six subjects, who had exhibited large differences in histochemically identified muscle fibre composition. Motor unit recruitment patterns were identified by sampling 310 motor units and counting firing rates in pulses per second (pps). During concentric wrist flexion at 30% of maximal exercise intensity the mean firing rate was 27 (SD 13) pps. This was around twice the value of 12 (SD 5) pps recorded during sustained static contraction at 30% of maximal voluntary contraction, despite a larger absolute force level during the static contraction. A similar pattern of higher firing rates during dynamic exercise was seen when concentric wrist flexion at 60% of maximal exercise intensity [30 (SD 14) pps] was compared with sustained static contraction at 60% of maximal voluntary contraction [19 (SD 8) pps]. The increase in dynamic exercise intensity was accomplished by recruitment of additional motor units rather than by increasing the firing rate as during static contractions. No difference in mean firing rates was found among subjects with different muscle fibre composition, who had previously exhibited marked differences in metabolic response during corresponding dynamic contractions. It was concluded that during submaximal dynamic contractions motor unit firing rate cannot be deduced from observations during static contractions and that muscle fibre composition may play a minor role.

EMG activity and voluntary activation during knee-extensor concentric torque generation

“…However, neural factors responsible for the reduction in neural drive inhibition (from 60°AEs -1 to 120°AEs -1 ) are not clearly understood and could be attributed to either or both additional recruited motor units and/or an increase in the firing rate. However, according to Søgaard et al (1998), the possibility of changes in firing rate could be excluded as a factor in the increase in neural drive. Indeed, these authors have observed that even during slow sub-maximal voluntary concentric wrist flexions (30°AEs -1 ; intensity ranging from 30% to 60% maximal voluntary efforts), firing rate could attain the leveling off of the sigmoidal force/firing rate curve (i.e., area causing only minimal torque augmentations with frequency intensification).…”

“…Thus, during both maximal and sub-maximal voluntary contractions, the firing rate for Con60 and Con120 could be close to maximal. According to this experiment (Søgaard et al 1998), performed in a different muscle group, it may be speculated that additional recruited motor units would regulate primarily the neural drive increase (with increasing concentric angular velocity beyond 60°AEs -1 ).…”

Does the frequency content of the surface mechanomyographic signal reflect motor unit firing rates? A brief review

“…Furthermore, when used to measure interspike intervals, intramuscular EMG provides information regarding the activity of only a few motor units at most. Bellemare et al [12] indicated that motor unit firing rates ranged from 12 to 60 Hz for the biceps brachii during an isometric maximum voluntary contraction (MVC) of the forearm flexors, and Søgaard et al [81] reported firing rates between 7 and 43 Hz for the flexor carpi radialis during an isometric muscle action of the wrist flexors at 60% MVC. Thus, it is unlikely that all motor units are firing at the same frequency, even during an isometric muscle action at a steady torque level.…”

Control of the wrist joint in humans

“…Subsequently, De Luca et al (1996) have shown ranges of ®ring rates of 7±30 impulses á s )1 for ECRL and 5±25 impulses á s )1 for ECU with changes in force from 0 to 50% maximum voluntary contraction (MVC). Sùgaard et al (1998) have demonstrated dependence of mean ®ring rates on dynamic compared to static isometric contractions. For example, during dynamic contractions, ®ring rates changed from 27 to 30 impulses á s )1 when force changed from 15% to 30% MVC.…”