Motor unit recruitment during prolonged isometric contractions

Abstract

Motor unit recruitment patterns were studied during prolonged isometric contraction using fine wire electrodes. Single motor unit potentials were recorded from the brachial biceps muscle of eight male subjects, during isometric endurance experiments conducted at relative workloads corresponding to 10% and 40% of maximal voluntary contraction (MVC), respectively. The recordings from the 10% MVC experiment demonstrated a characteristic time-dependent recruitment. As the contraction progressed both the mean number of motor unit spikes counted and the mean amplitude of the spikes increased significantly (P < 0.01). This progressive increase in spike activity was the result of a discontinuous process with periods of increasing and decreasing activity. The phenomenon in which newly recruited motor units replace previously active units is termed "motor unit rotation" and appeared to be an important characteristic of motor control during a prolonged low level contraction. In contrast to the 10% MVC experiment, there was no indication of de novo recruitment in the 40% MVC experiment. Near the point of exhaustion a marked change in action potential shape and duration dominated the recordings. These findings demonstrate a conspicuous difference in the patterns of motor unit recruitment during a 10% and a 40% MVC sustained contraction. It is suggested that there is a close relationship between intrinsic muscle properties and central nervous system recruitment strategies which is entirely different in fatiguing high and low level isometric contractions.

Motor Unit Substitution in Long-Duration Contractions of the Human Trapezius Muscle

“…By this it is meant that motor units alternate their activity in a cyclical fashion, such that substitution of one motor unit for another would be followed by back-substitution of the original unit. Although this concept is often mentioned within the clinical community, it has been reported by only a few researchers (Fallentin et al 1993;Sjøgaard et al 1986), and these reports have generated more controversy than conviction. Person (1974), Kato et al (1981), and Fallentin et al (1993) among others describe recruitment and silencing of motor units during a contraction, yet none of their studies provides evidence of constant joint torque or muscle force.…”

“…Although this concept is often mentioned within the clinical community, it has been reported by only a few researchers (Fallentin et al 1993;Sjøgaard et al 1986), and these reports have generated more controversy than conviction. Person (1974), Kato et al (1981), and Fallentin et al (1993) among others describe recruitment and silencing of motor units during a contraction, yet none of their studies provides evidence of constant joint torque or muscle force. De Forrest (1973) andDe Luca et al (1982) found no derecruitment during constantforce isometric contractions of moderate duration at different force levels.…”

Excitatory drive to the alpha‐motoneuron pool during a fatiguing submaximal contraction in man.

“…If, during a submaximal constant force contraction, active MUs fatigue and additional ones become recruited, a decrease of the superimposed twitch must occur, indicating an increased excitatory drive to the a-motoneuron pool. The idea of a progressive recruitment of new, unfatigued MUs is primarily based on data from single MU recordings (Moritani et al 1986;Fallentin, J0rgensen & Simonsen, 1993) and on the finding of a time-dependent increase in EMG amplitude (e.g. Edwards & Lippold, 1956;BiglandRitchie et al 1986a;Krogh-Lund & J0rgensen, 1991;L6scher, Cresswell & Thorstensson, 1994).…”

Spatial reorganisation of muscle activity correlates with change in tangential force variability during isometric contractions

“…Regarding activity of muscle subvolumes, dynamic reorganisation of the muscle activity within the muscle during sustained contractions has been described [Holtermann et al, 2010[Holtermann et al, , 2008. Moreover, it has been reported that an EMG increase is related with the number of newly recruited motor units during sustained low-level contractions [Fallentin et al, 1993], supporting the stronger impact on force variability by additional motor unit recruitment [Contessa et al, 2009].…”