Discharge pattern of human motor units during dynamic concentric and eccentric contractions

doi:10.1016/S0924-980X(98)00012-5

Electroencephalography and Clinical Neurophysiology/Electromyography and Motor Control

Volume 109, Issue 3, June 1998, Pages 245-255

Electroencephalograp...

https://doi.org/10.1016/S0924-980X(98)00012-5 Get rights and content

Abstract

Objectives: A total of 45 motor units (MUs) from the human biceps brachii muscle were investigated during isovelocity concentric and eccentric movements performed by means of a device implementing an external torque in the direction of the extension proportionally to the elbow angle changes. The effects of movement velocity on the recruitment and decruitment thresholds (RT and DT) and the corresponding discharge patterns were determined. Methods: A wire branched electrode placed subcutaneously was used to discriminate the potentials from a single MV. Results: The majority of MUs (91%) were recruited at lower torque values with the increase of movement velocity. The decrease of RT was statistically significant for 47% of the investigated MUs. A typical discharge pattern of short first interspike interval (ISI) followed by a longer one was observed for 93% of all MUs. After the first 2–3 spikes the rate of the MU discharge was approximately constant regardless of the fact that the muscle force gradually increased until the end of the concentric movement.

Conclusions: There are differences in the muscle force control during shortening and lengthening contractions. For 82% of the investigated MUs DT was smaller at faster movements and for 21 MUs (47%) the decrease of DT was significant. The gradually declined MU discharge rate throughout the entire movement with a very long last ISI was demonstrated for 93% of the investigated MUs.

Introduction

Since the pioneer study of Adrian and Bronk (1929)the main two mechanisms involved in the gradation of muscle force at voluntary contractions became well known. The first mechanism is known as recruitment and represents the number of active motor units (MUs) and the second one – the rate coding – relates to the firing frequency of the already active MUs. A number of investigations have studied the relative contribution of force grading mechanisms mostly during isometric contractions. Recently, more attention has been paid to movements accompanied by shortening and lengthening of the muscles (Nardone and Schieppati, 1988; Nardone et al., 1989; Theeuwen et al., 1994; van Groeningen and Erkelens, 1994; Garland et al., 1996; Søgaard et al., 1996). Despite the importance of the movements accompanied by lengthening of active muscles for ordinary daily activity only a limited number of investigations have been devoted to the study of MU activity during this kind of motor performance. The investigations of some authors suggest differences in muscle force control during shortening and lengthening contractions (Stuart et al., 1988; Nardone et al., 1989).

Since the formulation of the size principle by Henneman (1957)that the recruitment properties of MUs correlate to their motoneurone size a number of investigators have taken interest in studying the limits of its operation. However, the papers devoted to the decruitment properties of the MUs are scarce especially the ones observed during movements.

The purpose of this study is to examine the MU activity and discharge pattern during movements which involved shortening (concentric movements) and lengthening (eccentric movements) contractions of the human biceps brachii muscle.

Section snippets

Subjects

Eight subjects (aged 25–45 years) volunteered for the study. They were clinically healthy and none had revealed any history of neurological or muscular disorders. All subjects gave their informed consent prior to each study. The experimental procedures were approved by the local ethics committee. The subjects participated at least in two training sessions and in a number (up to 10) of experimental ones. The investigation was conducted with the subjects comfortably seated in a chair with a

Results

The majority of the volunteers after 2–3 training sessions succeeded to fulfil the motor task – constant-velocity concentric and eccentric movements with two different speeds. A total of 45 MUs from the right biceps brachii of 8 subjects were recorded during the experiments. Only MUs recruited during the concentric movements and recorded successfully during both flexion and extension of the elbow joint were further evaluated.

The mean maximal voluntary contraction torque in flexion direction for

Discussion

Different techniques have been elaborated for selective recording of MU potentials and the most impressive results have been obtained with the application of computer signal decomposition (Mambrito and De Luca, 1984; Masakado et al., 1995). An additional essential limitation for the selective recording of MU activity during movements is the variation in the shape and amplitude of the potentials due to the physical instability of the electrode and its shift relative to the position of the MU

Acknowledgements

This work was supported by Bulgarian National Science Fund – Grant B-635/1996.

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Discharge pattern of human motor units during dynamic concentric and eccentric contractions

Abstract

Introduction

Section snippets

Subjects

Results

Discussion

Acknowledgements

Brain Res.

Electroenceph. clin. Neurophysiol.

Electroenceph. clin. Neurophysiol.

Electroenceph. clin. Neurophysiol.

Brain Res.

Electroenceph. clin. Neurophysiol.

Electroenceph. clin. Neurophysiol.

Electroenceph. clin. Neurophysiol.

Electroenceph. clin. Neurophysiol.

Electroenceph. clin. Neurophysiol.

Prog. Neurobiol.

Exp. Neurol.

The discharge of impulses in motor nerve fibers. Part II. The frequency of discharge in reflex and voluntary contractions

J. Physiol. (London)

The effect of the stimulation pattern on the fatigue of single motor units in adult cats

J. Physiol. (London)

The relationship between the rate of rise of isometric tension and motor unit recruitment in a human forearm muscle

Pflug. Arch.

Topography of terminal motor innervation in striated muscles from stillborn infants

Am. J. Phys. Med.

Recruitment order contractile characteristics and firing patterns of motor units in the temporalies muscle of monkeys

Exp. Neurol.

Behaviour of human motor units in different muscles during linearly varying contractions

J. Physiol. (London)

Ballistic contractions in man: characteristic recruitment pattern of single motor units of the tibialis anterior muscle

J. Physiol. (London)

Ballistic contractions in fast or slow human muscles: discharge patterns of single motor units

J. Physiol. (London)

Spinal motoneuron recruitment in man: rank deordering with direction but not with speed of voluntary movement

Science