THE ANTAGONIST MUSCLE PATTERN IN ELBOW EXTENSION OFA THROWING TASK

Abstract

INTRODUCTION The myoelectric activity in fast human movements is characterised by a phasic pattern with two fundamental events: the first agonist burst, responsible for the initial limb acceleration, and an antagonist impulse. The accurate definition of the antagonist burst role, and the relative participation of peripheral and central sources on its control, are not clearly defined in the literature (Angel, 1977; Marsden et al., 1983). The purpose of the present study was (1) to characterise the antagonistic pattern on the elbow extension when subjects had to throw a dart onto a target, and (2) to analyse the antagonist changes when subjects throw with different accuracy/speed demands. Experiments were performed on 13 subjects. Surface EMG were recorded from triceps and biceps brachii with active bipolar electrodes. Electrogoniometers wore used on shoulder and elbow joints. RESULTS The antagonist EMG contained a phasic burst (ANT) which began between the movement onset and the end of the agonist activation, with an usual co-contraction duration of less than 30 ms. The time interval between the movement onset and the beginning of ANT was normally higher than the EMG latency necessary for spindle influences, considered to be less than 20 ms (Tarkka, 1986). In our results, ANT always began during the acceleration phase and, in most cases, developed its maximum intensity around the moment of elbow peak velocity. So, the antagonist burst represents more than an impulse braking preventing full extension, and we must admit its participation in the control of the duration of the acceleration phase. Antagonist modifications with velocity indicated an amplitude increase, measured by the integrated EMG. without relevant temporal changes. Two subjects presented an alternative way of braking the movement, when the accuracy constraints were absent, based, on the increase of the joint stiffness through the co-contraction of agonist and antagonist muscles. This kind of movement braking resulted in a reduced timing accuracy demand when compared with the active braking, produced by the antagonist phasic burst (Ghez & Martin, 1 982). CONCLUSION The timing of the antagonist phasic burst invite us (1) to associate it with the control of the end of the acceleration phase and (2) to accept that it could be influenced by the muscular response to stretch, although modulated by higher central commands. (3) The general tendency was to maintain the temporal structure of antagonist EMG among conditions and to increase its intensity with the increase of the throwing velocity. (4) Two subjects presented an alternative way of braking the movement when the accuracy constraints wers absent. REFERENCES Angel, R. (1977). Antagonist muscle activity during rapid arm movements. Central versus proprioceptive influences. J. Neuroi, Neuros., Psychia t., 40: 683-686. Ghez, C., & Martin, J. (1 982). The control of rapid limb movement in the cat. Ill - Agonist- Antagonist coupling. Exp: Br. Res., 45: 1 15 - 125. Marsden, C., Obeso, J., & Rothwell, J. (1983). The function of the antagonist muscle during fast limb movements in man. J. Physiol., 335: 1 - 13. Tarkka, 1. (1986). Short and long latency reflexes in human muscles following electrical and mechanical stimulation. Acta Physiol. Scand., 128. Supplem. 557: 1-32.