Analysis of the electromyographic activity of human elbow joint muscles during slow linear flexion movements in isotorque conditions

Electromyograms were recorded by surface electrodes from the mm. biceps bra chii (caput longum et breve), brachioradialis and triceps brachii (caput lo ngum) in 10 healthy human subjects during slow linear movements in the elbo w joint against a weak extending torque. The test movements were carried ou t under visual control through combining on a monitor screen a signal from a joint angle sensor with an appropriate command generated by a computer. T he movements were fulfilled against a weak constant extending torque (0.5-2 .5 Nm) and the extensor muscles were inactive. Surface electromyograms were full-wave rectified, filtered and averaged within sets of 10 identical tes ts. For the test movements in the range from 20 degrees to 100 degrees (0 d egrees corresponds to a completely extended joint in these designations) th e dynamic components of the efferent commands to actively contracted muscle s frequently had a well-expressed monotonous increase. The electromyography intensity during movement quite often increased exponentially in all three investigated flexors. At the same time, the averaged electromyograms in on e or two muscles could contain non-monotonous oscillations, thus showing a well-expressed decrease in the intensity of the efferent inflow within a mi ddle range of a movement phase. The non-monotonous oscillations could occur in some subjects under minimal loads (0.5-0.75 Nm); they usually appeared initially only in one muscle, whereas the frequency of their occurrence was not high. The probability of finding the non-monotonous oscillations in th e electromyograms from the muscles under study increased with heightening t he extending load. Under the loads of 2.0-2.5 Nm this type of reaction coul d be found in almost half of all records (for the three muscles in each of the 10 subjects under testing). The presence of noticeable non-monotonous c omponents in the electromyograms of the elbow flexors during their contract ion is probably connected to their biomechanical arrangement within the joi nt. It is known that the arms of the forces generated by elbow flexors are maximal in the middle range of joint angles and decrease with a change in a ngle in both directions. Thus, we can suppose that non-monotonous decrement components in the electr omyograms of the elbow flexors are presumably connected with an obvious nec essity for a subject to decrease the excitatory efferent inflow to the musc les in the middle range of the joint angles. The pattern of electromyograms in the flexors acting around the joint was also dependent on a redistribut ion of activity between agonists. In the subjects showing stable non-monoto nous components on the averaged electromyogram records in two agonists, a r edistribution of the activity between these muscles has been demonstrated w hen phases of the electromyogram diminishing in one of them coincided with the appropriate increments in the other. (C) 1999 IBRO. Published by Elsevi er Science Ltd.