Muscle activity patterns during quick change in elbow extensions

Abstract

In this study, we investigated a motor strategy for changing the amplitude or direction of movement during rapid extensions of the elbow joint. We focused on changes in the triphasic electromyographic (EMG) pattern, i.e. the first agonist burst (AG1), the second agonist burst (AG2) and the antagonist burst (ANT), in relation to changes in the amplitude and direction of movement required after the initiation of movement. Ten seated subjects extended the elbow 40 degrees in the Basic task and 80 degrees in the Wide task, and then returned to the starting position after reaching for a target in the Basic task (Return task). The tasks were performed under two conditions: while performing a predetermined task (SF condition), and while performing a task in response to a visual stimulus after movement commencement (ST-BW condition—the subject performed either the Basic task or the Wide task; ST-BR condition—the subject performed either the Basic task or the Return task). Kinematic and temporal parameters, and EMG activity from the agonist (triceps brachii) and the antagonist (biceps brachii) muscles were recorded. Comparing the Wide task with the Basic task, the onset latency of AG2 increased, and the duration in all bursts increased. In the ST-BW condition, the onset latency of AG1 and AG2 and the duration of AG1 increased compared with the SF condition. Comparing the Return task with the Basic task, the onset latency of ANT and AG2 increased. The duration of all bursts and the average EMG (a-EMG) of ANT in the Return task increased compared with the Basic task. There was no difference between the SF and ST-BR conditions. It is concluded that the motor strategy for changing the amplitude of initiated movement is to control the duration of AG1 depending on the amplitude of movement in order to change the timing to stop movement, and that the motor strategy for changing the direction of initiated movement is to increase the duration and amplitude of ANT in order to stop at the target and flex continuously.