Transition from slow to ballistic movement: development of triphasic electromyogram patterns

Summary

The purpose of this investigation was to determine how the triphasic electromyogram (EMG) pattern of muscle activation developed from the agonist muscle only pattern as movement time (t _mov) decreased. Six adult women produced a series of 30° elbow extension movements in the horizontal plane at speeds ranging from ballistic (< 400-ms t _mov) to very slow (> 800-ms t _mov). Surface EMG from triceps brachii (agonist) and biceps brachii (antagonist) muscles were recorded, together with elbow angle, on a microcomputer. The results showed that triphasic EMG patterns developed systematically as t _mov decreased from 1000 ms to < 200 ms. In trials with very long t _mov, many elbow extension movements were produced by a single continuous activation of the agonist triceps brachii muscle. As t _mov decreased however, agonist activation became predominantly burst-like and other components of the triphasic EMG pattern [activation of the antagonist (Ant) and second agonist activation (Ag₂)] began to appear. At the fastest movement speeds, triphasic EMG patterns (Ag₁-Ant-Ag₂, Ag₁ being first activation of agonist muscle) were always present. This data indicated that the triphasic pattern of muscle activation was not switched on when a particular t _mov was achieved. Rather, each component systematically developed until all were present, as distinctive bursts of activity, in most trials with t _mov less than 400 ms.