RECIPROCAL ACTIVATION AND COACTIVATION IN ANTAGONISTIC MUSCLES DURINGRAPID GOAL-DIRECTED MOVEMENTS

Seven normal subjects performed elbow extensions as rapidly as possibl e from an initial position to a visually defined target at 36 degrees in amplitude. In electromyograms, the reciprocal activation of the ago nist and then antagonist bursts was always followed by simultaneous ac tivation of the antagonistic muscles, i.e., coactivation. Instructions added to perform extensions ''as rapidly as possible'' changed coacti vation; the command to ''strongly fix the upper arm at the target'' in creased coactivation, whereas ''relax immediately after the start of m ovement'' made coactivation almost disappear. However, basic features of reciprocal activation remained the same. Other instructions given a lso changed coactivation on initiation and termination, while reciproc al activation was relatively unaltered. When subjects were encouraged to ''relax immediately after the start of movement, but fix the upper arm quickly after attaining the target,'' coactivation initiated short ly after reaching the target (< 200 ms). Following the instruction to ''relax the upper arm quickly after attaining the target,'' coactivati on terminated rapidly after reaching the target (< 280 ms). The result s show that instructions serve to change amplitude and timing of coact ivation while keeping reciprocal activation relatively unaltered, sugg esting that coactivation is controlled independently of reciprocal act ivation during rapid goal-directed movements.