The aim of this investigation was to determine how the CNS controlled seven segments of the human deltoid muscle during a change in the direction of shoulder joint motion. Specifically, we wished to determine how the prime mover, synergist and antagonist muscle segments of this muscle were manipulated to assume new functional roles as the direction of shoulder motion was rapidly changed from shoulder abduction to shoulder adduction. Seven bipolar surface electrodes (7 mm inter-electrode distance) were placed over the seven segments (D1'D7) of the right deltoid, in seven young (19'24yrs) male subjects, to detect changes in muscle segment activation as the subjects transitioned from a rapid shoulder-abduction to a rapid-adduction force impulse (MT = 1000 ms). For each subject, fifteen trials were recorded at an inter-trial interval of 30 seconds. Comparisons of muscle segment timing and intensity of activation were made across 6 equal time intervals between just before the peak of the abduction force impulse and the subsequent peak of the adduction force impulse. The results of this study have shown that segments of the deltoid were activated during both the shoulder abduction and shoulder adduction motor task. In addition, the pattern of muscle segment activation (timing and intensity), during the transition from shoulder abduction to shoulder adduction, was dependent upon the muscle's moment arm and line of pull in relation to the axis of shoulder joint rotation. Three distinct patterns of neuromotor activation were noted within the segments of the deltoid muscle. During abduction the agonist prime mover and synergist segments (D1'D5) were totally deactivated (< 10% MVC) as they became antagonist segments during adduction. The antagonist segment (D7), during abduction, was deactivated and then reactivated as it became a synergist segment during adduction. Finally segment D6 was shown to have a nearly continuous period of activation. The study has shownthat during a transition to a new movement direction, a muscle segment's line of pull and future function in the next phase of the movement appears to determine its period and intensity of activation.