The relation between posture and movement: A study of a simple synergy in a two-joint task

Human subjects performed fast, discrete elbow or wrist flexion or extension movements in a sagittal plane under the instruction to move one of the joints "as fast as possible". We hypothesize that, when an instruction requires voluntary movement in only one joint, the muscles controlling the other, postural joint will display changes in their levels of activation tightly coupled to the primary movement, i.e. a postural synergy. Joint angles and electromyographic (EMG) signals from two flexor and two extensor muscles were recorded and analyzed. Both muscle pairs demonstrated commonly observed tri-phasic EMG patterns. The elbow flexor and the wrist flexor tended to demonstrate simultaneous EMG bursts, while the elbow extensor and the wrist extensor also showed similar patterns of activation. This was confirmed by both visual analysis and also cross-correlation analysis of the EMG pairs. The timing of the antagonist burst in postural muscles frequently coincided with shortening of the muscle and thus could not be attributed to the action of the local reflexes to muscle stretch. We consider the EMG patterns in postural muscles to be primarily of a central origin. We suggest that the formation of postural synergies (postural anticipation) may be not a separate process, but a separate peripheral pattern of a single control process that may involve a number of joints and muscles.