Effects of voluntary rapid head rotation movement in horizontal plane on EMG activities of tonically contracting elbow extensor and flexor muscles were studied in five healthy subjects. Arm EMGs were averaged in relation to the EMG onset of the prime mover. M. sterno-cleidomastoideus (SCM). EMG variations could occur before SCM-EMG onset. Facilitatory and inhibitory effects were observed. They were not necessarily organized in reciprocal fashion but varied in every subject. We interpret the present findings as resulted from an incorporated part of the central motor program for head rotation which was specific to each subject.