Premovement posture and focal movement velocity affects on postural responses accompanying rapid arm movement

Coordination of intentional upper limb movement concurrent with supporting postural activity was investigated in adult males under varying task conditions. Seven subjects performed a 60 deg rapid elbow flexion (focal movement) to a target in movement times of 170, 195, or 220 ms while standing. Measurement of center of pressure via a force platform revealed that subjects adopted individual premovement postural preferences such that locus of center of pressure resided in one predominant quadrant of the foot. Each premovement postural preference was accompanied by one most common postural muscle onset sequence as indicated by bilateral EMG analysis of rectus femoris and biceps femoris. In addition, onset times for postural muscles exhibiting anticipatory postural activity occurred earlier relative to biceps branchii as focal movement velocity increased. The finding that each premovement postural condition was accompanied by one particular postural muscle onset sequence suggested that postural synergies were flexibly organized with respect to onset sequence.