EMG analysis of human postural responses during parabolic flight microgravity episodes.

Anticipatory postural activity in the trunk and legs precedes rapid shoulder flexion in unit gravity. We tested the hypothesis that anticipatory activity is a component of a single neural command for arm movement by monitoring the surface electromyographic activity of the biceps femoris, paraspinals and deltoid muscles of three subjects during the microgravity phase of parabolic flight. If part of a single command, anticipatory postural activity would be expected to remain intact despite the absence of the body's center of gravity in a reduced gravity environment. However, in at least 75% of the microgravity trials anticipatory biceps femoris activity was absent, indicating a separation of postural and agonist muscle activity. Such a finding suggests that anticipatory postural biceps femoris activity may be initiated independently of agonist (deltoid) activity.