Hand trajectories of vertical arm movements in one-G and zero-G environments. Evidence for a central representation of gravitational force.

The purpose of the present experiment was to study the way in which the central nervous system (CNS), represents gravitational force during vertical arm pointing movements. Movements in upward (against gravity) and downward (with gravity) directions, with two different mass loads (hand empty and with a hand-held 0.5-kg weight) were executed by eight subjects in a normal gravitational environment. Movements by two cosmonauts, in the two directions, were also tested in a state of weightlessness. Analyses focused upon finger trajectories in the sagittal plane. Subjects in a normal gravitational environment showed curved paths for both directions and weight conditions. In addition, downward movements showed significantly smaller curvatures than upward movements. Movement times were approximately the same for all the experimental conditions. Curvature differences between upward and downward movements persisted during space flight and immediately postflight. Movement times from both cosmonauts increased slightly during flight, but returned to normal immediately on reentry in a one-G environment. Results from the present study provide evidence that gravity is centrally represented in an anticipatory fashion as a driving force during vertical arm movement planning.