Joint position sense in simulated changed-gravity environments.

The proprioceptive position sense was investigated in an elbow-angle matching task in which the right forearm was moved passively by an experimenter and the left forearm actively by the subject. The right forearm could be immersed in water or loaded with a weight, to simulate micro- and hypergravity. We found that in simulated microgravity, matching performance was more variable than in normal gravity, and the right forearm deviated systematically upwards. The latter finding was limited to near-horizontal forearm positions (i.e., where the anticipated effects of gravity are strongest), and could be "reset" by intermittent visual feedback. The observed impairments of the proprioceptive position sense may adversely affect the manual performance of astronauts. In simulated hypergravity, we found no changes of variability and no systematic deviations. This outcome confirms our previous results that weight compensation is efficient even when only static (i.e., gravitational) cues are available.