Disturbances in human position sense during alterations in gravity: a parabolic flight experiment.

Under conditions of weightlessness human position sense appears to deteriorate. This was tested, employing three methods of measurement: two-arm matching, one-arm pointing and one-arm repositioning, carried out during parabolic flight. In hypergravity (1.8G), position sense errors in a matching task increased significantly from the value during horizontal flight (1G) of + 2.5° (± 3.8° SD), to + 3.5° (± 3.3°). For pointing, errors increased significantly from + 9.1° (± 4.4°) to + 11.2° (± 4.4°). In microgravity (0G), matching errors fell significantly to + 0.35° (± 3.5°), while in pointing the fall was not significant. For repositioning, there were no significant changes in errors in either hypergravity or microgravity. It is proposed that the errors in matching and pointing are a consequence of the force of gravity acting at the elbow joint to alter the position signal coming from muscle and joint receptors. For repositioning, memory of the test angle was stored centrally, to be reproduced independently of any changes in gravity.