Human limb position sense measured by repositioning during changes of gravity in parabolic flight

Our sense of limb position is believed to be signalled by muscle spindles. To confirm this, we previously sought evidence for spindles contributing to each of three different methods of measuring position sense. These included two-arm matching, one arm pointing and repositioning. The method of repositioning showed little evidence of spindle involvement. This study aims to seek further supporting information for the absence of a spindle contribution to repositioning. With this method, the passive limb is moved to a chosen joint angle and the blindfolded participant is asked to remember the angle and, subsequently, to reproduce it. It is known that responses of spindles are disturbed by changes in gravity. The present experiments were carried out during rises in gravity (hypergravity) and during falls (microgravity), in parabolic flight, to determine whether this disturbed position sense by repositioning. In order to maximise effects, the remembering stage, was carried out in one gravity state and repositioning in a different state. It was found that repositioning accuracy was unaffected by changes in gravity. These results suggest that the method of repositioning, used for measuring position sense, relies on a central memory of previous positions, rather than on peripheral input from muscle spindles.