Human whole-body reaching in normal gravity and microgravity reveals a strong temporal coordination between postural and focal task components.

Previous experiments by our group in normal gravity (1 G) have revealed spatial relationships between postural and focal components of whole-body reaching and pointing movements. We suggested that these relationships could be explained partly through the use of gravity to displace the CoM and attain the object or target position. In this study we compared human whole-body reaching in 1 G and microgravity (0 G) in order to more fully investigate how gravity contributes to strategies adopted for task execution and to determine possible invariant temporal relationships between multiple segments. Whole-body reaching movements made from the standing position in two experimental conditions of execution speed (naturally paced and as fast as possible) were recorded during periods of 1 G and 0 G in parabolic flight. Overall, at each speed of reaching, movement times were significantly slower when performed in 0 G than in 1 G for two of the three subjects, but all subjects were able to produce significantly faster movements in 0 G than in 1 G. Despite similar general trends across subjects observed in 1 G, angular displacements of reaching movements performed in 0 G differed greatly between subjects. There were changes at all joints, but above all at the shoulder and the ankle. However, despite a high intersubject and intratrial variability in 0 G, in both gravity conditions all subjects demonstrated times to peak curvilinear velocity for the finger (end effector) and the whole-body centre of mass (CoM) that coincided, regardless of the speed of execution. Moreover, cross-correlations between multiple segment curvilinear velocities and those of the CoM revealed tight, highly correlated temporal relationships between segments proximal to the CoM (which was expected). However, for more distal segments, the correlations were weaker, and the movements lagged behind movements of the CoM. The major and most interesting finding of this study was that although the finger was the most distal within the segment chain, with respect to the CoM, it was highly correlated with the CoM (0.99--0.98, all conditions) and with no time lag. Despite the large intersubject and inter-environmental variability recorded in this study, temporal relationships between postural task components (CoM displacements) and those of the focal movement (end-effector trajectory) were consistently conserved.