Motor coordination in weightless conditions revealed by long-term microgravity adaptation

The functional approach to studying human motor systems attempts to give a better understanding of the processes behind planning movements and their c oordinated performance by relying on weightlessness as a particularly enlig htening experimental condition. Indeed, quantitative monitoring of sensorim otor adaptation of subjects exposed to weightlessness outlines the function al role of gravity in motor and postural organization. The recent accessibi lity of the MIR Space Station has allowed for the first time experimental q uantitative kinematic analysis of long-term sensorimotor and postural adapt ation to the weightless environment though opto-electronic techniques. In t he frame of the EUROMIR'95 Mission, two protocols of voluntary posture pert urbation (erect posture, EP; forward trunk bending, FTB) were carried out d uring four months of microgravity exposure. Results show that postural stra tegies for quasistatic body orientation in weightlessness are based on the alignment of geometrical body axes (head and trunk) along external referenc es. A proper whole body positioning appears to be recovered only after mont hs of microgravity exposure. By contrast, typically terrestrial strategies of co-ordination between movement and posture are promptly restored and use d when performing motor activities in the weightless environment. This resu lt is explained under the assumption that there may be different sensorimot or integration processes for static and dynamic postural function and that the organisation of coordinated movement might rely stably on egocentric re ferences and kinematics synergies for motor control. (C) 2001 Elsevier Scie nce Ltd. All rights reserved.