Anticipatory postural adjustments associated with arm flexion: interest of vertical torque

Objective: To find a biomechanical parameter able to characterize postural adjustments in different movement conditions. Methods: The arm-raising movement performed during the upright human position imposes a vertical torque (Tz) that can be measured by a force plate-form. This torque was studied in ten healthy young subjects with opto electronic system Vicon 370. The subjects stood on a force platform, performed shoulder flexion of their right arm, to grasp a handle in front of them, in five conditions : self-paced at 3 different velocities (slow, medium, maximal), triggered by an auditory signal, loaded (1 kg attached to the wrist), all at maximal velocity. In a sixth condition, the arm was passively displaced by an experimenter. Results: Tz displayed a negative phase (counter-clock wise body rotation) in all conditions. A positive phase occur-red prior to the negative one, and preceded movement onset only if the movement was voluntary. In the triggered condition, the positive phase of Tz was delayed (- 60 ms) compared to the self-paced condition at maximal velocity (- 155 ms). However Tz onset latency was modified, neither by load nor velocity. Tz amplitude increased with increasing velocity, load and in a reaction time condition. Conclusions: The vertical torque Tz, especially its positive phase gives useful informations about the latency, duration and intensity of the postural preparation related to a voluntary movement, according to the movement parameters. Tz is therefore able to characterise postural adjustments in all conditions, even with low movement velocity. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.