[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 occurred 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.
Bleuse
Séverine
S
Service de neurophysiologie clinique, hôpital Roger-Salengro, CHU Lille, 59037 Lille, France.
Cassim
François
F
Blatt
Jean-Louis
JL
Defebvre
Luc
L
Guieu
Jean-Daniel
JD
D000222:Adaptation, Physiological / Q000502:physiology*; D000328:Adult; D001132:Arm / Q000502:physiology*; D001696:Biomechanical Phenomena; D005260:Female; D006801:Humans; D008297:Male; D009119:Muscle Contraction; D011187:Posture / Q000502:physiology*; D012016:Reference Values; D013997:Time Factors; D019415:Torque; D014836:Volition