Vertical torque allows recording of anticipatory postural adjustments associated with slow, arm-raising movements

Abstract

When performed in the upright position, voluntary arm-raising movements perturb balance. The maintenance of equilibrium requires postural adjustments, some of which can be anticipatory. It is usually suggested that the role of anticipatory postural adjustments is to stabilise the whole body centre of mass. During movements performed at low velocity (i.e. with a lower inertial perturbation), anticipatory postural adjustments have not systematically been detected by classical recording methods (mainly electromyography). The aim of this study was to use vertical torque to characterise anticipatory postural adjustments in slow movement and to determine the significance of this biomechanical parameter. Twenty healthy subjects performed self-paced, right arm-raising movements at low and high velocities. Movements were recorded by an optoelectronic system enabling the synchronization of video, force plate and electromyographic data. The force platform provided information on vertical torque and centre of foot pressure anteroposterior displacement. Electromyography activity was recorded from the right anterior deltoid and the bilateral biceps femoris, tibialis anterior and soleus muscles. Rapid, voluntary, unilateral movements were associated with an early centre of pressure backward shift, anticipatory vertical torque and electromyographic activities. In slow movements, only the anticipatory changes in vertical torque were consistently observed, with the same latency as in rapid movement. The existence of vertical torque in slow movement (when stabilisation of the centre of mass is not necessary) shows that this parameter does not serve to minimise the centre of mass displacement but rather contributes to the generation of arm movement.