Posturo-kinetic organisation during the early phase of voluntary upper limb movement. 1. Normal subjects.

The nature and organisation of anticipatory postural adjustments (APA) associated with the early phase of a voluntary upper limb movement were studied. Upper limb elevations, performed at maximal velocity, were studied according to three conditions: bilateral flexions (BF) and unilateral flexions without and with an additional inertia (respectively OUF and IUF). Activities of the anterior part of the deltoid (DA) and of main muscles of the lower limbs, pelvis, trunk and scapular girdle were recorded by surface electromyography. Miniature-accelerometers enabled the recording of the tangential acceleration of the arm at wrist level (Aw) and the antero-posterior accelerations of various body links. Systematic investigations allow a precise description of the segmental phenomena which precede the onset of the voluntary movement. Before the activation of the anterior deltoid, a sequence of EMG modifications occurred in muscles of lower limbs, pelvis and trunk. The onset of Aw was preceded by anticipatory local accelerations of all the body links. Anticipatory EMG activities and local accelerations were organised according to patterns which were specific to the forthcoming voluntary movement. By comparing anticipatory EMG activities with anticipatory local accelerations, the nature of anticipatory postural movements can be determined. They appear to counteract the disturbing effects of the forthcoming voluntary movement. Because of their reproducibility and specificity, the anticipatory postural movements can be considered as preprogrammed. Postural adjustments and voluntary movement appear to be parts of the same motor program. Anticipatory postural movements should result from muscular functional synergies selected from a pre-evaluation of the perturbative aspects of the forthcoming movement.