[Relationship between postural support and intentional movement: biomechanical approach].

The presentation is in two parts. The first, more theoretical, section reviews the basic biomechanical features underlying the movement of all bodies, both animate and inanimate, being in contact with the ground. Application of biomechanical laws shows that intentional movement can only be executed if the external environment can react to the movement of members relative to the body. Thus, it is theoretically necessary that the part of the body, including the body segment(s) that are located between the ones which are voluntarily moved and the supporting surfaces, ie, the "postural support", is involved in the motor activity. The general equilibrium conditions of a complex mechanical system such as the human body are then examined to show why movement itself disturbs balance stability and that postural reactions are, a priori, required to preserve this stability. The second sections includes an analysis of postural activities for several types of sensory-motor tasks. They are shown to be synergistically organized, and the major features of this organisation are described. The anticipatory postural adjustments (APA) which occur BEFORE the start of voluntary movement, and are thus "preprogrammed", are then examined. It is shown that they depend on the parameters of the planned movement, posture and the uncertainty about the tasks. This is followed by a discussion of the reasons why APA may be considered as a counter-perturbation, opposing, in advance the perturbation of equilibrium brought about by the voluntary movement. The presentation concludes with several hypotheses for explaining the organisation of postural activity associated with voluntary movement. The most likely appears to be a process of "parallel" command of the voluntary and postural components.