Anticipatory Control of Voluntary Action: Merging the Ideas of Equilibrium-point Control and Synergic Control

This chapter presents an overview of two major developments in the field of motor control: the referent configuration hypothesis (equilibrium-point hypothesis) and the theory of synergies (using the computational apparatus of the uncontrolled manifold hypothesis). It is suggested that a control hierarchy based on referent configurations at the whole-body, joint, and muscle levels may use synergic mechanisms to stabilize shifts of the task-related referent configuration expressed in salient for the motor task variables. On the other hand, control with referent configurations can itself lead to synergic relations within an ensemble of elemental performance variables that stabilize important performance variables produced by the ensemble as a whole. Relations between the notions of referent configuration and synergy and the control of vertical posture are discussed. Changes in synergy indices are described prior to a planned quick action or a reaction to a self-triggered perturbation. These phenomena, termed anticipatory synergy adjustments, share many common features with the well-known phenomena of anticipatory postural adjustments. A hypothesis is offered that anticipatory postural adjustments represent a superposition of two processes: anticipatory synergy adjustments with a minimal net mechanical effect and the generation of net forces and moments of force that are expected to counteract the anticipated perturbation.