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Anticipatory Postural Adjustments (APAs) are commonly described as unconscious muscular activities aimed to counterbalance the perturbation caused by the primary movement, so as to ensure the whole-body balance, as well as contributing to initiate the displacement of the body center of mass when starting gait or whole-body reaching movements. These activities usually create one or more fixation chains which spread over several muscles of different limbs, and may be thus called  inter-limb APAs . However, it has been reported that APAs also precede voluntary movements involving tiny masses, like a flexion/extension of the wrist or even a brisk flexion of the index-finger. In particular, such movements are preceded by an  intra-limb  APA chain, that involves muscles acting on the proximal joints. Considering the small mass of the moving segments, it is unlikely that the ensuing perturbation could threaten the whole-body balance, so that it is interesting to enquire the physiological role of  intra-limb  APAs and their organization and control compared to  inter-limb  APAs. This review is focused on  intra-limb  APAs and highlights a strict correspondence in their behavior and temporal/spatial organization with respect to  inter-limb  APAs. Hence it is suggested that both are manifestations of the same phenomenon. Particular emphasis is given to  intra-limb  APAs preceding index-finger flexion, because their relatively simple biomechanics and the fact that muscular actions were limited to a single arm allowed peculiar investigations, leading to important conclusions. Indeed, such paradigm provided evidence that by granting a proper fixation of those body segments proximal to the moving one APAs are involved in refining movement precision, and also that APAs and prime mover activation are driven by a shared motor command.

The Organization and Control of Intra-Limb Anticipatory Postural Adjustments and Their Role in Movement Performance