ANTICIPATORY POSTURAL ADJUSTMENTS: FROM POSTURE TO MOVEMENT

In all the experiments we investigated the relationship between the voluntary movement and the postural control. We particularly focused our attention on the role played by the APAs, since the general hypothesis is that the postural feed forward control of the movement is strictly bound to the voluntary movement itself in a complex and flexible way. In the first experiment we tested the hypothesis that APAs preceding an upper-limb target reaching movement could play a role also in controlling the movement accuracy. The aim of this study was seeking a direct proof of the relationship between the APAs amplitude and the endpoint of a target reaching movement. The aim of the second study was to determine whether a short term immobilization (12 h) interferes in parallel with both the activation of the prime mover muscle, responsible for a given movement, and the postural muscles that are recruited to stabilize the limb. In the third experiment was aimed at verifying whether the postural activation is affected by the phenomenon of the motor resonance as well as already described for the prime mover activation. The results of the first experiment reinforce the hypothesis that a successful on-target pointing movement relies upon a specific tuning between APAs and prime mover activation, as that obtained at the end of the adaptation phase. The most important result of the second experiment is that, although the prime mover activation remains unchanged after the immobilization, the trajectory described by the index finger is most likely changed between the two sessions due to the modification in the postural control that led to a less effective stabilization of the proximal joint, as was suggested by the mechanical model designed by Caronni and Cavallari (2009a). In the last experiment we demonstrated that the resonant response in resting subjects replicates, under threshold, both the primary movement and postural activity. The precocious increase in excitability observed in BB may be the expression of the anticipatory activation observed during the execution of the movement. Given that MR reflects aspects that are intrinsic to motor programming also this result strongly support the idea that primary movement and the postural command are essential components of the same neural process.