## 3 Overview of Anticipatory Postural Adjustments
in 1991 [47], who reported a pattern of muscular activity in various arm muscles about 50 to 60 ms before the movement. Moreover, [PERSON] and [PERSON] [33] described an anticipatory postural chain developing in several upper-limb muscles, that stabilizes the segmental equilibrium of the arm during index-finger tapping. With the prone hand, a brisk finger flexion was preceded by an excitatory burst in Extensor Carpi Radialis (ECR), Triceps Brachii (TB), and Superior Trapezius (ST), while Flexor Carpi Radialis (FCR), Biceps Brachii (BB), and Anterior Deltoid (AD) showed a concomitant inhibition of their tonic activity. The coupled activities of TB-BB and ST-AD counterbalanced the elbow and shoulder flexion torques produced by the reaction force that the index-finger flexion discharged on the metacarpophalangeal (MP) joint. Interestingly, when the hand posture was changed from proneto supine, the APA pattern reverted in sign in the elbow and shoulder postural muscles but not in ECR and FCR. Therefore, BB and AD showed an excitation pattern, while TB resulted inhibited. These observations demonstrate the ability of APAs to adapt to the mechanical requirements of the postural context. Despite the different classification, inter- and intra-limb APAs share similar control mechanisms. Indeed, APAs create fixation chains in several muscles to avoid the destabilization produced by the reaction forces. As mentioned above, APAs revert in sign when movement direction is reverted [33; 40] and adapt to changes in the postural context and also in movement speed [48; 49; 50]; moreover, they have an important link with the movement precision [34; 51]. Besides the kinematic aspects, inter- and intra-limb APAs also share the same neural structures. The involvement of Primary Motor Cortex (PMC) in both inter- [52] and intra-limb APAs has been studied [33]. For example, the inhibition of PMC through transcranial magnetic stimulation on the left M1 induced a delay of the APA onset in the contralateral Latiissimus Dorsi muscle while the subject abducted his left arm [53]. Even the Supplementary Motor Area (SMA) has been correlated to the modulation of APAs. In fact, during a bimanual task, patients suffering unilateral SMA lesion showed impaired inter-limb APAs in the forearm contralateral to the lesion [54]. Furthermore, a study carried out with the transcranial direct current stimulation highlighted the role of SMA in modifying intra-limb APA amplitude associated to the index-finger tapping motor task [55]. [PERSON] et al. (2005) [56] reported the involvement of Sensorimotor Areas, using functional Magnetic Resonance Imaging, while [PERSON] and [PERSON] (2004) [57] reported that the Pontomedullary Reticular Formation, a site of integration of signals from both cortical and subcortical structures, is able to mediate APAs in time and magnitude, to optimize motor control of posture and movement in the cat. Moreover, basal ganglia have been reported to contribute to APA control (Figure 1); in particular, a disruption in intra-limb APAs associated with the index finger flexion [58] and an impairment in inter-limb APAs during a bimanual load-lifting task [59] have been found in patients with Parkinson's disease. Figure 1: Role of Cerebellum and Basal Ganglia in APA programming. Cerebellum and Basal Ganglia receive information about the environmental context and body schema from the Parietal Associative Cortices, as well as about the motor goal from SMA and Premotor Cortices. In turn, Basal Ganglia define the pattern of muscular recruitment (i.e., the muscles which should be excited or inhibited), while the timing and the coordination of such actions is defined by the cerebellum. Through their Thalamic projections, the _final_ motor program reaches the Primary Motor Cortex, which routes it to both the focal and postural muscles. Note that the APA command reaches the postural muscles before the voluntary command activates the focal muscles.