## 3 Overview of Anticipatory Postural Adjustments  
Considering the human body as an \"articulated chain\" in which a combination of anatomical rigid structures work together, we should consider that every anatomical segment is closely linked to others. Therefore, when a segment is voluntarily moved (primary movement), a reaction force occurs which perturbates all adjacent segments. For this reason, each motor action should be associated with a specific anticipatory program which is able to counterbalance the reaction force caused by the prime mover [30; 31]. This anticipatory program consists of unconscious muscular activities, called Anticipatory Postural Adjustments (APAs), which create one or more fixation chains spreading over several muscles of the same limb or of different limbs, in order to counteract the perturbating reaction force [32]. When voluntary actions involve large masses, APAs spread over several muscles of different limbs and they are called _inter-limb APAs_. They aim to stabilize the whole-body postural equilibrium. When voluntary actions concern tiny masses, APAs develop also in the same limb in which one of the distal segments is moved. These latter activities are called _intra-limb APAs_ and they have been shown to optimize movement performance by stabilizing the limb's proximal segments [33; 34]. Therefore, basically, the aim of APAs can be summarized as the stabilization and fine-tuning of the whole-body equilibrium as well as of the local equilibrium of body segments. The majority of APAs literature has been focused on the inter-limb postural chains which precede voluntary actions such as shoulder flexion and extension or elbow flexion [35; 36; 37] and also movements involving the lower limbs, hips, and trunk [38; 39]. For example, when flexing both arms at the shoulder level, a dorsal muscle postural chain develops, comprising Erector Spinae (ES), Biceps Femoris (BF), and Soleus (SOL), in order to counterbalance the reaction force due to arms movement [40]. In fact, the perturbation induced by this primary movement may dislocate the projection on the ground of the body Center of Mass (CoM) and cause a whole-body equilibrium disturbance [30; 31; 36]. The maintenance of the body's dynamic stability needs inter-limb APAs; this is particularly evident during gait initiation since they create the propulsive forces to move the CoM forwards. Gait initiation is the transient period between quiet standing posture and steady-state walking. This is the most used functional task to investigate how the Central Nervous System (CNS) controls equilibrium during a whole-body movement involving modifications in the base of support and CoM progression [41]. During gait initiation in healthy subjects, the Centre of Pressure (CoP, the barycenter of the ground reaction forces) first moves backwards and towards the future swing foot. The onset of such CoP shift is usually defined as APA onset. The horizontal gap created between CoP and CoM produces an \"imbalance\" torque that drives the CoM forwards and towards the future stance foot (imbalance phase). Subsequently, the CoP moves laterally towards the stance foot (unloading phase), so that the body weight is supported on that side and the swing foot can execute the first step [42]. The CoP shift during APA is associated with a typical electromyographic sequence involving inhibition of the activity in the Soleus (SOL) muscles, tonically active during quiet stance, shortly followed by activation of Tibialis Anterior (TA) muscles, normally silent during quiet stance. In particular, in the stance leg, the SOL inhibition precedes TA excitation by about 100 ms [39]. In parallel, the intra-limb APAs associated with movements that involve little masses optimize the movement performance by preserving the local equilibrium of the limb [43]. Examples of intra-limb APAs are reported for elbow movements [44; 45; 46], as well as wrist flexions as documented by [PERSON] et al.