Disruption of intra-limb anticipatory postural adjustments in parkinsonian patients

Voluntary movements induce postural perturbations which are counteracted by anticipatory postural adjustments (APAs). These actions build up long fixation chains toward available support points, to grant whole body equilibrium, but also develop short chains within the same limb where a distal segment is moved (intra-limb APAs), to stabilize the proximal segments. The neural structures generating intra-limb APAs still need investigations; the present study aims to address the role of the basal ganglia. APAs that stabilize the arm when the index-finger is briskly flexed were recorded in 13 parkinsonian (PS) and in 10 age matched healthy subjects (HS). In agreement with the literature, in HS the activation of prime mover Flexor Digitorum Superficialis (FDS) was preceded by an inhibitory APA in Biceps Brachii (BB) and Anterior Deltoid (AD), and almost simultaneous to an excitatory APA in Triceps Brachii (TB). In contrast, only 4 PS showed a similar pattern, while other 4 did not show APAs in BB and 5 actually developed a BB excitation. The frequency of normal sign, inverted and lacking BB APAs was different in HS vs. PS (p=0.0016). No significant differences were found for TB and AD APA timings, however index-finger movement was smaller in PS (p=0.01) and more delayed with respect to FDS onset (p<0.0001). These findings support a role of basal ganglia in shaping the motor program not only for what concerns the prime mover, but also for the anticipatory postural component.