*4.1. Participant Recruitment*

Two cohorts of subjects, consisting of fourteen PD patients and twelve Healthy Controls (HC) respectively were recruited. The PD patients were assessed for the LA, AC, Gait, PSretrop and Po tasks (UPDRS tasks 3.8, 3.9, 3.10, 3.12 and 3.13, respectively) by two neurologists (N1, N2) expert in movement disorders. The postural stability score (PSPIGD) was assessed by the PIGD subscale score obtained from the AC, Gait, PSretrop and Po UPDRS tasks [47]. Motor impairment is sensitive to the time passed after the last drug intake; therefore, the OFF state (practically defined as that after 12 h without medication) was chosen as the reference for disease severity scoring. PD patients were excluded if they had previous neurosurgical procedures, tremor severity > 1 or cognitive impairment (Mini–Mental State Examination Score < 27/30). PD patients met the following criteria: Hoehn and Yahr average score 2.1 (min 1, max 3); age range 53–80 years (mean 69, std. dev. 7.5). disease duration range 3–10 years (mean 5.8, std. dev. 2.5), gender 8 men and 6 women. The HC subjects performed the same tasks, in the same environmental conditions and with the same system setup of PD patients. The HC cohort was selected trying to approximately match the PD cohort in age and gender, excluding subjects affected by neurological, motor and cognitive disorders.

Informed consent was obtained in accordance with the Declaration of Helsinki (2008). The study's protocol was approved by the Ethics Committee of the Istituto Auxologico Italiano (Protocol n. 2011\_09\_27\_05).
