*2.1. Participants*

We recruited six patients with idiopathic PD according to the UK Brain Bank criteria who did not suffer from any other disease, including cognitive decline (i.e., Mini-Mental State Examination score of >27), vestibular disorders, and orthopedic impairments, that could interfere with walking. An additional inclusion criterion for this preliminary study was the ability of the patient to walk continuously and without assistance for at least three minutes. Disease severity was evaluated using MDS-Unified Parkinson's Disease Rating Scale motor part (UPDRS-III), and the stage of the disease was evaluated using the Hoehn and Yahr (H&Y) scale. Using items 3.15–3.17 of UPDRS-III (hands and feet), a sum rest tremor sub-score was created for the right and left sides separately. Similarly, a sum bradykinesia-rigidity score (items 3.3–3.8) was obtained for each side.

Demographic and clinical features are listed in Table 1. The study was approved by the Ethics Committee of University of Würzburg and conformed to the declaration of Helsinki. All patients gave their written informed consent to participate.
