Inclusion Criteria:  For PD participants: Neurologist-diagnosed idiopathic PD, H&Y stages 1.0-3.0 Ability to stand independently for 1 minute and ambulate 100 yards independently Healthy (not under ongoing medical care for health problems that could impact performance on study tasks) for controls: age-matched healthy adults  Exclusion Criteria:  Idiopathic PD in Hoehn & Yahr stages >3.0 Previous surgical management of PD (pallidotomy, deep brain stimulation Central or peripheral nervous system disorders other than PD including, but not limited to multiple sclerosis, cerebral palsy, Alzheimer's diseases, or chronic fatigue syndrome Cognitive impairment as defined by the Mini-Mental Status Examination (<26 for subjects with PD and <24 for control subjects) Unable to stand independently for 1 minute and to walk independently for 100 yards Body weight >400lbs