Maintaining balance against force perturbations: impaired mechanisms unresponsive to levodopa in Parkinson's disease

We introduce a new method to investigate postural instability in Parkinson's disease (PD) using computer-controlled motors to deliver precise pulls to the shoulders of subjects while standing. It mimics the clinical pull test but uses forces with unpredictable timing, direction, and magnitude. It revealed a number of balance control deficits in PD. Notably, the identified deficits were not significantly altered by levodopa medication, suggesting that disruption to nondopaminergic systems contributes to postural instability in PD.