Postural abnormalities to multidirectional stance perturbations in Parkinson's disease.

OBJECTIVE We investigated trunk control, protective arm movements, and electromyographic responses to multidirectional support-surface rotations in patients with Parkinson's disease (PD), aiming to better understand the pathophysiology underlying postural instability in PD, on and off antiparkinson medication. METHODS Ten patients with PD were compared with 11 age matched healthy controls. Seven patients were also tested without (OFF) antiparkinson medication. All subjects received rotational perturbations (7.5 deg amplitude) that were randomly delivered in six different directions. RESULTS The PD patients had decreased trunk rotation and ankle torque changes, consistent with a stiffening response. Stiffness appeared to be caused by the combined action of three factors: co-contraction that interfered in particular with the normal response asymmetry in trunk muscles; increased response amplitudes in agonist and antagonist muscles at both medium (approximately 80 ms) and balance correcting (approximately 120 ms) response latencies; and increased background activity in lower leg, hip, and trunk muscles. Although the patients had significantly earlier onset of deltoid muscle responses, this gave no functional protection because the arm movements were abnormally directed. Most instability in PD occurred for backward falls, with or without a roll component. Medication provided partial improvement in arm responses and trunk roll instability. CONCLUSIONS Our results confirm previous findings in ankle muscles, and provide new information on balance impairments in hip, trunk, and arm responses in PD.