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

There is evidence that postural instability associated with Parkinson's disease (PD) is not adequately improved by levodopa, implying involvement of non-dopaminergic pathways. However, the mechanisms contributing to postural instability have yet to be fully identified and tested for their levodopa responsiveness. Here we investigate balance processes that resist external forces to the body when standing. These include in-place responses, and the transition to protective stepping. Forward and backward shoulder pulls were delivered using two force-feedback-controlled motors and were randomised for direction, magnitude and onset. Sixteen PD patients were tested OFF and ON levodopa and 16 healthy controls were tested twice. Response behaviour was quantified from 3-D ground reaction forces and kinematic measurements of body segments and total body centre of mass (CoM) motion. In-place responses resisting the pull were significantly smaller in PD as reflected in reduced horizontal antero-posterior ground-reaction force and increased CoM displacement. Ankle, knee and hip moments contributing to this resistance were smaller in PD, with the knee extensor moment to backward pulls being the most affected. The threshold force needed to evoke a step was also smaller for PD in the forward direction. Protective steps evoked by supra-threshold pulls showed deficits in PD in the backward direction, with steps being shorter and more steps being required to arrest the body. Levodopa administration had no significant effect on either in-place or protective-stepping deficits. We conclude that processes employed to maintain balance in the face of external forces show impairment in PD consistent with disruption to non-dopaminergic systems.

from Physiology via xlomafota13 on Inoreader http://ift.tt/1SKyPUU
via IFTTT