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Control of arm movements in Friedreich's ataxia patients: role of sensory feedback

While major clinical features of movement disorders in FA patients have been identified, the underlying impaired neural control is not sufficiently understood. To elucidate the underlying control mechanism, the authors investigated single-joint movements of the upper limb in FA patients. Small, tolerable force perturbations were induced during voluntary single-joint arm movements to examine the compensatory reaction of the FA patient's motor system. Movement kinematics were measured, and muscle torques were quantified. As in healthy subjects, unperturbed single-joint movements in FA patients preserved similar temporal profiles of hand velocity and muscle torques, however, scaled in duration and amplitude. In addition, the small perturbations were compensated for efficiently in both groups, with the endpoint error < 0.5° (maximum displacement of 5-15°). The differences in movement time, torque response, and displacement between patients and controls were further quantified. To distinguish whether these differences were caused by a malfunction of top-down control or a malfunction of feedback control, the responses were fitted with a detailed model of the stretch reflex. The model simulations revealed that the feedback delay, but not the feedback gain was affected in FA patients. They also showed that the descending control signal was scaled in time and amplitude and co-contraction was smaller in FA patients. Thus, this study explains how the motor deficits of FA patients result from pathological alterations of both top-down and feedback control.

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