The reaching movements of patients with Parkinson's disease under self-determined maximal speed and visually cued conditions.

Two-dimensional kinematic analysis was performed of the reaching movements that six subjects with Parkinson's disease and six healthy subjects produced under self-determined maximal speed and visually cued conditions. Subjects were required to reach as fast as possible to grasp a ball (i) that was fixed stationary in the centre of a designated contact zone on an inclined ramp (self-determined maximal speed condition), or (ii) that rolled rapidly from left to right down the incline and into the contact zone (visually cued condition). Parkinson's disease subjects displayed bradykinesia when performing maximal speed reaches to the stationary ball, but not when they reached for the moving ball. In response to the external driving stimulus of the moving ball, Parkinson's disease subjects showed the ability to exceed their self-determined maximal speed of reaching and still maintain a movement accuracy that was comparable to that of healthy subjects. Thus, the bradykinesia of Parkinson's disease subjects did not seem to be the result of a basic deficit in their force production capacity or to be a compensatory mechanism for poor movement accuracy. Instead, bradykinesia appeared to result from the inability of Parkinson's disease subjects to maximize their movement speed when required to internally drive their motor output. The occasional failure of Parkinson's disease subjects to successfully grasp the moving ball suggested errors of coincident anticipation and impairments in grasp performance rather than limitations in the speed or accuracy of their reaches. These results are discussed in relation to the notion that the motor circuits of the basal ganglia play an important role in the modulation of internally regulated movements.