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.

The coordination between the trunk and arm of six subjects was examined during unrestrained pointing movements to five target locations. Two targets were within arm's length, three were beyond. The trunk participated in reaching primarily when the target could not be attained by arm and scapular motion. When the trunk did contribute to hand transport, its motion started simultaneously with arm movement and continued until target contact. Redundancy in the degrees of freedom used to execute the movement had no effect on the configuration of joints and segments used to attain a specified target; no difference in variability was noted regardless of whether redundancy existed. However, different configurations were used to achieve the same wrist coordinates along a common endpoint path, depending on the final position of the hand. The addition of trunk flexion, rotation and scapular motion did not alter the coupling between the elbow and shoulder joints and had no effect on the path of the hand or the smoothness of its velocity profile. Thus, trunk motion was integrated smoothly into the transport phase of the hand. As the trunk's contribution to hand transport increased, it played a progressively greater role in positioning the hand close to the target during the terminal stage of the reach. Of the movement components measured, trunk flexion was the last component to complete its motion when target reaches were made beyond arm's length. Hence, the trunk not only acts as a postural stabilizer during reaching, but becomes an integral component in positioning the hand close to the target.

Training that restricted compensatory truncal motion during TRT improved the precision of reaching more than during RE. Truncal restraint during rehabilitation of reaching may be an effective therapeutic strategy in patients with moderately severe hemiparetic stroke, especially when combined with TRT.

Walking while carrying a hand-held object requires the generation of appropriate grip forces to offset the inertial forces produced during locomotion. The present study examined the interaction between grip forces and locomotion-induced inertial forces across the gait cycle. Eight subjects transported a container under three conditions: self-paced transport with and without accuracy constraints and a velocity-constrained condition. The results showed that the trunk and transported container moved in a synchronized, sinusoidal pattern during all conditions. Grip and inertial forces of the transporting hand were highly coupled in an anticipatory fashion, regardless of task demands. The inertial forces were higher and the coupling was greater in the faster, unconstrained condition. However, grip force modulation was observed even when the inertial forces acting on the container were small and applied indirectly to the container through the locomotor effects originating in the legs and trunk. We suggest that continuous grip force adjustment is used as a generalized strategy to maximize efficiency during object transport regardless of the size or origin of the inertial forces.

No abstract