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North American Congress on Biomechanics Canadian Society for Biomechanics - American Society of Biomechanics University of Waterloo Waterloo, Ontario, Canada August 14-18, 1998 |
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It has been hypothesized that the basal ganglia and their cortical loops are involved in the control of complex movements, as evidenced by impairments that Parkinson's disease (PD) patients show in the execution of complex movements. In this experiment, we vary the level of coordination required by having subjects perform single and two joint movements to targets in an effort to elucidate the underlying mechanisms of this impairment in the execution of complex movements.
Teulings and colleagues (1997) have shown that PD patients are differentially impaired in the coordination of handwriting movements involving multiple degrees of freedom. They found that movement smoothness (measured by jerk) decreased for movements requiring motion of the fingers and the wrist as opposed to the fingers or the wrist in isolation for PD patients, but not for age-matched controls. In addition, Alberts and colleagues (1998) found that PD patients exhibit identical kinematics for the two arms reaching simultaneously to objects of different sizes, while in contrast control subjects scale the movement of each arm to the accuracy requirements of the target objects. These data suggest that PD patients compensate for impairments in the execution of complex movements by reducing the number of joints to be controlled when possible.
This potential reduced capability to control complex movements could contribute substantially to bradykinesia and increased variability in PD patients. The purpose of this investigation was to compare the control of single and two joint movements for PD patients and control subjects to determine whether the patients show the same coordinated relative timing at each joint.
Seven PD patients and 5 age matched controls served as subjects in this experiment. Subjects were seated at a table with the chair height adjusted such that when the arm rested on the table, both the upper and the lower arm were parallel with the table surface. The subjects pointed to four targets on the tabletop, initiating each movement from the same start position directly in front of the subject's midline, 24.5 cm from the table edge. The four targets were arranged such that Target 1 required elbow extension only, with Targets 2-4 requiring increasing amounts of shoulder flexion (cf. Sainburg et al., 1995). Target 1 was located 45° to the right of the midline, 34 cm from the start position. Subjects used approximately 45° of elbow extension to achieve the target. Target 2 was 28 cm anterior to the start position. Subjects used approximately 45° of elbow extension and 8° of shoulder flexion. Target 3 was 45° to the left of the midline, 25 cm from the start. Subjects used approximately 33° of elbow extension and 15° of shoulder flexion. Target 4 was 90° to the left of the midline, 42 cm from the start position. Subjects used an approximately equal amount of elbow extension and shoulder flexion (25° for both). Infrared light emitting diodes were placed on the trunk, shoulder, elbow, and index fingernail of the right arm to record movements. Subjects wore a wrist brace to eliminate wrist contribution to the task. Each subject performed 20 trials to each target, presented in 2 blocks of 10 trials each. The sequence of target presentation was counterbalanced across subjects. Joint angular excursions and velocities were determined. In addition, jerk scores were computed for the endpoint (fingertip) trajectory and normalized for movement distance and duration as follows:
Figure 1 displays the mean jerk scores for both subject groups across the four target conditions. The PD patients exhibited higher jerk scores than their age-matched controls for Target 4 (significant contrast, p<.05).
Figure 1. Mean jerk scores for each group are plotted for each target condition.
Sample velocity phase plots of movements to Target 4 for one representative PD patient and one control subject are shown in Figure 2. The plots reveal that the control subjects accelerated and decelerated both joints simultaneously. The PD patients also accelerated both joints simultaneously, but decelerated them sequentially. The patients first decelerated the elbow joint and then the shoulder joint. The mean magnitude of temporal asymmetry between elbow and shoulder peak deceleration was 47 (15) ms for controls on Target 2 and 111 (41) ms for the PD patients. Values for Target 3 were 47 (9) ms for controls and 88 (29) ms for PD patients while for Target 4 the values were 41 (11) ms for controls and 108 (32) ms for PD patients.
Figure 2. Velocity phase plots for two representative subjects moving to Target 4.
PD patients have been shown to be impaired in force control, especially in conditions with precise spatial targets (Rand et al., 1998). Decelerating each joint separately may simplify the control of multijoint movements, assisting PD patients in the achievement of targets. It does, however, decrease the fluency of the movement, resulting in higher jerk scores.
Alberts JL et al. Brain, in press.
Bastian et al. J Neurophys, 76, 492-509, 1996.
Rand MK et al. In prep.
Sainburg et al. J Neurophys, 73, 820-835, 1995.
Teulings et al. Exp Neurol, 146, 159-170, 1997.
Supported by NIA AG 14676 awarded to G. E. Stelmach.
