Age- and speed-related differences in harmonic ratios during walking.

Harmonic ratios (HRs), derived from trunk accelerations, measure smoothness of trunk motion during gait; higher ratios indicate greater smoothness. Previous research indicates that young adults optimize HRs at preferred pace, exhibiting reduced HRs at speeds faster and slower than preferred. Recent studies examining HRs and other trunk acceleration measures challenge this finding. The purpose of this study was to examine age-related differences in HRs across a range of self-selected overground walking speeds. Anteroposterior (AP), vertical (VT), and mediolateral (ML) HRs were examined in 13 young adults (ages 20-23), 13 healthy older adults (ages 60-69), and 13 healthy old-old adults (ages 80-86) while walking overground at very slow, slow, preferred, fast, and very fast speeds. Young and older adults exhibited similar HRs in all directions of motion across speeds, while old-old adults exhibited lower AP- and VT-HRs. All groups exhibited reduced HRs at speeds slower than preferred. However, there were no differences in HRs between preferred and faster speeds, with the exception of reduced VT-HRs in the very fast condition for the older groups. The ML-HR was not different between groups, and varied less across speeds. Stride time variability exhibited inverse relations with, and independently contributed to, HRs across speeds; lower stride time variability was associated with greater smoothness of trunk motion. Older groups were not disproportionately affected by walking more slowly and smoothness of trunk motion did not show a clear pattern of optimization at preferred pace for any group.

Planning and control of sequential rapid aiming in adults with Parkinson's disease.

Eight people with Parkinson's disease (PD), 8 age-matched older adults, and 8 young adults executed 3-dimensional rapid aiming movements to 1, 3, 5, and 7 targets. Reaction time, flight time, and time after peak velocity to the 1st target indicated that both neurologically healthy groups implemented a plan on the basis of anticipation of upcoming targets, whereas the PD group did not. One suggested reason for the PD group's deficiency in anticipatory control is the greater variability in their initial force impulse. Although the PD group scaled peak velocity and time to peak velocity similarly to the other groups, their coefficients of variation were greater, making consistent prediction of the movement outcome difficult and thus making it less advantageous to plan too far in advance. A 2nd finding was that the PD group exhibited increased slowing in time after peak velocity in the final segments of the longest sequence, whereas the other 2 groups did not. The increased slowing could be the result of a different movement strategy, increased difficulty modulating the agonist and antagonist muscle groups later in the sequence, or both. The authors conclude that people with PD use more segmented planning and control strategies than do neurologically healthy older and young adults when executing movement sequences and that the locus of increased bradykinesia in longer sequences is in the deceleration phase of movement.

Control of aperture closure during reach-to-grasp movements in Parkinson's disease.

This study examined whether the pattern of coordination between arm-reaching toward an object (hand transport) and the initiation of aperture closure for grasping is different between PD patients and healthy individuals, and whether that pattern is affected by the necessity to quickly adjust the reach-to-grasp movement in response to an unexpected shift of target location. Subjects reached for and grasped a vertical dowel, the location of which was indicated by illuminating one of the three dowels placed on a horizontal plane. In control conditions, target location was fixed during the trial. In perturbation conditions, target location was shifted instantaneously by switching the illumination to a different dowel during the reach. The hand distance from the target at which the subject initiated aperture closure (aperture closure distance) was similar for both the control and perturbation conditions within each group of subjects. However, that distance was significantly closer to the target in the PD group than in the control group. The timing of aperture closure initiation varied considerably across the trials in both groups of subjects. In contrast, aperture closure distance was relatively invariant, suggesting that aperture closure initiation was determined by spatial parameters of arm kinematics rather than temporal parameters. The linear regression analysis of aperture closure distance showed that the distance was highly predictable based on the following three parameters: the amplitude of maximum grip aperture, hand velocity, and hand acceleration. This result implies that a control law, the arguments of which include the above parameters, governs the initiation of aperture closure. Further analysis revealed that the control law was very similar between the subject groups under each condition as well as between the control and perturbation conditions for each group. Consequently, the shorter aperture closure distance observed in PD patients apparently is a result of the hypometria of their grip aperture and bradykinesia of hand transport movement, rather than a consequence of a deficit in transport-grasp coordination. It is also concluded that the perturbation of target location does not disrupt the transport-grasp coordination in either healthy individuals or PD patients.

Motor learning processes in a movement-scaling task in olivopontocerebellar atrophy and Parkinson's disease.

Nine Parkinson's disease (PD), seven olivopontocerebellar atrophy (OPCA) patients and two age-matched control groups learned a linear arm movement-scaling task over 2 days, requiring movements proportional in length to visually presented target-bars. Scaling was acquired through knowledge of results (KR concerning the direction and magnitude of errors) following every second acquisition trial. Initial acquisition of both groups was significantly worse than their respective controls (poorer movement scaling), but rapidly improved to nearly identical levels. Retention for the PD group's movement scaling was as good as controls initially, but markedly poorer after 24 h. The OPCA group did not show this deficit. Both patient groups extrapolated accurately to longer, previously unpracticed target distances (no KR provided), suggesting an unimpaired capacity to generate and use an internal representation of the movement scaling. They also rapidly learned a new scaling relationship when the gain was changed. Overall, the learning of this movement-scaling task was not adversely affected in OPCA, and the impairment was restricted primarily to longer-term retention in PD. The study suggests that: (1) the ability to acquire movement scaling in a task that requires conscious use of error feedback and no new coordination may depend little on the cerebellum, and (2) the basal ganglia may participate in longer-term storage of scaling information.

Peripheral constraint versus on-line programming in rapid aimed sequential movements.

The purpose of this investigation was to examine how the programming and control of a rapid aiming sequence shifts with increased complexity. One objective was to determine if a preprogramming/peripheral constraint explanation is adequate to characterize control of an increasingly complex rapid aiming sequence, and if not, at what point on-line programming better accounts for the data. A second objective was to examine when on-line programming occurs. Three experiments were conducted in which complexity was manipulated by increasing the number of targets from 1 to 11. Initiation- and execution-timing patterns, probe reaction time (RT), and movement kinematics were measured. Results supported the peripheral constraint/pre-programming explanation for sequences up to 7 targets if they were executed in a blocked fashion. For sequences executed in a random fashion (one length followed by a different length), preprogramming did not increase with complexity, and on-line programming occurred without time cost. Across all sequences there was evidence that the later targets created a peripheral constraint on movements to previous targets. We suggest that programming is influenced by two factors: the overall spatial trajectory, which is consistent with Sidaway's subtended angle (SA) hypothesis (1991), and average velocity, with the latter established based on the number of targets in the sequence. As the number of targets increases, average velocity decreases, which controls variability of error in the extent of each movement segment. Overall the data support a continuous model of processing, one in which programming and execution co-occur, and can do so without time cost.

Distribution of programming in a rapid aimed sequential movement.

Studies indicate that rapid sequential movements are preprogrammed and that preprogramming increases with complexity, but more complex sequences that require on-line programming have been seldom been studied. The purpose of this investigation was to determine whether on-line programming occurs in a 7-target sequence in which there is a unique target constraint and if so, to determine how different task constraints affect the distribution of additional programming. Subjects contacted seven targets with a hand-held stylus as quickly as possible while maintaining a 90% hit rate. Initiation-band execution-timing patterns and movement kinematics were measured to determine when the additional programming took place. Results indicated that additional programming occurred before initiation and during movement to the first target when the constraint required more spatial accuracy (small target). A different type of unique target (a triple hit of one target) caused the additional programming to occur on-line one or two segments before its execution. Different positions of the unique target also affected timing patterns. Results were discussed in terms of: (1) capacity of processing; (2) control of movement variance; and (3) mean velocity as a programmed parameter in sequential aiming movements.