| The performance of standing reach requires the maintenance of postural stability and the coordination of multiple joints. Although aging is associated with declines in postural stability, the impact of workspace target heights, reaching with the dominant versus non-dominant arm, and movement context on limb-posture control is not well understood in older adults. The first study of this dissertation examined anticipatory and dynamic postural control during standing reach to different heights with the dominant and non-dominant arm. Compared to younger individuals, older adults produced larger anticipatory postural adjustments (APAs), and center of pressure (COP) trajectories were less smooth, particularly when returning to an upright posture (Chapter 2). These results suggested that older adults used an active "over-control" strategy to increase the safety margin for balance, rather than relying on later, potentially inadequate compensatory postural responses. Older adults exhibited significant increases in APA amplitude and COP trajectory smoothness when reaching with the dominant compared to the non-dominant hand, perhaps reflecting handedness. In contrast, no differences between age groups were found when examining hand trajectory curvature, indicating that planning of multi-joint, standing reach movements was not affected by age (Chapter 3). Hand trajectories were more curved during reaching to low compared to higher targets regardless of age, suggesting that the biomechanical demands associated with controlling the trunk affects hand trajectory formation. The second study examined whether the movement context (pointing versus grasping) would affect postural control (Chapter 4). In older adults only, grasping was associated with a decrease in COP trajectory linearity, suggesting that aging affects the ability to anticipate and counteract the internal perturbations generated by grasping an object. From a rehabilitative perspective, the results of these studies indicate that standing balance training in older adults should incorporate different workspace locations, functional goals, as well as tasks involving reaching with both the dominant and non-dominant hands. |
