Shifts in Key Time Points and Strategies for a Multisegment Motor Task in Healthy Aging Subjects.

In this study, we compared key temporal points in the whole body pointing movement of healthy aging and young subjects. During this movement, subject leans forward from a standing position to reach a target. As it involves forward inclination of the trunk, the movement creates a risk for falling. We examined two strategic time points during the task-first, the crossover point where the velocity of the center of mass (CoM) in the vertical dimension outstripped the velocity in the anteroposterior dimension and secondly, the time to peak of the CoM velocity profile. Transitions to stabilizing postures occur at these time points. They both occurred earlier in aging subjects. The crossover point also showed adjustments with target distance in aging subjects, while this was not observed in younger subjects. The shifts in these key time points could not be attributed to differences in movement duration between the two groups. Investigation with an optimal control model showed that the temporal adjustment as a function of target distance in the healthy aging subjects fits into a strategy that emphasized equilibrium maintenance rather than absolute work as a control strategy.