Age induced modifications in the persistency of voluntary sway when actively tracking the complex motion of a visual target.

Movement persistency, reflected in systematic cycle to cycle fluctuations of a rhythmical task such as walking or voluntary sway, is compromised with increasing age, making older adults more susceptible to falls. In the present study, we tested whether it is possible to improve rhythmic voluntary sway persistency in old age by actively tracking the complex (i.e. persistent) motion of a visual target. Twenty healthy young and 20 older adults performed 132 cycles of anterior-posterior sway under two conditions: a) self-paced sway and b) sway while tracking the vertical motion of a complex visual target. The persistency of sway cycle amplitude and duration, detected from the center of pressure displacement, was quantified using the Fractal exponent α. We also recorded body kinematics in order to assess the intersegmental coordination that was quantified in the Mean Absolute Relative Phase (MARP) and the Deviation Phase (DPh) between the trunk and the lower limbs. In self-paced sway, older adults showed a lower persistency of cycle duration and a higher MARP and DPh between the trunk and the lower limbs compared to young adults. Tracking the complex visual target motion increased the persistency of cycle amplitude, in young but not in older adults, when compared to the self-paced sway while it decreased the persistency of cycle duration in both groups. The relative phase measures showed a moderate to strong relationship with the persistency of cycle amplitude and duration when older adults swayed in their self-pace. These findings suggest older adults cannot exploit active tracking of the complex visual motion cue to improve voluntary sway persistency. This could be related to the less stable and out of phase intersegmental coordination characterizing rhythmic voluntary sway in old age.
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