Targeting a specific motor control process reveals an age-related compensation that adapts movement to gravity environment

Abstract

As the global population ages, it is crucial to understand sensorimotor compensation mechanisms that allow older adults to remain in good physical health, i.e. underlying successful aging. Although age-related compensation has long been conceptualized and despite important research effort in varied gerontological subfields, behavioral compensatory processes and their underlying neural mechanisms remain essentially chimeras. This study investigates age-related compensation at the behavioral level. It tests the basic hypothesis that age-related compensatory processes may correspond to an adaption process that changes movement strategy. More specifically, we focused on the ability of younger (n = 20; mean age = 23.6 years) and older adults (n = 24; mean age = 72 years) to generate movements that are energetically efficient in the gravitational environment. Previous results, from separate studies, suggest that aging differently alters energy efficiency in arm movement and whole-body movement tasks. With aging, energy efficiency seems to remain highly functional in arm movements but was shown to decrease in whole-body movements. Here we built on recent theoretical and experimental results demonstrating a behavioral process that optimally adapts human arm movements to the gravitational environment. Analyzing phasic muscle activation patterns, previous studies provided electromyographic measurements that quantified the output of an optimal strategy using gravity effects to discount muscle effort. Using these measurements, we probed the effort-minimization process in younger and older adults during arm movement and whole-body movement tasks. The key finding demonstrates that aging differently alters motor strategies for arm movements vs whole-body movements. Older adults used gravity effects to a similar extent as younger ones when performing arm movements, but to a lesser extent when performing whole body movements. These results provide clear experimental support for an adaptation strategy that down-regulates effort minimization in older adults.