Comparing arm to whole-body motor control disambiguates age-related deterioration from compensation.

Background: As the global population ages, it is crucial to understand sensorimotor compensation mechanisms. These mechanisms are thought to enable older adults to remain in good physical health, but despite important research efforts, their precise nature remains elusive and has not been definitively demonstrated. A major problem with their identification is the ambiguous interpretation of age-related alterations. Whether a change reflects deterioration or compensation is difficult to determine. Methods: To address this challenge, we examined movement efficiency in younger and older adults using two complementary approaches. In Experiment 1 (Younger, n = 20; mean age = 23.6 years, and older adults, n = 24; mean age = 72 years), we quantified energetic efficiency through the negativity of phasic EMG activity-an established marker of how the nervous system exploits gravity to minimize muscular effort-during both single-joint arm movements and whole-body actions (sit-to-stand/back-to-sit and whole-body reaching). In Experiment 2 (younger adults, n = 20; mean age = 22.9 years; older adults, n = 20; mean age = 70.6 years), we directly measured energetic cost using exhaled-gas analysis during treadmill walking under varying balance constraints. Results: In Experiment 1, older adults preserved efficient planning during arm movements, but they showed reduced gravity-related efficiency during whole-body tasks. Complementary center-of-mass analyses and optimal control simulations indicated that this reduced efficiency aligned with movement strategies favoring stability over energy minimization. In Experiment 2, older adults exhibited a disproportionately larger increase in metabolic cost and perceived effort when equilibrium demands were elevated, despite performing the same tasks as younger adults. Discussion: This supports a causal role of equilibrium constraints in decreasing walking efficiency in older adults. Overall, these results suggest that reduced movement efficiency in healthy older adults does not reflect a deterioration but rather a compensation process that adapts movement strategy to the task specificities. When balance is at stake, healthy older adults prefer stability to energy efficiency.