Healthy aging is associated with reduced lateralized expression but preserved gravity-related motor planning during vertical arm movements

Abstract Motor lateralization, often expressed behaviorally as interlimb asymmetry, reflects distinct specializations of the dominant and nondominant motor systems. Although healthy aging has been associated with reduced motor lateralization, most previous studies have characterized this effect using coarse performance measures rather than specific motor-control processes. Here, we examined whether healthy aging alters the lateralization of gravity-related motor optimization, a motor-planning process through which the nervous system exploits gravitational torque to minimize muscle effort during vertical movements. Forty-one healthy young adults (24.3 ± 3 years) and forty-two healthy older adults (72.0 ± 6 years) performed single-degree-of-freedom upward and downward arm movements with their dominant and nondominant arms. Arm kinematics and electromyographic activity of the prime movers were recorded, and directional kinematic asymmetries together with negative epochs in phasic EMG activity (i.e., phases where EMG activity is inferior to the theoretical value needed to compensate gravity torque) were analyzed as hallmarks of gravity-related motor optimization. Strong age × arm interaction effects were found for relative durations to peak acceleration and peak velocity, as well as for the negativity of antigravity muscles’ phasic signals. In young adults, the results generally indicated an advantage for the dominant arm, whereas no such effect was observed in older adults. Importantly, values obtained in older adults with both arms were generally equivalent to those of the dominant and non-dominant arms in young adults. These findings suggest that healthy aging is associated with a reduced lateralized expression of gravity-related motor optimization rather than with a global decline in motor planning. More broadly, they support the view that some anticipatory control processes may remain functional in healthy aging despite reduced motor lateralization.