Aging decreases the lateralization of gravity-related effort minimization during vertical arm movements

Motor lateralization refers to differences in the neural organization of cerebral hemispheres, resulting in different control specializations between the dominant and the non-dominant motor systems. Multiple studies proposed that the dominant hemisphere is specialized for open-loop optimization-like processes. Recently, comparing arm kinematics between upward and downward movements, we found that the dominant arm outperformed the non-dominant one regarding gravity-related motor optimization in healthy young participants. The literature about aging effects on motor control presents several neurophysiological and behavioral evidences for an age-related reduction of motor lateralization. Here, we compare the lateralization of a well-known gravity-related optimal motor control process between young and older adults. Forty-one healthy young (mean age = 24.3 {+/-} 3 years) and forty-two healthy older adults (mean age = 72.0 {+/-} 6 years) performed single degree-of-freedom vertical arm movements between two targets (upward and downward). Participants alternatively reached with their dominant and non-dominant arms. We recorded arm kinematics and electromyographic activities of the prime movers (Anterior and Posterior Deltoids) and we analyzed parameters thought to represent the hallmark of the gravity-related optimization process (i.e directional asymmetries and negative epochs on the phasic EMG activity). We found strong age x arm interaction effects on all parameters; i.e., relative durations to peak acceleration and peak velocity and the negativity of antigravity muscles phasic signals. Although all three parameters showed a lateralization effect where the dominant arm was superior to the non-dominant arm in young adults (as in Poirier et al. 2022), we found no such effect in older adults. With both arms, the results of older adults lied between those of the dominant and non-dominant arm of young adults. These results add to those of recent literature showing that feedforward motor control remains functional in older adults. More, the results obtained with the non-dominant arm may support a previously hypothesized increased reliance on predictive mechanisms in older adults.