Adults up to 80 years old maintain effective movement planning when facing complex body dynamics

Abstract

Aging can significantly impact motor performance, especially in highly complex tasks such as multi-joint movements where the nervous system needs to adequately coordinate mechanical interactions between joints. This coordination is inherently challenging for the brain. Effective coordination of multiple joints relies on intact feedforward control to predict movement dynamics in the initial phase of the movement, and on feedback control to fine-tune the execution in the final phase. However, the effect of aging on these specific control mechanisms remains controversial. In our experiment we investigated a pure elbow motion task using the KINARM exoskeleton. A group of 50 young (20-35 years old), 80 old (55-70 years old) and 30 older-old (80+ years old) healthy participants were recruited. Each participant performed 30{degrees} elbow rotations while stabilizing the shoulder joint. Movements were directed toward two distinct targets in both flexion and extension directions. The task was performed under two controlled speed conditions to maximally challenge the motor system, as higher elbow velocities increase interaction torques at the shoulder, demanding greater neuromuscular effort for stabilization. The timing and magnitude of anticipatory EMG activity of the agonist shoulder muscle, necessary to counteract interaction torques, were preserved across all age groups. Moreover, increasing elbow velocity did not result in any performance differences between young and older old adults, indicating that shoulder stabilization during movement initiation remained intact with age. However, older adults exhibited reduced ability to stabilize the shoulder position until the end of the movement, leading to decreased reaching accuracy with older age. These results suggest that feedforward control, essential for movement planning, which is essential for shoulder stabilization during initiation, is preserved during healthy aging and remains resilient to increased motor demands, even in older old adults. In contrast, feedback control appears to deteriorate with age, potentially contributing to reduced movement precision in the final phase of the multi-joint movement.