Altered interlimb coupling during unimanual and bimanual force control in aging and Parkinson's disease

Bimanual coordination depends on the ability to flexibly regulate interlimb coupling while maintaining independent control of each limb. Aging and Parkinson's disease (PD) may disrupt this balance by altering sensorimotor regulation and shared neural input between limbs. This study examined force control, coordination dynamics, and physiological coupling during unimanual and bimanual isometric force tasks in healthy young adults (HYA), healthy older adults (HOA), and individuals with Parkinson's disease (PD). Participants completed 30 s unimanual and bimanual force tasks with target forces set at 20% maximal voluntary contraction (MVC). During unimanual tasks, participants produced the target force with one limb while the uninstructed limb rested on a force transducer. During bimanual tasks, both limbs produced the target force. Beginning 15 s after trial onset, visual feedback for the instructed limb in the unimanual tasks or one designated limb in the bimanual tasks was removed for 10 s. Behavioral measures included constant error (CE), coefficient of variation (CV), and interlimb force-force cross-correlation, while physiological coupling was quantified using EMG-EMG coherence between homologous first dorsal interosseous (FDI) muscles. PD participants demonstrated greater force variability, greater unintended force production in uninstructed limbs, and stronger interlimb force-force coupling relative to HYA and HOA participants, with group differences most pronounced during bimanual conditions. Whereas HYA and HOA participants generally increased force variability following removal of visual feedback, PD participants demonstrated reduced variability when feedback was removed. EMG-EMG coherence was elevated in PD across frequency bands and task conditions, while HYA and HOA participants demonstrated comparatively lower and more feedback dependent coherence patterns. Behavioral and physiological measures provided converging evidence of stronger and less adaptable interlimb coupling in PD. These findings indicate altered and less adaptable regulation of sustained force output in PD and are consistent with greater shared neural input to homologous muscles and reduced flexibility in the independent regulation of force across the limbs.