Asymmetric Bilateral Deficit and Facilitation in Maximal-Speed Bimanual Finger Coordination

Bilateral deficit, reduced performance during bimanual movements relative to unimanual execution, has been consistently reported in healthy individuals, yet whether this effect is uniform across hands remains unclear. We examined hand-specific bilateral effects during maximal-speed cyclic finger movements in 73 right-handed participants across unimanual, bimanual in-phase, and bimanual anti-phase coordination conditions. The dominant hand exhibited bilateral deficit, whereas the nondominant hand demonstrated facilitation during in-phase coordination. Anti-phase coordination significantly reduced frequency in both hands, accompanied by increased movement amplitude. Multivariate regression analysis revealed a hand-specific dissociation: unimanual frequency asymmetry predicted dominant hand deficit, whereas amplitude modulation predicted nondominant hand facilitation. These opposing effects align with complementary hemispheric specialization and asymmetric interhemispheric interactions during synchronization. An extended coupled oscillator model incorporating hand-specific natural frequencies predicted these asymmetric effects through weighted frequency convergence during in-phase coordination and amplitude-dependent stability constraints during anti-phase coordination, with predictions consistent with the experimental observations. These findings demonstrate that bilateral deficit and facilitation can coexist within the same individual, with implications for understanding asymmetric limb function and for theoretical models of bimanual coordination.