Dynamics of bimanual rhythmic coordination in the coronal plane

We investigated the 1:1 frequency locking of two hand-held pendulums oscillated parallel to the body's coronal plane. In this configuration, anti-phase defined muscularly is in-phase defined spatially, and vice versa. Coordination equilibria measured by average relative phase were shifted less from muscular anti-phase than from muscular in-phase by detuning (unequal uncoupled pendulum frequencies) and were shifted less in both modes with vision than without. Variability of the equilibria, however, was ordered opposite to their degrees of shift and was unaffected by vision. Demonstrated subcritical pitch-fork and tangent bifurcations conformed to the variability classification of anti- and in-phase coordination. Implications for dynamical models, hierarchical control, and definitions of coordination modes were discussed.