Manipulating symmetry in the coordination dynamics of human movement.

J. A. S. Kelso and J. J. Jeka (1992) demonstrated that symmetry is a useful conceptual tool to distinguish the coordination between components with similar versus different anatomical properties. The present experiments studied human arm-leg patterns to test whether their coordinative asymmetry was changed by manipulating the inertial properties of a single limb. The results showed that (a) consistent with model predictions, adding weight to the arm or the leg minimized or enhanced coordinative asymmetry, respectively and (b) the response to a perturbation slowed as movement frequency increased but in a fashion that reflected the underlying coordinative asymmetry. The observed coordinative effects suggest the influence of neural phase relationships and emphasize that symmetry plays an important role in understanding coordination in systems in which control cannot be traced unequivocally to a single end-effector or a neurophysiological substrate.