Asymmetric dynamics of human limb coordination

The dynamics of pattern formation and change are studied in a complex, multicomponent system, specifically the arms and legs of human subjects. Previous studies (Kelso & Jeka, in press) have demonstrated novel features in the coordinative dynamics of an arm-leg pair, including: (1) differential stability of coordinative modes produced by limbs moving in the same (S-mode) versus different (D-mode) directions; (2) a slow drift in relative phase preceding transitions from the D- to the S-mode; (3) preferred transition routes between patterns; and (4) spontaneous emergence of non 1:1 frequency- and phase-locked patterns, in addition to periods of relative coordination. These phenomena have been encompassed theoretically in a model of coupled oscillators which includes a symmetry-breaking term to represent the difference in the uncoupled frequencies of the arm and leg (Kelso et al., 1990). To test predictions of the (Kelso et al., 1990) model, the first of two studies was aimed at whether manipulation of the inherent biophysical differences between the arm and leg, through inertial loading, would be reflected in their coordinative dynamics. The results showed that loading the leg led to the highest percentage of: (1) D- to S-mode transitions in the down direction (i.e., with decreasing values of relative phase); and (2) transitions to phase wandering. Loading the arm led to: (1) an approximately equal number of transitions in either the up or down direction; and (2) very few transitions to phase wandering. The conclusion was that adding weight to the arm or leg was influential in minimizing or enhancing the coordinative asymmetry, respectively. A second study used the same loading conditions as Experiment 1 within a perturbation paradigm to study possible differences in relaxation time and perturbation-induced transitions, as additional measures of the asymmetry of the coordinative dynamics. Relaxation time and perturbation-induced transition pathways showed no effects of inertial loading. Pretransition relative phase showed a steady decrease when the leg was loaded and very little drift in the arm load condition. Pretransition relaxation time increased systematically with required frequency and relative phase variability, but only with perturbations in the up direction (i.e., increasing values of relative phase). These effects were consistent with model predictions and showed that asymmetric dynamics characterized the coordinative patterns of anatomically different components.