The contribution of inherent and incidental constraints to intentional switching between patterns of bimanual coordination

This article addresses the contribution of inherent (arising from the structure of the neuromuscular-skeletal system) and incidental (arising from specific features of a task or environment) constraints/resources to intentional modifications of coordination dynamics. An experiment was conducted to determine the extent to which the organization of voluntary transitions between locally stable modes of bimanual coordination was influenced by the frequency of movement, and by the provision and phase of external auditory pacing. Subjects performed rhythmic pronation and supination movements of the forearm in four bimanual patterns. In two in-phase patterns, either maximum pronation (inphase pronation) or maximum supination (inphase supination) was synchronised with each beat of an auditory metronome. In two antiphase patterns, either maximum excursion to the left (antiphase left) or to, the right (antiphase right) was synchronised with the metronome. In an initial session, mean (spontaneous) transition frequencies (MTF) were established. In a second session pacing was provided at MTF -0.5 Hz, MTF -0.25 Hz, MTF and MTF +0.25 Hz. A visual signal presented early or late in the trial indicated that subjects were to switch to the opposite mode of coordination. On some trials, metronome pacing was removed following an induction period. Transition durations decreased in a linear fashion with increases in pacing frequency, both when the metronome was present and when it had been removed prior to the imperative signal, suggesting that movement frequency acts as an inherent constraint upon the coordination dynamics. Transitions were of greater mean duration when the metronome was present than when it was absent, indicating that external pacing signals may impose incidental constraints. Transitions from the antiphase to the inphase pattern were generally of shorter duration than those in the reverse direction. These results are discussed with reference to both dynamical and physiological models of multilimb coordination.