PHASE-TRANSITIONS AND CRITICAL FLUCTUATIONS IN RHYTHMIC COORDINATION OF IPSILATERAL HAND AND FOOT

Four subjects performed rhythmic movements of the ankle and the wrist in time with an auditory metronome, in two modes of coordination, anti phase and in-phase. The forearm was placed in either a prone or a supi ne position. When movements were prepared in the antiphase mode, spont aneous transitions to the in-phase mode or to phase wandering were obs erved as the metronome frequency was increased. When prepared in the i n-phase mode, transitions between in-phase modes or to phase wandering were occasionally observed. Predicted signature features of nonequili brium phase transitions were noted, including loss of stability and cr itical fluctuations. The stability of the movement patterns was determ ined by spatial (dependent upon the direction of movement) rather than anatomical (dependent on the coupling of specific muscle groups) cons traints. The position of the forearm had no consistent bearing upon th e variability of the phase relations between the limbs, the frequency of phase transitions, or the time of onset of transitions. These resul ts are discussed with reference to the coordination dynamics (e.g., mu ltistability, loss of stability) of multijoint movements.