WHY CHANGE GAITS - DYNAMICS OF THE WALK RUN TRANSITION

Why do humans switch from walking to running at a particular speed? It is proposed that gait transitions behave like nonequilibrium phase tr ansitions between attractors. Experiment 1 examined walking and runnin g on a treadmill while speed was varied. The transition occurred at th e equal-energy separatrix between gaits, with predicted shifts in stri de length and frequency, a qualitative reorganization in the relative phasing of segments within a leg, a sudden jump in relative phase, enh anced fluctuations in relative phase, and hysteresis. Experiment 2 dis sociated speed, frequency, and stride length to show that the transiti on occurred at a constant speed near the energy separatrix. Results ar e consistent with a dynamic theory of locomotion in which preferred ga its are characterized by stable phase relationships and minimum energy expenditure, and gait transitions by a loss of stability and the redu ction of energetic costs.