Biomechanical and energetic determinants of the walk–trot transition in horses

We studied nine adult horses spanning an eightfold range in body mass(M_b) (90–720 kg) and a twofold range in leg length(L) (0.7–1.4 m). We measured the horses' walk–trot transition speeds using step-wise speed increments as they locomoted on a motorized treadmill. We then measured their rates of oxygen consumption over a wide range of walking and trotting speeds. We interpreted the transition speed results using a simple inverted-pendulum model of walking in which gravity provides the centripetal force necessary to keep the leg in contact with the ground. By studying a large size range of horses, we were naturally able to vary the absolute walking speed that would produce the same ratio of centripetal to gravitational forces. This ratio,(M_bv²/L)/(M_bg),reduces to the dimensionless Froude number(v²/gL), where v is forward speed, L is leg length and g is gravitational acceleration. We found that the absolute walk–trot transition speed increased with size from 1.6 to 2.3 m s^–1, but it occurred at nearly the same Froude number (0.35). In addition, horses spontaneously switched between gaits in a narrow range of speeds that corresponded to the metabolically optimal transition speed. These results support the hypotheses that the walk–trot transition is triggered by inverted-pendulum dynamics and occurs at the speed that maximizes metabolic economy.