Griffin, T.M.*; Garcia, S.; Wickler, S.J.; Hoyt, D.F.; Kram, R.: Determinants of the walk-trot transition and preferred walking speeds: Insights from intra-specific size comparisons of horses
Much of our understanding of the effects of size on locomotion has come from inter-specific size comparisons (i.e. mouse-to-elephant). To control for phylogenetic and morphological differences associated with inter-specific comparisons, we used intra-specific size variation within horses to understand the determinants of the walk-trot transition (W-T) and preferred walking (PW) speeds. We addressed two questions: 1) Are W-T and PW speeds determined by inverted pendulum mechanics? 2) Do W-T and PW speeds minimize the metabolic cost required to travel a unit distance? We studied 9 horses (Equus caballus) spanning an 8-fold range in mass (90 – 720kg) and a 2-fold range in leg length (0.7 – 1.4m). W-T speeds and rates of oxygen consumption were measured during treadmill locomotion. PW speeds were measured during overground walking. To compare inverted pendulum mechanics across size we calculated the Froude number, = v2/gL, where v is forward speed (m/s), g is gravity, and L is leg length. We found that the absolute W-T speed increased with size (1.6 to 2.3 m/s), but occurred at nearly the same Froude number (0.34). The W-T speed concurred with the metabolically optimal transition speed. The absolute PW speed increased with size up to 400kg (0.9 to 1.5 m/s) but curiously did not increase further. The PW speed coincided with the minimum metabolic cost of transport, except in the smallest horses that preferred slower than energetically optimal speeds. In general, W-T and PW speeds are tightly coupled to both inverted pendulum mechanics and metabolic energy minimization. Supported by NSF IBN-0073020, NIH S06 GM53933 and NIH AR44688.
