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First published online November 5, 2004
Journal of Experimental Biology 207, 4215-4223 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01277

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

Timothy M. Griffin^1,*, Rodger Kram², Steven J. Wickler³ and Donald F. Hoyt³

¹ Department of Integrative Biology, University of California, Berkeley, CA 94720-3140, USA
² Department of Integrative Physiology, University of Colorado, Boulder, CO 80309, USA
³ Equine Research Center and Departments of Animal and Veterinary Sciences and Biological Sciences, California State Polytechnic University, Pomona, CA 91768, USA

^* Author for correspondence at present address: Orthopaedic Bioengineering Laboratory, Department of Surgery, Duke University Medical Center, Durham, NC 27710, USA (e-mail: tmgriff{at}duke.edu)

Accepted 3 September 2004

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.

Key words: equine, Froude number, gait, inverted pendulum, locomotion, oxygen consumption

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