title: Biomechanical and energetic determinants of the walk-trot transition in horses.
abstract: 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(b)v2/L)/(M(b)g), 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.
authors.affiliation: Department of Integrative Biology, University of California, Berkeley, CA 94720-3140, USA. tmgriff@duke.edu
authors.forename: Timothy M
authors.initials: TM
authors.lastname: Griffin
authors.forename: Rodger
authors.initials: R
authors.lastname: Kram
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Wickler
authors.forename: Donald F
authors.initials: DF
authors.lastname: Hoyt
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D001696:Biomechanical Phenomena; D055592:Biophysical Phenomena; D001703:Biophysics; D001837:Body Weights and Measures; D006736:Horses / Q000502:physiology*; D008124:Locomotion / Q000502:physiology*; D010101:Oxygen Consumption / Q000502:physiology*