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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

View larger version (28K): [in a new window]   Fig. 1. Walk–trot transition speed (closed circles) versus leg length for different-sized horses. Isolines represent speed and leg length combinations for specific Froude numbers. The walk–trot transition occurs at faster absolute speeds in horses with longer legs but at nearly the same Froude number (0.35). Values are means ± S.D.

View larger version (17K): [in a new window]   Fig. 2. Cost of transport for a range of walking and trotting speeds (closed and open circles, respectively). Data are shown for the miniature (A–C), Arabian (D–F) and draft (G–I) horses. Dashed lines are least-squares second-order polynomial curve fits. Bars indicate the observed gait used continuously by the horse during the transition speed measurements. Hatched bars indicate speed range where the horses switched gaits at least once during the 1 min observation period. Horses were then trained to use a prescribed gait for oxygen consumption measurements at speeds within the hatched bar region after the transition speed measurements were completed. However, we were not able to obtain walking and trotting O2 data at the same speeds in the transition region for the miniature and draft horses. In these cases, the curve fits were extended beyond the data as shown to calculate the optimal metabolic transition speed.

View larger version (16K): [in a new window]   Fig. 3. Observed range of gait-transition speeds plotted against the metabolically optimal transition speed for the different-sized horses. Horses switched from a walk to a trot at speeds corresponding to the optimal metabolic transition speed (i.e. the speed at which the cost of transport for walking intersects that for trotting). The top of the bars are the slowest speeds that the horses trotted at for 1 min, and the bottom of the bars are the fastest speeds that the horses walked at continuously for 1 min. Therefore, the bars represent the observed range of speeds in which the horses spontaneously switched between walking and trotting. Top and bottom error bars are 1 S.D. Our method of extrapolating polynomial curve fits for several of the horses' data produces some error in determining the optimal metabolic transition speed. This error, however, is likely to be only about a quarter the magnitude of the observed transition speed range (i.e. the vertical bar). Dashed line is the line of identity.

View larger version (14K): [in a new window]   Fig. 4. (A) Percent recovery of mechanical energy via the inverted-pendulum-like mechanism in walking as a function of the dimensionless Froude number. Data from the literature are presented for children and adults walking at a range of speeds in normal gravity, and for adults walking at a range of speeds in simulated reduced gravity. Percent recovery is similar at equal Froude numbers, suggesting that at equal Froude numbers the inverted-pendulum dynamics are mechanically equivalent. Child data from Cavagna et al. (1983) for 3–4, 7–8 and 11–12-year olds. Adult and reduced gravity data from Griffin et al. (1999). (B) Percent recovery for a biped (humans) and quadruped (dogs) versus Froude number for a range of walking speeds. Percent recovery decreases more precipitously at faster speeds for the quadruped compared with the biped at similar Froude numbers. Quadruped data are from Griffin et al. (2004), and biped data are for adults walking in normal gravity from Griffin et al. (1999).

View larger version (14K): [in a new window]   Fig. 5. Cost of transport (J kg–1 m–1) decreases with body mass among >90 different species of birds and mammals (Langman et al., 1995; Taylor et al., 1982) but does not change with body mass within horses. For clarity, only the mouse and elephant data points are shown for the inter-specific relationship, although the line is derived from data representing >90 species. Open squares are mean values for each size range of horses from the present study and closed squares are mean values from the literature (N=3, Hoyt and Taylor, 1981; N=5, Eaton et al., 1995; N=4, Minetti et al., 1999; N=4, Pagan and Hintz, 1986; N=7, Potard et al., 1998). The cost of transport was calculated as the linear slope of the rate of oxygen consumption versus trotting speed for horses. We assumed an energetic equivalent of 20.1 J ml–1 O2. The scaling relationship for the horse data was calculated using a least-squares linear regression, following Taylor et al. (1982). The 95% confidence limits of the exponent for the horse line are ±0.138.