Effect of load on preferred speed and cost of transport.

Horses have a tendency to utilize a relatively narrow set of speeds near the middle of a much broader range they are capable of using within a particular gait, i.e., a preferred speed. Possible explanations for this behavior include minimizing musculoskeletal stresses and maximizing metabolic economy. If metabolic economy (cost of transport, CT) and preferred speeds are linked, then shifts in CT should produce shifts in preferred speed. To test this hypothesis, preferred speed was measured in trotting horses ( n = 7) unloaded on the level and loaded with 19% of their body weight on the level. The preferred speed on the level was 3.33 ± 0.09 (SE) m/s, and this decreased to 3.13 ± 0.11 m/s when loaded. In both conditions (no load and load), the rate of O2 consumption ( n = 3) was a curvilinear function of speed that produced a minimum CT (i.e., speed at which trotting is most economical). When unloaded, the speed at which CT was minimum was very near the preferred speed. With a load, CT decreased and the minimum was also near the preferred speed of horses while carrying a load.