Effect of load on preferred speed and cost of transport

Horses have a tendency to utilize a relatively narrow set of speeds near th e middle of a much broader range they are capable of using within a particu lar gait, i.e., a preferred speed. Possible explanations for this behavior include minimizing musculoskeletal stresses and maximizing metabolic econom y. If metabolic economy (cost of transport, C-T) and preferred speeds are l inked, then shifts in C-T should produce shifts in preferred speed. To test this hypothesis, preferred speed was measured in trotting horses (n = 7) u nloaded 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 decr eased to 3.13 +/- 0.11 m/s when loaded. In both conditions ino load and loa d), the rate of O-2 consumption (n = 3) was a curvilinear function of speed that produced a minimum C-T (i.e., speed at which trotting is most economi cal). When unloaded, the speed at which C-T was minimum was very near the p referred speed. With a load, C-T decreased and the minimum was also near th e preferred speed of horses while carrying a load.