Gait, estimated net cost of transport and heat production at different speeds in three-day-event horses.

Heart rate and gait characteristics (stride length and frequency) were studied in 6 horses subjected to a standardised incremental exercise test, involving moving at the trot and increasing speeds up to a fast gallop and subsequently during the steeplechase phase of a 3-day-event. The studies were performed in hot conditions. Appropriate scaling, based on hindleg length (hh), stride length (L), stride frequency (f) and speed (Sp) for nondimensional stride length (lambda = (L/hh), nondimensional stride frequency (phi = f(hh/g)1/2) and nondimensional velocity (û = Sp/(ghh)1/2), where g is the gravitational acceleration, demonstrated that there were no major differences in characteristics over the full range studied lambda = 2.3 û0.68. However, there were subtle differences in some horses that could endow a benefit in locomotory efficiency when compared to others exercising at the same absolute speed. There were clear changes in the relationship between nondimensional stride length and frequency with increasing speed, from trot to canter (at û approximately 1.4) and to full gallop (at û approximately 2.3); when trotting, lambda was less than 2.2 and the transition from canter to gallop took place at lambda approximately 3.2. The cost of transport/kg/m, estimated from the heart rates measured continuously during each study, decreased with increasing speed and bodyweight. In some animals, there appeared to be a weak minimum around the canter-gallop transition speed. When interpreted as oxygen cost, using published values for the oxygen consumption-heart rate relationship, the cost fell from an average of 0.201 ml/kg/m at the trot to 0.161 mlO2/kg/m when galloping during the incremental exercise tests. During the steeplechase, the cost was approximately 7.5% higher than at the same speed in the exercise tests; this was probably due to jumping effort. Estimated power consumption increased linearly with speed. In the steeplechase, power consumption was also 7.5% higher than during the exercise tests at the same absolute speed; this was equivalent to an average rate of heat production of 346 kcal/min (24 kW) or 59.5 kcal/min/m2 of the measured body surface area.