59.5  Friday, Jan. 6  Jump racing: do horses slow down due to a force limit? SELF, ZT*; SPENCE, AJ; WILSON, AM; The Royal Veterinary College; The Royal Veterinary College; The Royal Veterinary College zself@rvc.ac.uk

Running animals face the challenge of using limb force to support body weight over the time in which their feet are in contact with the ground. As they go faster, the proportion of the stride that the limbs are in contact with the ground (duty factor) drops. It has been proposed, in humans and horses, that one factor that limits speed is the maximum force which the limbs can withstand. This is supported by the observation that both humans and horses run slower on bends (simulating increased gravity), although interestingly this is not true in dogs. Jumping is a task that requires increased impulse (to raise the COM) and therefore the manner in which animals jump can be used to investigate what limits locomotor performance. Horses race over courses with jumps which requires a longer aerial phase between stances. If horses are constrained by limb force, they would have to use a longer limb contact time before and after the jump to maintain duty factor (which is inversely proportional to force). Here we test the hypothesis that jumping horses maintain duty factor in jump strides by increasing stance time and slowing down.

First, we examined speed and position data during jump races and found that horses slow down by up to 2ms^-1 at fences (a much greater drop in kinetic energy than the gain in potential energy required to clear the jump) which could avoid a limb force limit through increased contact time. However, high speed video data collected during jump training reveals that duty factor is lower in jump strides, suggesting increased limb force. Limited force plate data shows high peak forces during take-off, greater than those experienced during level galloping. These results suggest that limb force may not be a direct limit to racehorse jumping performance.