Summary: Reasons for performing study: The effect of racetrack surface (dirt or synthetic) on distal hindlimb kinematics of racehorses running at competition speeds is not known. Objectives: To compare distal hindlimb and hoof kinematics during stance of breezing (unrestrained gallop) racehorses between dirt and synthetic surfaces. Study design: Two-dimensional kinematic video analysis of 5 Thoroughbred racehorses galloping at high speeds (12-17m/s) on a dirt racetrack and a synthetic racetrack. Methods: The positions of kinematic markers applied to the left hindlimb were recorded at 500Hz. Position, velocity and acceleration of joint angles and hoof translation during stance were calculated in the sagittal plane. Peak translational and angular kinematic values were compared between the dirt and synthetic race surfaces using mixed model analyses of covariance. Results: Maximum and heel-strike metatarsophalangeal (fetlock) angles were greater (P<0.05) on the dirt surface than on the synthetic surface. Maximum fetlock angle occurred earlier during stance on the dirt surface (P<0.05). Greater horizontal displacement of the heel during slide occurred on the dirt surface (P<0.05). Conclusions: During high-speed gallop, hindlimb fetlock hyperextension and horizontal hoof slide are greater on a dirt surface than on a synthetic surface. Synthetic race surfaces may mitigate risk of injury to hindlimb fetlock structures by reducing fetlock hyperextension and associated strains in fetlock support structures. Differences in hoof slide may contribute to different distal hindlimb kinematics between surfaces.
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