BIOMECHANICS OF HORSE JUMPING

Abstract

INTRODUCTION: During jumping, a horse’s body changes its direction of angular movement twice and hence has to undergo the necessary accelerations/ decelerations in order to perform the jump. Clayton (1989) describes the terminology of horse jumping. The aim of this study was to calculate the angular acceleration of the horse’s body, the path of the center of gravity, and to provide an explanation of jump failures. METHODS: Videofilms were made during an international tournament. The fence chosen for this analysis was an oxer of 1.6m height and 1.2m distance between the elements (rails). A video camera Sony DXC-9100 (100 fr./s) and video software (Adobe Premiere 4.2) were used. For 33 horses we measured the angle of a reference line (crupper - withers) relative to the environment. The angular data set vs. time was splined (non-linear least squares, taking consideration of the linear segments) and first and second differentiations were performed to calculate the angular velocity and acceleration using Mathematica 3.0. The path of the center of gravity (constructed according to Sprigings and Leach, 1986) during the suspension phase was also analyzed by means of a parabolic function. RESULTS: Related to angular acceleration, 5 phases can be distinguished: phase 1: acceleration in order to lift the body, 1a) initiated by the front limbs and 1b) continued by the rear limbs, with zero acceleration during the short time interval between 1a and 1b. Phase 2: deceleration of body lift and acceleration of negative angular motion of the jump suspension (generated by the rear limbs). Phase 3: zero acceleration during jump suspension. Phase 4: deceleration of jump suspension and acceleration of body descent (front limbs). Phase 5: deceleration of body descent (rear limbs). The mean values of angular accelerations of phases 1-5 (in rad/s2) were: phase 1a: +30, phase 1b: +20, phase 2: -55, phase 3: 0, phase 4: +40, phase 5: -30. For positive acceleration, the ground reaction force vector is situated in front of the center of gravity, in negative acceleration, behind. The angular impulse (considering body mass, radius of gyration, Dangle and Dtime of the suspension phase) amounted to 200-250 Nms. The knocking down of the rails by the front limbs was mainly due to the jump-off position, the angular body lift and the path of the center of gravity. The knocking down of the rails by the hind limbs was due to flexion in the hip joint instead of extension. CONCLUSIONS: Decisive factors for horse jumping are acceleration during body lift (phases 1a and 1b), the take-off position, the path of the center of gravity, and limb movements, especially for high fences and high speeds (for a reduced winning time). The measurement of the mentioned parameters by means of a videotechnique is hence a valuable tool for aptitude tests. REFERENCES: Clayton, H. M. (1989). Terminology for the Description of Equine Jumping Kinematics. J. Eq. Vet. Sci. 9, 341-348. Sprigings, E., Leach, D. (1986). Standardised Technique for Determining the Centre of Gravity of Body and Limb Segments of Horses. Eq. Vet. J. 18, 43-49.