20) A Biomechanical Theory on How a Horse Jumps Part 2
By Coralie Hughes
Last Friday we looked at the first part of an analysis of the biomechanics of jumping by MMCP and Advanced Instructor Coralie Hughes. The jump was broken into the Approach, Take-off, Suspension and Landing phases. Last week we looked at the Approach and Landing phases.
Today we’ll see the horse finish the jump as we look at the Suspension and Landing phases.
To bascule over the fence, the horse has to lower his head and raise his withers (thoracic sling). The shape of the horse’s head, neck and back is the bascule. In the air, the horse’s body rotates around his center of gravity. The trapezius, brachiocephalic and latissimus dorsi muscles contract to flex the shoulder and elbow joints and lift the scapulae. This helps lift the forelimbs to attain the necessary height to clear the jump.
The folded forelegs bring the horse’s weight closer to his center of gravity, which increases speed. Tension on the nuchal ligament and the supraspinous ligament (that run from the poll to the sacrum along the topline of the neck and the back) also helps to raise the center of gravity further to increase height because the head and neck are extended forward and down.
Just after the bascule of the jump, the head and neck start to come up which shifts the center of gravity back. This acts as a reflex to flex the hind limbs in order to clear the jump and to extend the forelimbs in preparation for landing. As the forelimbs extend for landing, the abdominal muscles contract to help lift the back while the gluteals and other hip flexor muscles flex the hips, hock and stifle to clear the jump.
The front limbs take the full weight of the horse as it lands. The thoracic sling (especially the pectoral muscles) plays a crucial concussive role that protects the musculoskeletal system as well as the vital organs encased in the ribcage.
The horse must lift his head and neck up to shift the body weight back and slow down the rotation of the body axis, thus ensuring good balance on landing. The trailing fore limb tends to be perpendicular to the ground (it lands first because the horse is cantering) and absorbs most of the impact (up to 2 ½ times the body weight of the horse). The suspensory ligament and the deep digital flexor tendon are stretched to the extent that the fetlock often touches the ground. Strain is put on the navicular bone via the deep digital flexor tendon as the horse lands on his heel with the toe turned upwards. The trailing forelimb quickly rebounds off the ground.
As the front legs come to land, the thoracic sling along with all the muscles of the forelimbs contract to brace the leg and support the joints. As the forefeet touch the ground after the jump they push the body up into the next canter stride. This reverses the rotation of the body axis and allows the hind legs to step under the body to canter on.
Jumping at the high levels of competition is especially demanding on the horse’s musculo-skeletal system. Excessive muscular tension in any of the structures mentioned in this review of the biomechanics of jumping disables the normal muscular ability to fully contract and fully relax. Without the normal ability to contract and relax, the muscles are less effective in acting on the skeleton to move the body as required for performance. An unhealthy musculature can result in compensatory patterns of movement which increase the strain on the musculo-skeletal system of the horse: joints, muscles, tendons, and ligaments.
Gillian Higgins, How your Horse Moves
Sara Wyche, Muscles in Motion
Slow motion jump videos