Power Failure

What might cause a loss of hind-end propulsion? Dr Charlotte Oastler MRCVS explains

THE horse’s hind-end is often said to be his engine, providing the energy and impulsion that is required for forward movement.

Both the fore and hindlimbs share the role of moving the horse across the ground, through a combination of synchronized muscle contractions causing joint flexion and extension. The forelimbs carry the majority of his weight and have an essential role in weight-bearing and shock absorption, particularly in jumping. The hind-end, however, provides the bulk of the power to push the horse’s entire body forward.

There is a much greater muscle mass in the hindlimbs in order to achieve this, with these muscles working to support and move the skeleton. In addition, the tendons within the limbs are able to store energy and reuse it to push the horse off the ground.

THE WORKING ENGINE

THE horse’s hindlimbs essentially work as a series of levers, with the joints — such as the hock and stifle — acting as the fulcrum (balance point). The muscles provide the force to move the lever (in this case, the limb).

When a muscle in the upper limb contracts, its associated tendon pulls on the lower joint, resulting in the lower limb moving backwards and pushing off the ground. This strong force at the joint allows the lower limb to be moved incredibly fast and to its maximum extent, as anyone who has ever been on the end of a double-barrelled kick would agree.

Additionally, the long bones of the upper hindlimb — the femur and tibia provide long lever arms, making movement more efficient.