What causes a loss of hind-end propulsion? *H&H Plus*

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 synchronised 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. [subscriber-content]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. When the horse’s foot strikes the ground, the leg becomes weight bearing and the muscles of the limb start to contract. This in turn flexes the joints of the hindlimb. The tendons within the limb store elastic energy, like a compressed spring. The horse uses this stored energy to extend his hip, stifle, hock and fetlock, moving himself off the ground and propelling himself forward. This can be best visualised in a horse in extended trot, or taking off over a large fence. At the final, propulsive part of the movement, the limb is in full extension and the full force of the movement pushes the body forward. A horse performs this action constantly; however, problems may develop that reduce his ability to move with maximum efficiency. If a horse lacks propulsion, he will usually move on his forehand and give the rider the feeling of riding downhill. He may also appear to be pulling himself along with his forelegs, rather than pushing through from the hindquarters. The rider may feel the horse is moving unevenly behind, maybe more so on one rein. The horse may struggle to strike off on a certain canter lead, or “bunny hop” with his hindlimbs rather than stepping through clearly to track up. This is often more pronounced when stopping or collecting, when he should be bringing his hindlegs underneath him. A horse may also uncharacteristically begin to knock fences down, particularly with his hindlegs. This is perhaps more noticeable in combinations, where significant repetitive power is required to clear the second obstacle so soon after the first. Muscle matters Problems with hind-end propulsion can stem from numerous locations within the body; the most common reason can be a lack of hindquarter strength, especially in an unfit or young horse developing muscle. As the horse builds muscle mass, he gains greater inherent power for forward movement. Good conformation also plays a significant role; a horse born with a larger pelvis will have the structure to support greater musculature. And correct conformation enables placement of the hindlegs well underneath the body, allowing balanced forward propulsion. Unfortunately, inadequacies of conformation can compromise this movement. A horse with straight, cow or sickle hocks, for example, will find engagement difficult and inevitably struggle to push with the same power as a horse with better conformation. In some cases, pain may prevent a horse from using himself properly and creating the propulsion required for effective movement. Any musculoskeletal pain originating from the back, sacrum, pelvis or limbs can affect the way a horse uses himself and therefore the power he can produce from behind. Narrowing down the cause will usually involve a thorough lameness examination. Your vet will look for any swelling, pain or loss of muscle, particularly around the horse’s back and pelvis, perhaps using flexion tests and watching him work on different surfaces — both in straight lines and on a circle. Consideration of movement, both in-hand and ridden, is essential for assessing symmetry of movement and establishing any overt lameness. Assessing saddle fit may also assist in the investigation. Nerve or joint blocks can help determine the location or significance of any abnormal findings. Imaging can also help clarify the issue, be it an X-ray revealing arthritic changes, an ultrasound scan showing soft tissue injury or scintigraphy (bone scanning) to identify an area of significance within the pelvis or elsewhere in the hindquarters. Imaging of the pelvis, sacroiliac joint and femur are difficult due to the large muscle mass in this area, so scintigraphy and ultrasound are often the best way forward. Building blocks Treatment depends largely on the inciting cause. If a specific joint pain is identified, for example, anti-inflammatories may be injected into the affected area. If causes of pain are ruled out or addressed, correct training and strengthening work can be used to improve power, range of motion and flexibility. Polework, cavalletti, water treadmills, hillwork and lungeing can all help to develop hind-end power. Muscle gain also requires appropriate nutrition, so feeding a balanced and high-quality source of protein will ensure the building blocks are in place to develop the musculature needed for work. The hind-end is perhaps the most essential element in the creation of forward movement, so a problem in any component can lead to poor performance. Although in many cases correct work and nutrition can improve hindquarter strength and propulsion, continued difficulties may indicate pain. Working together with your vet will enable any underlying problems to be identified and addressed. When Billy Lemon lost her zing After slipping on a corner in a 1.50m jump-off, Billy Lemon caught the top rail of the following fence. “She immediately lost power — it just didn’t feel like her as she pushed off the ground,” says Alice Watson of her Cevin Z mare, now 14. “She seemed OK, but at the next show she had a few fences down and I knew something was wrong. Unfortunately, it took two years to find out what.” It was equine vet Rory O’Shea who eventually diagnosed a ruptured cruciate ligament in her left stifle, which was tidied up by arthroscopy (keyhole surgery) and treated with an adipose tissue graft. Alice feels that Billy Lemon’s stoicism made diagnosis more difficult. “She is very ‘busy’ behind and her hindlegs are like pistons at trot, so it was hard to see or feel the difference,” she says. “She was lame in both by the time she was diagnosed, although one was worse. Her jumping had felt such an effort; it was her heart that took her round.” With power restored, Billy Lemon has bounced back to form — finishing second this summer in a 1.45cm four-star class at Hickstead. The author: Dr Charlotte Oastler MRCVS, of Emerson & Watson, has a particular interest in performance horses and equine medicine. The practice covers Berks, Bucks, Oxon and Surrey, with vets carrying equipment to provide a mobile service 365 days a year – for patients ranging from children’s ponies to elite competition horses. emerson-watson.com 01344 937937 Ref Horse & Hound; 28 November 2019 [/subscriber-content]