The stifle is the largest, most complex joint in the horse, and probably one of the weakest.
The bones that make up the stifle are the femur (thigh), tibia (shin), and patella (kneecap). A thin capsule surrounds the entire stifle joint that has a specialized fluid to help with shock absorption and lubrication. There are also a number of ligaments that provide stability.
In the centre of the stifle joint are two large crossing ligaments (the cruciate ligaments). These two ligaments form an X inside the joint by attaching to the femur and tibia. They prevent the leg from bending excessively but can unfortunately they can easily be torn or damaged.
While standing, the horse is able to lock his hind legs by shifting his weight and rotating the patella. This action which locks the three patella ligaments over a ridge located on the femur allows the horse to sleep while standing. However some horses are find it difficult to release these ligaments once engaged, hence the condition – locking stifle.
A weak stifle is often due to a general lack of muscle tone in the hind limbs and pelvis. This can be compounded by poor confirmation (very straight and upright hind legs), poor shoeing or trimming (too much toe or extended heels), arthritis or cruciate ligament injury.
Anatomically the stifle corresponds to the knee joint in humans. In humans it is well established that strengthening our gluteal muscles, along with our quadriceps and hamstrings plays a pivotal role in maintaining proper knee function. The same is true of the horse.
So just as a doctor will prescribe physiotherapy to a human after a knee injury or recommend pilates to someone suffering from arthritis, so our horses with weak stifles need more movement, not less (unless your vet specifically recommends box rest).
Research into stifle injuries showed that the affected horses are often clumsy, lack balance and are overweight. It was also noted that they frequently had weak abdominal and gluteal muscles. So we need to help address the horse’s balance and to tone muscle.
The more balanced, supple and flexible the horse is, the easier it will be for him to build correct muscles. When a horse becomes supple, he will relax in his work, which alleviates tension, allowing him to stretch further and become even more supple.
There are a number of exercises that can help strengthen the muscles that support the stifle these include rein-back, hill work and lateral movements.
A quality rein-back that is slow and precise will build strength in the hind end. A few even steps backwards during regular sessions will improve muscle tone in the horse’s haunches and hind legs. This exercise can be done under saddle or in-hand. However it is important that you avoid pulling on the reins to back your horse, as this will causes him to raise his head and hollow his back which in turn will restrict the hind end. Either gently raise each rein individually, or use a rope around the base of his neck to help him move more correctly. If working on the ground you can use a whip to lightly tap against his chest. A rein-back in a straight line will work on both hind legs equally whilst a rein-back on a curved line will work on the inside hind more.
Working over raised poles (cavaletti) either under saddle or in-hand is also great for weak stifles or hocks. The slow action of lifting the hind legs up and over the pole will strengthen the abdominal muscles and pelvis as well as the Tensor muscle and Long Digital Extensor which are responsible for the stability of the stifle.
Other exercises that work really well are enlarging a circle whilst retaining bend and turn on the forehand in motion.