Horse's Stay Apparatus

Passive Stay Apparatus-Pelvic Limb

Often you will hear someone refer to the "stay apparatus" in the horse. What is that? What is its function and how does it work? These structures allow a horse to rest while standing with little muscular activity and, therefore, little fatigue. The joints do not collapse because the muscles are bypassed and connective tissues take over (tendons and ligaments. This ability allows instant action when danger threatens. It is interesting to note that a horse can immediately spring into action if threatened. This locking mechanism does not put them at risk for flight.

There is a stay apparatus both in the forelimbs and pelvic region of the horse. "The horse's foreleg supports the front part of its body weight towards the centre of the top edge of the scapula (shoulder blade). If there were no stay apparatus, when the muscles relaxed the horse's weight pushing down on this point would cause the shoulder, elbow and carpal ('knee') joints to bend and the fetlock to overextend, resulting in collapse of the leg." The stay apparatus is an intriguing part of horse anatomy which enables horses to rest or doze whilst standing up.

"The horse uses its pelvic limb stay apparatus to support the weight of the caudal end of its body while using a minimal amount of muscular activity. The pelvic stay apparatus is more significant than that in the pelvic limb. When employed by one pelvic limb, the stay apparatus allows the other pelvic limb to be placed in a 'resting' position with just the tip of the hoof touching the ground. Although the stay apparatus reduces the amount of energy required to remain standing, the amount of muscular effort is not reduced. This explains why you see horses switching their weight from one hind limb to another.

How does the stay apparatus function?

The pelvic limb stay apparatus has three essential elements. The first element, the stifle joint locking mechanism, allows the weight of the caudal body to rest, essentially, on the locked joint. The second element, the reciprocal mechanism, ensures that the stifle and hock joints will move in unison, and the leg will move in a smooth, coordinated manner. The first and second elements work together. The third element involves other ligaments/tendons in the distal limb." University of Pennsylvania Education Projects

When your horse is sleeping while standing up the stay apparatus keeps the horse upright.

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