The stay apparatus in horses is a remarkable adaptation that allows them to stand and even rest with minimal muscular effort by “locking” their limbs through a coordinated system of tendons, ligaments, and muscles.

In the forelimb, it prevents flexion of the shoulder, elbow, carpus, and fetlock. The biceps brachii tendon with its lacertus fibrosus connects to the extensor carpi radialis, resisting shoulder and elbow flexion. The suspensory ligament, distal check ligament, and branches of the digital flexor tendons stabilize the fetlock and pastern, preventing overextension. Flexor tendons further reinforce joint stability.

The hindlimb stay apparatus is more complex, relying on two unique features: the reciprocal apparatus and the patellar locking mechanism. The reciprocal apparatus, formed by the peroneus tertius and superficial digital flexor, ensures that the stifle and hock move together, locking the limb in extension. The patellar locking mechanism occurs when the medial patellar ligament hooks over the medial trochlear ridge of the femur, securing the stifle and hock in a weight-bearing position with little muscular effort.

The “Bow and String” mechanism specifically supports the distal limb. The cannon bone acts as the “bow,” while the suspensory ligament, superficial digital flexor tendon, deep digital flexor tendon, and their check ligaments form the “string.” When weight is applied, this system becomes taut, preventing overextension of the fetlock, pastern, and coffin joints. This spring-like support absorbs shock, conserves energy, and enables horses to endure prolonged standing and high locomotory stress efficiently.