Passive stay apparatus

The passive stay apparatus is a mechanisms to take the weight of the relaxed standing animal that conserves energy which would otherwise be expended is using musculature and is achieved in the forelimb by a series of tendons, ligaments and fascia. The horses distribution of body weight in the forelimb can be described by a vertical line which passes through mid serratus ventralis, caudally to the shoulder, through the elbow joint and cranially to the carpus, metacarpophalangeal and interphalangeal joints. The biceps brachii tendon travelling from the supraglenoid ubercle cranial to the intertubercular groove via the intermediate tendon locks the joint by inserting medially on the radius close to the axis of rotation of the elbow and being pulled by lacertus fibrosus and extensor carpi radialis to prevent buckling and joint collapse. The architecture of the carpus is closely packed with a strong plantar ligaments preventing overextension of the carpus as well as the abaxial and axial ligaments of the proximal interphalangeal joint and the distal sesamoidean ligaments in the distal interphalangeal joint. The superficial and deep digital flexor tendons tighten acting as check ligaments. All these ligaments are countered by the extensor branch of extensor digitorum communis.

The horses distribution of body weight in the hind limb can be describe by a vertical line which travels caudally to the stifle joint and cranially to the tarsal, metatarsophalangeal and interphalangeal joints. The stifle joint is locked when the patella is in resting position as the stifle is in extension. The patella is medially positioned when locked which is achieved by the parapatellar and medial patella ligament as well as peroneus tertius which is a cord like tendon as well as the superficial digial flexor which all work in unison. The architecture of the tarsus is closely packed with a strong plantar ligaments preventing overextension of the tarsus as well as the abaxial and axial ligaments of the proximal interphalangeal joint and the distal sesamoidean ligaments in the distal interphalangeal joint.