The stay-apparatus is a collection of tendinous ligamentous structures in the forelimbs and hind limbs of horses which help keep the limbs from collapsing and allow the horse to remain standing with a minimal amount of muscular effort. In the lower leg, the stay apparatus is similar between forelimbs and hind limbs. The pastern and fetlock are supported by the suspensory ligament, the sesamoidean ligaments, and the flexor tendons. The upper forelimb is supported by a complex system of tendons, muscles, and collateral ligaments. In the hindlimb, the patella can lock so that the stifle remains extended, while tendinous structures link the stifle and hock in such a way that they can only move in unison. Therefore if the stifle is locked in extension, the hock will be also. This unique anatomical adaption allows horses to rest without actually lying down.