The passive stay apparatus in the horse's hindlimb allows the horse to stand using minimal muscular energy. The apparatus uses a combination of two anatomical features: the first is the stifle locking mechanism which enables the stifle to be 'locked' (immobilised) by hooking the patella and the medial patellar ligament over the medial trochlear ridge of the femur. The second element is the reciprocal apparatus which dictates that the stifle, the hock and the fetlock joints always flex and extend synchronously, so immobilisation of the stifle will also lock the hock and the fetlock in position. This system allows the horse to rest a large portion of the weight of its hindquarters on the 'fixed' hindlimb and rest the collateral hindlimb.