Reciprocal Apparatus - Overview

The reciprocal apparatus of the equine hind limb is a unique anatomical feature that facilitates the movement of horses, particularly during locomotion. The arrangement is brought about by the peroneus tertius tendon on the front of the hind limb, which runs from the lower femur & inserts into the front of the hind cannon bone (normally a hock flexor) & the Gastronemicus (normally a hock extensor) & the superficial flexor tendon on the back of the limb, which run from the lower femur and insert into the tuber calcis or point of hock. The opposing action of this reciprocal system, allows for flexion & extension of the stifle & hock to be in unison while aiding in the stabilisation of the stifle & hock joints when the limb is firmly planted on the ground.

This mechanism also plays a vital role in allowing the horse to sleep standing up. This forms part of the horse's "stay apparatus". When the horse's stifle locks into position the hock is also stable and immobilised. This position can be maintained causing no muscle fatigue, enabling the horse to rest. As a horse falls asleep while standing they will normally rest one hind limb with the toe on the ground, this initiates the stay apparatus of the opposite hind limb.

Summary

In summary, the reciprocal apparatus of the equine hind limb is a sophisticated system that enhances a horse's locomotion by synchronising the movements of key joints, contributing to the horse's agility and efficiency, whilst allowing the horse to sleep standing up.