Overview of Passive Stay Apparatus -Thoracic Limb
Left thoracic limb- lateral view

What is the function of the stay apparatus?

The horse uses its thoracic limb stay apparatus to support the weight of the front end of its body while using a minimal amount of muscular activity.   The stay apparatus helps prevent fatigue of the limb muscles by reducing the amount of energy required to keep the limb stable.

How does the stay apparatus function?

The stay apparatus transfers weight from the thoracic limb muscles to connective tissue structures that do not tire, namely tendons, ligaments or bone.  With the weight shifted from muscle to connective tissue elements or bone, the horse requires less muscular activity to keep the limb stable and hold up the front end of the body.
The stay apparatus helps the limb resist gravitational forces that would otherwise cause the thoracic limb joints to flex and allow the body to collapse to the ground.