Last year at university I didn't choose to take the anatomy module as I find anatomy quite hard to memorise. Those that did take the module had to write an essay on "the stay apparatus" and I thought it might be an interesting topic to talk about this Sunday.
The stay apparatus is why the horse can rest whilst standing. This is present in both the front and hind legs and is due to a number of muscles and ligaments that lock the leg into position and hold it there. This mechanism is slightly different in the fore and hind limbs but the basic idea behind it is the same for both.
The tendons and ligaments
The horses' leg below the knee or hock is supported by three elastic ligaments at the back. The suspensory ligament supports the fetlock and pastern joints. The superficial flexor tendon supports the fetlock and pastern joints. And the deep flexor tendon supports the fetlock, pastern and coffin joints.The superficial and deep flexor tendon are both supported by check ligaments. All three of these tendons work in a series, with the body weight pressing down through the joint, the fetlock joint moves down. The suspensory ligament tightens first, then the superficial flexor then the deep flexor and these hold the leg in place.
The front legs
In the front legs, above the knee, there are a number of muscles involved in enabling the horse to stand whilst resting. The serratus ventralis muscle is the major one attaching the limb to the body, it supports the body when relaxed. The weight of the body pressing down through the leg closes the shoulder joint. There are also some mechanisms involving muscles to stop the shoulder joint over flexing. The knee is able to weight bear without extra effort in its normal position. This is due to the vertical line going through the radius and large metacarpal bones. The knee is prevented from buckling forwards due to an inelastic tendon inserted onto the large metacarpal bone.
In the hind legs there is a similar mechanism to stop the hock becoming too flexed. The greater the weight going through the limbs there is a greater tendency to flex the hip and the tighter it will lock. Little or no muscles are needed to maintain this posture. There is also a mechanism in which the pelvis locks on one side in order to allow the hind leg on the other side to rest. It is said that horses still need to rest their legs despite the stay apparatus as the amount of energy used is reduced but muscular effort is still required.
The stay apparatus in foals
When looking at newborn foals the stay apparatus has been looked at. Foals are born at a more advanced stage of development than human babies, it is also thought that they have an advantage over human babies when standing as they have four legs and not two. However, it is also thought that the foal may have the ability to have automatic limb control whereas the human baby needs to learn this. If the muscles in the legs failed to maintain a state of contraction then the foals collapse to the floor. It suggests that a moderate level of contraction needs to be maintained at rest in order to stay standing. The stay apparatus becomes better as the foal grows and develops.
The stay apparatus is not controlled by muscular control alone, and it is not entirely automatic. Balance is also involved and this includes the central nervous system. Reflexes are also involved.
Once the legs have locked into place, only limited muscle use is required. In order for them to unlock, the femoris muscle contracts to lift the patella up off the ridge it is resting on.
A lot of this information is from the book Horse Structure and Movement by Symthe and Goody. This is a really good book if you want to learn more about the anatomy of the horse and how it effects their movement.