Or How the Stay Apparatus is for More than Sleeping While Standing
The horse works a lot like a mechanical puppet. The bones and muscles act like levers to power movement at a distance through tendons and ligaments that activate the lower limb. All the gears and power are centered close to the body and the extremities are controlled remotely by cables. But some of these cables are designed to be springy enough to act as energy stores to automate certain moments of movement, maximizing speed and efficiency.
This keeps the horse nimble and much lighter than it would need to be if it did it all through brute force.
I had only heard of the stay apparatus in the context of allowing the horse to sleep standing up but in my 'satiable curiosity I noticed people mention how it actually does more than that. So I got curious and started down the rabbit hole. This is what I found.
There are 3 intertwined mechanisms that automatically stabilize the limbs and store energy:
the Stay Apparatus, the Reciprocal Apparatus in the hind limb, and the Suspensory Ligament.
These systems allow the horse to avoid using muscle.
The key actors are the Biceps Brachii, the Peroneus Tertius, the Gastrocnemius, the Deep Digital Flexors (DDF), the Superficial Digital Flexors (SDF) and the Suspensory ligaments. These have developed to be more fibrous in one way or another to be able to withstand great tensions.
The Stay Apparatus on Front and Hind Limb
A horse can use the mass of his trunk as a counterweight to lock all its limbs in place so efficiently that it can sleep that way.
For the front limb, as the horse relaxes, the weight of the torso pulls on the Serratus Ventralis which holds the front limb to the trunk. This is all it takes to lock up all the joints. As the joints are stacked at opposing angles, there can be no collapse and the horse can rest peacefully.
The collateral ligaments (a series of bands) of the elbow are set further back on the horse than on most animals. The elbow defaults to an extended position and must be willfully flexed by
the horse. The tension from the weight of the trunk will keep the limb straight through the fibrous band of the Biceps Brachii and a series of ligaments including the check ligaments whose only job is to protect the joints from overextension. The suspensory ligament goes from the knee to the toe. In other animals it is a muscle, but on a horse it has become completely fibrous and serves to protect from overextension.
It is literally a cable. It just holds.
As nature would have it, there is more than one use for this energy saving mechanism.
The stay apparatus also means that the horse conserves energy while moving. With each step the horse is spring-loading muscles and ligaments from those same systems that will activate upon release and effortlessly move the limb with more speed and power than the muscle alone could provide. This allows the horse to keep the big muscles nearest its center of mass and keep its extremities light.
More weight on the end of a long limb adds wear and tear on everything above it. Just like wearing heavy shoes for a jog, the whole body has to compensate.
The suspensory ligament acts as an elastic spring that supports all the fetlock joint. There is no muscular component to adjust the length of the suspensory ligament. As the horse moves, the fetlock extends and stretches the ligament. This allows the fetlock to sink under the horse’s weight, absorbing shock, and then spring back as the horse moves off the leg.
In the same way the deep digital flexor in the front and hind limbs will stretch up to a max in the mid stance and create a driving force in the second half of the stance.
The hind end doesn't lock as efficiently as the front and so you will see the horse shift its weight occasionally as one leg fatigues.
The patella can be locked into position by a ligament managed by a thigh muscle when the horse decides to rest. The Vastus muscle pulls the ligament over the inner ridge of the stifle joint which engages the reciprocal mechanism and the suspensory ligament to keep the whole leg in position.
This truly locks the leg and can cause problems when things go wrong in the joint. It must be unlocked by shifting the weight to the other leg to unload and allow the ligament to then be pulled back to center and the patella will slip back into its groove.
In motion, the reciprocal apparatus's job is to keep the stifle and the fetlock in synch without spending any energy. The stifle and the hock stay locked in parallel to each other when the hoof lifts from the ground.
This means that the muscles that act on the stifle also act on the hock. This allows for more and greater muscles to be involved in the movement. Each segment of muscles involved adds to the overall power of the movement, all while keeping the motion harmonious. This is called the ‘summation of velocities’ where each set of muscles adds power to the next and creates a greater force than can be made by just one set. This requires a perfect synchronization. In human athletes this is the flow that a boxer seeks from his foot to his fist or a baseball player using his whole body to throw a ball.
Locking the stifle to the hock allows for greater accuracy while this force is applied, The horse controls very well where his hind feet go and there is the stability in the joint to follow this through. The horse can also see things closer up from the bottom half of the eye and with a slight tilt of the head has the hind in full view. This is why horses prefer to defend themselves from the rear rather than the front end. They can see more back there and have the accuracy to act precisely. They have evolved for this as they are built to run away from predators with that powerful hind end but also to kick the attacker off before it has a chance to reach its more vulnerable areas, such as the throat and the back.
The horse's limbs are a series of springs and cables that are activated at a distance by motors located near the center of mass. Refined mechanisms have evolved to maximize precision, strength and energy. This creates a very efficient, powerful and fast system of interactions that give the horse the ease of motion and grace that has been moving the hearts of humans for a very long time.
The Dynamic Horse by Hilary M. Clayton
Warning - graphic content but so very interesting: