Horses do some of their sleeping standing up, because they are prey animals, and (originally) needed to be able to run from predators in a heartbeat.
This is enabled by the 'stay apparatus' which locks their limbs when they are unconscious and upright.
It's also an energy-saving mechanism, in that the 'stay' is supported by tendons and ligaments that don't require blood supply, whereas a horse in motion is using muscles, which use vasculature, burn energy, and give them explosive power.
It's very structurally elegant, and part of what makes them so magical.
As long as it's working the way it's supposed to.
This intricate system of ligaments and tendons essentially locks the horse's limbs in place, allowing them to rest without exerting unnecessary energy. It relies on the joints stacking directly above one another.
Understanding the Function
"In the lower leg, the stay apparatus is similar between forelimbs and hind limbs. The pastern and fetlock are supported by the suspensory ligament, the sesamoidean ligaments, and the flexor tendons. The upper forelimb is supported by a complex system of tendons, muscles, and collateral ligaments. In the hindlimb, the patella can lock so that the stifle remains extended, while tendinous structures link the stifle and hock in such a way that they can only move in unison. Therefore if the stifle is locked in extension, the hock will be also. This unique anatomical adaption allows horses to rest without actually lying down."
The Stay can be disrupted by:
Injury or trauma: Damage to muscles, tendons, or ligaments involved in the stay apparatus can impair its function.
Arthritis: Joint degeneration can affect the stability and flexibility.
Neuromuscular diseases: Conditions affecting the nerves or muscles can disrupt the coordination required for the stay apparatus to function properly.
Pain: Pain from other areas, such as the hooves, back or pelvis, can cause the horse to avoid using the stay apparatus.
Overuse or fatigue: Excessive strain or fatigue from prolonged standing or intense exercise can lead to temporary or permanent dysfunction.
Congenital abnormalities: Some horses may be born with structural issues that affect the stay apparatus.
Infections or inflammation: Conditions like tendonitis or joint infections can compromise the integrity of the stay apparatus components.
Conformation in Action
When a horse stands still, the stay apparatus helps distribute their weight efficiently, reducing strain on muscles and minimising fatigue. If the horse is unable to stand square, either because of conformation faults, or a pain-stance, like a box-stance (cant-in stance), or splayed-out, they will be using muscles to maintain their posture rather than tendons and ligaments. This causes more muscle strain, is fatiguing, and so they may lie down more, have swelling or heat in their lower limbs, develop joint pain earlier, and/or have trouble maintaining weight, because they are expending excess energy.
You can see when a horse is standing square and the stay apparatus is engaged they are 'level headed' which the origin of the expression meaning balanced, dependable, even-tempered, rational or sensible.
If the horse is box-stance or splayed-out they may be trying to avoid weighting either the front or back of the hoof. That would be a good first place to start.
Bear in mind this splayed-out stance is selectively bred-for in some breeds like American saddlebreds and morgans, but if your horse is not one of those breeds - it's unusual.
Importance of Proper Hind Leg Conformation
This is a great article that talks about hind leg conformation. It discusses what to look for when assessing the hind limbs and the consequences of different faults. Definitely worth a read.
This article argues that one muscle - vastus medialis, which is a knee muscle, is active when the stay apparatus is activated. It has implications for patella locking, so worth a read of patella lock is an issue for your horse.
Scaling
Absent human intervention, wild horses in Australia (brumbies) are generally between 13hh and 14hh. Some American mustangs reach up to 15hh but they generally have a short life. Scale matters. Larger and smaller horses tend to have less efficient biodynamics and are subject to more frequent lameness.
My equine biodynamics teacher taught me - if a horse was a bridge, with 4 upright supports and a centrally-located load that requires bearing - there are limits in height before you would need to add supporting struts.
Knut Schmidt-Nielsen Cambridge University Press 1984
This is a whole book about the scale of creatures. Animals must live within the boundaries set by the laws of physics and chemistry on this planet.
"Physical laws provide possibilities and opportunities, yet they impose constraints and set limits as to what is physically possible."
Effect of differences in tendon properties on functionality of the passive stay apparatus in horses
This article looks at the stay apparatus in miniature horses.
If your horse is not engaging the stay apparatus properly, the first port-of-call would be an experienced bodyworker like an equine chiropractor or osteopath.
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