It might be nice to doze off while waiting in the checkout line at the grocery store, but unlike horses, humans cannot sleep standing up. Having evolved to flee in an instant, horses are equipped with a "stay apparatus" that allows them to remain upright for long periods of time. But this mechanism isn't foolproof and sometimes it causes more harm than good.

Gerald Pijanowski, DVM, PhD, a professor of bioengineering and biosciences at the University of Illinois College of Veterinary Medicine in Urbana, notes that although horses may look like they are in a deep sleep while standing, "they really are just resting." In order to truly fall asleep, horses need to lie down either on their sternum (breastbone) or their side. This is because to go into REM (rapid eye movement) sleep, they must have relaxed muscles, which cannot happen when they are standing.

"In order to maintain a standing position in the hindlimb horses must prevent flexing of the stifle, maintain extension of the hock, and prevent overextension of the digital joints," explained Pijanowski. In short, there are several steps that happen at once to allow a horse's hindlimb to lock, and it all starts with lifting the patella (kneecap) over a bony ridge on the end of the femur (thigh bone). By activating the stay apparatus in one hind leg, that leg can bear the brunt of the hindquarter weight, allowing the horse to rest the opposite leg with just the tip of the hoof resting on the ground.

While the idea of a human popping their kneecap out of place to rest standing sounds painful, it certainly does not seem to bother horses.

But this mechanism, which, in most cases, is a normal activity for horses, can become a problem. Upward fixation or in layman's term, a "locking patella," can be seen in horses who accidentally lock their patella into place. "This happens particularly in ponies," said Pijanowski, "where they can pathologically lock both stifles at the same time and are unable to move."

In some horses recovering after orthopedic surgery, upward fixation of the patella spontaneously develops. The most recent study report involved a horse receiving post-operative morphine, with no previous history of upward fixation until after surgery. Once the morphine was discontinued, the horse's patellas unlocked.

Horses that have accidentally locked their patella in place and cannot undo it will present with an immovable extended hindlimb. If the problem is less severe, the locking might come and go and riders might notice subtle signs such as a bouncy canter or lead changes.

If you have questions about your horse's stay apparatus, or are concerned that upward fixation of the patella may be occurring, contact your local veterinarian.--Ashley Mitek

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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