The stay apparatus first appeared in the modern horse’s ancestor dinohippus, 5 to 13 million years ago. This unique evolutionary trait enables the horse to sleep standing up. Horses actually require very little sleep, and even less REM sleep, which is probably a good thing because REM sleep can only be achieved when the horse is lying down and its muscles are completely relaxed.
Horses have evolved to sleep standing up so they can flee dangerous situations immediately, without having to expend the time or energy required to get up from the ground. The biomechanics of the stay apparatus are as follows.
The Stay Apparatus in the Front Limbs
The long combined tendon holds the shoulder and knee in steady with little muscular effort while standing. Some effort from the fatigue-resistant parts of the triceps muscles is needed to steady the elbow, while the knee locks in a straight position and the long tendons of the suspensory apparatus support the fetlock in its bent resting position.
The Hindlimb Stay Apparatus
The 3 components of the hindlimb stay apparatus are:
- the stifle locking mechanism (green bands in below image)
- the reciprocal apparatus which locks the hock when the stifle is locked (illustrated in dark red in below image)
- and the suspensory apparatus, which supports the fetlock
The Stifle Lock
The patella (kneecap) is embedded in the tendon of the quadriceps muscle which straightens the stifle. Below the patella the tendon has 3 parallel straps. The horse can lock the stifle by looping the patella and 2 of the straps over a prominence on the end of the thigh bone.
Once the stifle is locked, the reciprocal apparatus locks the hock as well because of the two tendinous muscles of the reciprocal apparatus.
With a locked stifle and hock, and the fetlock supported by the suspensory apparatus, the horse can stand on one hind leg with little effort.
Biggs Waller, S. (2013). “Asleep on His Feet.” The Horse Dec. 2, 2013. Retrieved from http://www.thehorse.com/articles/13971/asleep-on-his-feet
Florida Museum of Natural History. Dinohippus, Florida Museum of NaturalHistory. Retrieved from http://www.flmnh.ufl.edu/fhc/dinohippus1.htm
Schuurman, S.O. et al. 2003. “The equine hind limb is actively stabilized during standing.” Journal of Anatomy Vol. 202(4) 2003. P.355-362
The Atlanta Equine Clinic. “The Stay Apparatus of the Pelvic Limb.” Retrieved from http://www.atlantaequine.com/pages/client_lib_pstayapparatus.html
University of Guelph. 2016. Functional Anatomy, Week 8 Presentation: Musculoskeletal System – The Legs
University of Illinois College of Veterinary Medicine. 2009. “Horses’ Sleep ‘Stay Apparatus’ Can Cause Limb to Lock.” The Horse Aug 11, 2009. Retrieved from http://www.thehorse.com/articles/23936/horses-sleep-stay-apparatus-can-cause-limb-to-lock