Horses, as we all know, have many special abilities. Among the less dramatic, but no less important, of these abilities is their ability to sleep standing up. Horses have a complex system called the passive stay apparatus that allows them to do this while using minimal muscular effort. One of the keys to this system is the ability to lock the kneecap (patella) in place, which keeps the stifle extended. Normally, the horse can lock and unlock the patella with no resistance.