A system of bones, tendons and ligaments within the horse's legs lock the limb into a resting weight bearing position. Normal horses can doze on their feet, exerting minimal muscular effort. This system of ropes and pulleys is known as the "passive stay apparatus" in the front limb and the "reciprocal apparatus" in the hind limb. Normal spinal and nerve function are also required for this system to function properly.

If any component of this system is damaged, then limb function is compromised and the horse might collapse when they bear weight. Loss of this function usually causes a horse great distress as they repeatedly try to bear weight, only to have the limb collapse under their weight.

I differentiate this from a non-weight bearing lameness. In this case, it is the lack of structural integrity that is causing the horse distress. Pain may be playing a role but it is less this than the loss of weight bearing ability. Examples of injuries that can cause this collapse to occur is radial nerve paralysis in the forelimb, sciatic paralysis or the severing of the gastrocnemius muscle in the hind limb.