STEVE: Shivers are a hyperreflexia, meaning that when the hind limbs in particular, are brought up, they tend to constantly flex and extend, flex extend, flex, extend, repetitively. It’s a nervous disorder, the origin of which is not really well understood. Basically it doesn’t really hurt a horse. It doesn’t really affect performance, but it sure can give your blacksmith fits with his back if he’s got a bad back.