A sloping shoulder helps the horse carry itself and the rider's weight through the maneuvers. A substantial forehand aids the horse. A long neck helps the horse move in balance. A horse that carries his head naturally very high will not be competitive as a reining horse. Long pasterns and weak fetlocks can limit the horse's ability to turn hard or slide.
Trainers gallop prospects for months in early training, and horses take a lot of pounding.
A lot of reining horses eliminate themselves early on, even if they have the ability and the try, because they can't hold up. The reining horse needs a strong back and loin, with a low tail set. The hindquarters help him to get deep in the ground when he stops and pivots. Bartlett noted that he likes to see a horse which stands wide in the stifles. The stifles need to be strong, and you can't have a lot of laxity in the stifles.