This horse might not require as much galloping in training, so he might be able to hold up despite his conformation. The too-lazy horse won't succeed in reining, as he'll resist the fast work. The horse has to stay fresh on the patterns, willing to turn on the spot and never anticipating the stop. He has to be very willing and very forgiving to tolerate that amount of control. Soundness The reining horse must be physically tough.
Trainers gallop horses a great deal in training, in circles, figure eights, and rundowns.
The horse which is bench-kneed or back at the knee won't hold up to the stresses of reining. We see concussion-type injuries on the fetlock and knee. The forehand absorbs the strain if the horse doesn't get into the ground to stop. Circles also can lead to problems in the front end in a horse which doesn't have correct conformation. We see knee chips on those horses.