The hindquarters have nothing to do with race track training or TB's. Most TB racers have a strong hindquarter bc that is what gives them the speed and not all TB's have non musclesd hindends as a trait. it all depends on the horse, and it's been ridden and how it's learned to carry it self. working on collected halts and trotting will def help build the muscle in that area.
"The horse you get off is not the same as the horse you got on; it is your job as a rider to ensure that as often as possible the change is for the better."