What you do need to concern yourself with is whether he carries himself straight. No other physical competency is anywhere near as important as straightness. The rider's prime task is to learn to identify crookedness by feeling it and seeing it in the horse's body, and then to eliminate it by teaching the horse to move straight. Before any horse can be straightened, he must first be calm and supple. In the absence of this, he cannot even be properly assessed.
Suppleness means range of motion at the horse's joints .
Helping the horse find suppleness is the rider's job. She can do it herself or employ therapists and veterinarians to help. When fear is added to the mix, even a horse that would otherwise have been reasonably straight and supple will present himself as stiff and crooked. There is a physical side to crookedness as well as a psychological side. I finish, however, by discussing the more important matter of recognizing and eliminating the psychological-emotional causes.