Many  “behavior problems” are the horse trying to communicate about pain or tension in the equine body. Hesitation , refusal. rearing, bucking are ways the horse can communicate.
Stress contributes to muscle tension, and such tension- acute or chronic can  lead to injury due to inhibited range of motion.  The horses muscle's provide the earliest indication of anxiety

It is important to recognize that emotional equilibrium is as vital to optimum performance as the physiological readiness of the mechanical structures,  unless the horse is comfortable and confident he can not perform to his full athletic ability. .