I agree with Sarahver. When a horse is running at that speed they put a tremendous amount of weight on their legs with each stride. I don't know much about the horse racing industry but IMO itll be hard to find an race horse that doesn't experience atleast some hyperflexion during a race. Like i said TBs usually have longer pasterns, and like Sarahver pointed out pasterns are shock absorbers. They are meant to flex like that, perhaps to not such a degree to the point where the fetlock almost touches the ground. But what do you expect with a long pastern-ed horse running like that?