There should be no protrusions on the inner rail of a riding arena.? Otherwise, it's easy to hook a heel, knee or stirrup and cause serious injury to the rider?s leg. If the rider?s leg just touches something hanging and starts it swinging, it could also spook the horse.? Ring posts should always be on the outside, with the rails lining the inside.? This is true even for pipe arenas with very thin posts, because a rider?s toe can easily hook any post inside the rails.