What does Anderson teach about horses that kick out with a hind leg when leg pressure is applied?

A horse that kicks out with a hind leg when the rider's leg is applied — a quick swish-and-kick rather than a full buck — is expressing objection to leg pressure, and Clinton Anderson identifies two distinct causes that require different responses. The first and more serious cause is pain. A horse that kicks at the rider's leg when it is applied near the flank or girth area may have ulcers, flank sensitivity, or soreness that makes the leg pressure painful rather than merely unwanted. Anderson treats this as a veterinary concern first — a horse that suddenly begins kicking at leg pressure that previously produced no such response should be evaluated for gastric ulcers, which are common in performance horses and which can cause precisely this response when leg pressure touches the tender flank area. The second cause is learned disrespect for leg pressure — the horse has discovered that kicking out removes the leg, because the rider pulled the leg away when the horse kicked. This is the same pattern as any other learned avoidance: the horse tried something, the rider's response confirmed it worked, and the horse continues using it. Anderson's correction requires keeping the leg on when the horse kicks — maintaining contact or reapplying immediately after the kick — combined with strong reinforcement for forward response. The horse learns that kicking does not remove the leg but that moving forward does. He is careful to distinguish between the two causes because applying the training correction to a horse that is kicking from pain makes the pain situation worse and typically produces escalating behavior. The assessment question is straightforward: did this behavior appear suddenly after years of normal response to the leg? If yes, investigate pain first. Has the horse always kicked at the leg and the rider has been removing their leg in response? If yes, address the training.