Or a more creative use of them:
Run a wire along the sharp ridge of the horseback and electrify it. Replace the foot blocks with spring-loaded blocks that must be pressed down in order to keep the circuit open. One block to the pussy wire, the other to nipple wires. The toes must be pressed against the blocks with some force to avoid the current. In this way the rider is forced to bury the sharp ridge as deeply as she can within herself. At the same time, it strains her calf muscles and may lead to cramping.
If she eases off of either of the blocks to release the strain or adjust her position, the current flows. If she loses contact with both, it may be a while before she can get them back under control. It would be more like watching her try to stay atop a bucking bronco with pleasant periods of rest in between, as opposed to clomping slowly down a bridle path.