How do you desensitize a horse to bridges, elevated surfaces, and unusual footing?

Bridges, elevated wooden platforms, metal grates, and unusual footing surfaces are common obstacles on competitive trail courses and practical challenges on real trails, and desensitizing a horse to them requires addressing two separate concerns: the visual and auditory stimulus of the unusual surface and the proprioceptive challenge of walking on footing that feels different under the horse's feet. Clinton Anderson's approach begins with a ground-level platform — a wooden board or low pallet — before introducing anything elevated. The horse is directed across the flat board using his Sending Exercise, with the handler behind the drive line directing the horse forward while the horse figures out the footing on its own. Anderson specifically does not pull the horse across unusual surfaces, because a horse that is dragged across something frightening is not learning that the surface is safe — it is being forced across something that remains frightening. As the horse becomes comfortable walking across flat boards, slightly elevated surfaces are introduced. The sound changes — a wooden bridge sounds dramatically different from solid ground — and Anderson allows the horse to stop and process the sound difference before asking it to continue forward. The hollow sound under the horse's feet, the slight flex of a wooden bridge, and the visual of looking down through gaps in the decking are each addressed as separate stimuli if the horse shows concern about any of them. For metal grates — common on ranch gates and some trail obstacles — the visual of seeing through the surface to the ground below is often more alarming than the footing itself. Anderson approaches this by first allowing the horse to investigate a grate laid flat on the ground, then walking it across, before introducing grates that are elevated enough that the visual drop is apparent. Parelli uses his Squeeze Game framework for bridge desensitization, treating the bridge as a narrow passage through which the horse builds confidence with the Squeeze Game before the unusual footing aspect is addressed.