Despite what you may have heard, it is unnecessary to stand in the stirrups when the horse you are riding urinates. This enduring myth is based on the notion that a horse's kidneys and/or bladder should bear no weight as he urinates. But those structures are located deep beneath layers of muscle and bone, far beyond where the saddle rests, so a rider's weight has no effect on them. In theory, it may be easier for a horse to stretch out to urinate if his rider stands, but sitting very still may have the same effect. In the end, the only compelling reason to stand may be to avoid rebukes from fellow riders who mistakenly think you must.