Maybe you're thinking about walking or cantering. Trotting involves a constant up-down motion, using your thigh muscles and core extensively in order to keep in rhythm with the horse. Even cantering is a fair amount of exercise if you're a real rider. You barely use the reins - it's all thigh squeezing to direct the horse - and you have to stay centered and slightly holding yourself up and away from the horse in order to have equilibrium. Throw in jumping and you have a whole-body workout.