Horses are herd animals who take their cues about potential dangers from their herd mates and the head horse. Since most horses no longer live in a herd situation, they transfer head horse status to their riders. And they are incredibly sensitive to minute changes of the rider’s weight and tension in the rider’s body. A tense rider can be accidentally telling the horse that there is something potentially dangerous in the immediate vicinity just by tensing up. It never occurs to the horse that the rider is reacting to what the horse is doing, because the head horse would not do so. Learning to control body tension on a tense horse is not easy but doable. Joe-Joe’s suggestion about a calming agent for the rider could actually help, but you will want to try it out off the horse before you try it out on the horse. You can also teach yourself to relax muscle groups off the horse as well. Most of us carry a lot more tension in our bodies than we realize.