When we humans get concerned or frightened, a flood of adrenaline can be released, and we tend to tense muscles through out our bodies. This is a problem when riding, because we need to relax and open muscles so that our body can flow with the horse’s movement & so that the energy the horse creates can flow through our body. When we tense muscles, we block the flow of energy, and instead of moving with the horse, we bounce. It is much for comfortable for the horse when we move with him, and bouncing on his back can be actually painful for the horse, who is genetically pre-programmed to run away from pain, which could be a predator who has managed to land on his back. Off the horse, experiment with relaxing the heavy muscles through your back until you can control them easily, then try it on the horse. Also teach yourself to recognize and control tension in your fingers, hands, arms & shoulders.
The 2nd problem with unintentionally tensing muscles is that horses read body language only too well. We have, to some extent, reprogrammed horses to consider their human riders as their “herd” leader, and when they feel their “leader” tense, they assume that is happening because the leader has spotted some danger they have not yet perceived, and the fact that there is not only something dangerous enough to catch the head horse’s attention is then magnified because to your horse’s way of thinking, not only is there is some type of danger somewhere in the vicinity, but your horse can’t find the source, so he doesn’t know which way to run.
Controlling your body language and even being amused (or making the horse think you are amused by staying relaxed), can reassure your horse that there is nothing to fear so he will relax. A horse having a panic attack does not think things through, he reacts, because in the wild that reaction could mean the difference between being alive and being dinner for some predator.