If your horse is taking shorter strides, or looks slightly lame at the extended trot, or is jumping flat with a leg hanging, the first place I check is the triceps muscle. If I find a knot there, usually I can release it with a couple of sessions of stress point therapy.
If the knot is on the lower end of the muscle, you will often see all of the above symptoms, plus a reluctance to pick up the canter on that side. The lower the spasm, the more sessions it will take to release, in my experience. I have worked on quite a few horses with low triceps issues, and in every case, the magic number seemed to be 5. By the fourth treatment, there was marked improvement, and by the fifth treatment, there was total resolution. While I do regular bodywork on those horses, that particular problem has not returned in any of them in the last few years.
If possible, walking up hills is great exercise following a bodywork session for the triceps.