When I read this - I thought of how my son - who has tourettes, spd and fibromyalgia, does this when he is frustrated, and he will also do this when he is in pain. I mean the tightening of his fist, or stiffens his arms. I have spd, and fibromyalgia, and I do the same. When I am in pain, I try to redirect my physical feeling into the muscle - usually my hands. I do the same when I am frustrated and in major pain. I am thinking of how I will grip a shopping cart - so hard - because I am trying to get through the shopping trip, but am so overstimulated and in so much pain. My son, is 12 and has trouble pouring glasses of anything. He spills quite often. But I have seen him do it the most when he is mad. Could it be a sense of pent up energy, from using such control to pour a cup of something? That would make sense to me. Like an outlet? A bit like the getting home after school, and keeping control, only to have a meltdown, to let it out. A mini sort of that? Not that its bad for sure. What is bad, is what my son has started to do. He gets so mad, and frustrated with his dad, that he fists up, and starts pounding his hips! I am trying to redirect him, to a punching bag we have. It worked this weekend. It seems like a release that he needs. Have you asked your son about what it feels like to him when he does it? Does it give him a sense of relief, or comfort?