Do you ride regular? And what angles do you have your bindings at now?
From what you posted, it sounds like you need to reduce your back binding angle. I had the same problem with the muscle group you describe, to the point where the muscle was cramping after 4-5 hours. I was riding with a duck stance, with the back binding at about
20 deg and dialed it back to about 15 and it was all good.
Hope this helps.