Failing at the corpse pose in yoga

The corpse pose, or Savasana in Sanskrit, is the most restful pose in yoga. It's the final pose used in most yoga classes, but can also be done on its own to facilitate meditation or relaxation. You lie on your back, legs slightly spread, arms slightly spread out.
This is supposed to be the easiest pose in all of yoga and I can't do it because my bicep and pectoral muscles are spastically contracting and pulling my arm up onto my lap. This also means that when I try to sleep on my back I have to tuck my hand under my butt to try to get the whole arm to relax. I tried an air splint but by morning I had always bent my arm forcing the air out.