potassuim possibly,Ill have to check the multis im taking for that,Drink plenty of water,This isnt like a spasm,only happens when i move my arm back quickly to reach back,when muscle tightens,it has a temp tight lockup that doest allow the arm to move back,I stretch through it and its fine,but its getting old. Might have to lay off bench press for a while,laying down lifting heavy isnt working for me well anymore,Im getting pressure in my temples that lasts for days,and pressure in my stomach and throat from acid reflux,only happens on bench days.Getting old sucks