I can't quiet figure out how to do these correctly. Some days i can do 80lbs on both sides and feal like im doing them correctly some days i can hardly do 50lbs. Some people i see move there heads forward and lift there shoulders which seems to be working a different muscle than if you leave your head in the normal position. Also what muscle is this isolating?I could not find it on the EXRX list thats why i posted it thanks