Why do your arms lift after pushing out for a while?

This is to do with the way in which your muscles control their length because hidden inside each of our muscles is a tiny thing called a muscle spindle and this is like a miniature muscle which has nerve fibres in it and those nerve fibres can tell how much the muscle is being stretched or lengthened. When you put force through a muscle it's trying to register, 'how hard should I contract, am I contracting at the right rate?' If the muscle is contacting hard but not going anywhere then this spindle doesn't stretch and it keeps sending messages back to the muscle saying you're not getting any longer, work harder. So when you're pushing up against the wall and your arm isn't going anywhere , it's trying to contract harder and harder to move the wall which, of course, doesn't move. Then when you move away from the wall and you switch off the voluntary drive to the muscle the spindle is still set to say I'm too short, I need to be longer. It tells your nervous system automatically like a reflex to make your arms longer.