You know when you are working out and you get muscle pumps and your muscles explode inside the gym...what causes muscles to be pumped even when you are not working out...For example when I do back my back gets huge...when I leave the gym it feels fatigued and small...after a protein shake a shower and some rest I go out at night or for the next couple days and it feels like back is huge and pumped again..What causes your muscles to do this?