I agree but not with the tensing part seeing as some muscles do actually have to flex in order for the arm to move.
Ok, yes to this and to Kevin. It's true that muscles are doing "something". But it isn't at all what people usually are doing and it's a relaxed rather than tense movement. What I should have said is that one should be able to allow the arm to bend and then straighten it pretty much at will with a third person placing his hands on your biceps triceps and your forearm. That third person shouldn't feel any muscles firing. It's the difference between extension and what most people do which is pushing.