Semi-involuntary muscle spasms when excited?

I've noticed a friend of mine when hes really excited or happy about something he emits a little squeaky noise and his arms/shoulders/chest seem to twitch and has a weird smile on his face. he says he can control whether or not it happens but that it feels good to let it happen and once it does, he has no control over it. it lasts maybe no more than a second or two. he is otherwise completely 100% normal and not weird at all- in fact, hes a very smart guy. ive only seen it twice- he says it rarely happens- only when he is really really happy about something and he usually does it in private- what gives?