Why do part of my body beats?

Question

sometimes i be my a litttle part of my leg just starts beating and i can see it moving up and down. my arms too though

Answer 1

This is most likely a muscle twitch (spasm). Your muscle cell, for whatever reason, has reached threshold and depolarized enough to cause a twitch in the cell. There is nothing wrong with this happening, though, so don't worry about it.  

Answer 2

haha, that happens to me too.  It could just be a tiny spasm but I would like to hear the other answers to this question.