its not weak muscles, its wear the skin has stretched. its caused by gaining or losing alot of weight, most teens get them, because u put on a bit of weight turing puberty. they will eventually fade, but use some vitamin e cream eevery night where the stretchmarks are, u have to rub it in for quite a while, to get the full benefit..also dont expect them to be gone within a week, it takes quite a few months, of rubing ur skin with the cream every day. dont worry bout them too much, they are horrible, and alot of people get them, but you sholdnt feel embarrased. good luck