 In order to be able to evaluate the effect of GH on the muscle a comparative group is needed. Therefore it will be proposed to delay in a group of patients the start of Growth HormoneGH treatment by 6 months. As most publications have shown a maximum effect of GH within the first year of treatment, six months should be enough to evaluate short-term effect of GH on the muscle. Therefore, this study will be a randomized trial: immediate start of Growth Hormone GH treatment versus start of Growth Hormone treatment 6 months later. After 6 months all children will be treated with GH. Therefore, the follow-up will be one year after baseline. 