After anorexia, how can one rebuild the weak muscles?

Not sure: Although "anorexia nervosa" is a well-known cause of weight loss, other medical conditions can cause anorexia. If you have been medically treated for your anorexia, only your treating doctor can help you rebuilt your weak muscles. On the other hand, if "anorexia" is a diagnosis of your own, please seek medical help to assess and monitor your "post-anorexia" status safely.