I personally don't think the problems with muscle weakness is just a symptom of fibro or associated conditions. Any type of illness or medical problem that involves lack of use of the body for a period of time can cause weakness in the muscles. If there is a lot of pain in any area of the body because of stiffness when trying to use it, it causes a lot of pain and the temptation is to say "ouch" and leave the attempted exercise alone. Then it follows that the muscles in that particular area are going to be weakened and therefore will need extra work to build them up.
If for instance you have say a bad bout of flu and you have to stay in bed, then the longer you are in bed, the weaker you will become. Thing is too that overdoing things with mucles causes problems, underdoing things causes problems too. Even fluid loss—from severe sweating, vomiting or diarrhea—can also cut off the power to muscles. When you lose fluid, you drain away salt and also trace amounts of potassium, magnesium and calcium. Muscles also need nutrients to perform properly. So diet needs to be looked into.
Thing is if muscle weakness occurs, then you must be very careful to take care in building them up again and do it slowly. If it becomes a very big problem, in that it is getting to be debilitating, then you need to speak to your doctor to discount anything serious going on.
I actually have a nerve and muscle wasting illness, so I am interested in any research on this type of thing. Whilst mine is progressive and not curable, there are so many types of problems that if worked on can be overcome. Just a few things that I thought might be of interest to you.