Look here for a start.
“Some research has found muscle abnormalities in fibromyalgia patients. These problems can be classified as the following:
* Biochemical abnormalities: Fibromyalgia patients may have lower levels of the muscle-cell chemicals phosphocreatine and adenosine triphosphate (ATP). Such chemicals regulate the level of calcium in muscle cells. Calcium is an important component in the muscles’ ability to contract and relax. If ATP levels are low, calcium is not “pushed back” into the cells, and the muscle remains contracted.
* Functional abnormalities: The pain and stress of the disease itself may harm muscle function.
* Structural and blood flow abnormalities: Some researchers have seen overly thickened capillaries (tiny blood vessels) in the muscles of fibromyalgia patients. The abnormal capillaries could produce lower levels of compounds essential for muscle function, as well as reduce the flow of oxygen-rich blood to the muscles.
To date, none of these abnormalities has a clearly defined relationship with fibromyalgia.”