I had a real hard time with sore muscles during my sicker part of being mercury poisoned. After heavy research I concluded it wasn't due to lactic acid but actually tartaric acid...which is produced by yeast breaking down sugar. Tartaric acid builds up in the muscles and causes the soreness. One way to overcome high levels of tartaric acid is to supplement with malic acid...and after I started supplementing with malic acid my muscles felt much less sore and it really made a huge difference. I can't say it would work for everyone but you won't know unless you try.

Here is some more information that explains a little more about tartaric acid in connection with fibromyalgia...remember that one of the biggest symptoms of fibromyalgia is just muscle tightness and soreness - http://www.greatplainslaboratory.com/fibromyalgia.html