I tend to have aching and weekness all over 50% of the time no matter what I'm doing. If I'm standing or walking, my calves and thighs burn out very quickly. It's the same feeling as getting lactic acid build up when lifting weights. My shoulder muscles burns out and feels week when I pour coffee or life my lunch bag etc. It's pretty pathetic. I used to maintain the philosophy that this meant that my muscles needed to rest. I gave up on that theory after a while and started lifting weights and targeted the exact muscles that were ailing me. This was painful but highly beneficial in a short period of time. This may not work for all CFS/Fibro sufferers as some are too sick to exercise.