I've had the same problem and not found anything natural to help. I took a small dosage of supplemental testosterone for a while and it helped GREATLY! My husband wanted to send a dozen red roses to the doctor, lol. As a side effect it improved my muscle tone and gave me a nice feeling of well-being and motivation. The only drawback I've heard of (and believe me I've done a lot of research!) is that once you start on supplemental hormones your body gets lazy and quits producing it's own so you have to stay on it. And when I quit it I sure noticed a difference! My muscles got real soft and 'loose', I gained weight, and my husband is willing to pay cash to the doctor when our insurance company didn't renew him.
If you do find something natural that helps please let us know. I think it's a very common problem for middle age women. I had high hopes for Maca, but that was a bust too.