Just study photographs of athletes and you'll get a good idea. It's also good to study some of the more realistic art in comic-books, although I wouldn't recommended anything anime unless its using the more human-realistic style and proportioning rather than the traditional big-eyed, long-legs, and tiny barbie-doll waist look. You won't get good references on muscle tones or shapes there the body's are usually too flat, too young-looking, and too generic.
You'd get better results looking at Marvel comics of the female heroines from like X-Men and so on, even Wonder Woman since you can pretty much see her body shape, since she's wearing the equivalent of a lycra body-suit.
I would recommend probably the old comics the most that used hyper-realistic comic-book art which you don't really see anymore. Great stuff, provided you can find anything from them printed in the 80's, now, you might find a really good graphic-novel that's a reprint of the old coloured comics drawn before PC's became common, like Pini's graphic novels of all the major EQ story-lines.
They are hard to find though since the comic-book companies don't generally reprint old editions unless its under special circumstances. Otherwise yah you're stuck with athlete photographs mostly, which can be difficult unless you've got a real good eye for changes in musculature due to body-weight, height, etc which would make your drawings more accurate. If you can get the base feminine muscular shapes down, you'll be fine anyway, since it really depends on how realistic you're going for in your artwork. Hyper-photographic real and viewers can tell where the mistakes are, just ordinary drawing and minimal realism, well, you can get away with more.
~ Pyre