There are two problems with the picture (or three, depending on what you are trying to accomplish with the image as a whole).
First, the light is too soft for the kind of definition you are describing. Soft light is generally flattering, but it's flattering precisely because it minimises the difference between highlight and shadow. In this case, you want to emphasize (without going overboard) the contours, and harder light will help a lot. That doesn't need to mean going to a bare bulb or an unmodified flash—using a strip bank (perhaps with an egg crate) rather than a softbox or umbrella would do, as would moving a soft light source (softbox or brolly) further away from the subject (although the falloff characteristics will be different). No strip bank? Block off (flag) part of the softbox/umbrella output to narrow the source.
(A strip, by the way, will emphasize contours in only one direction without an egg crate; you get a combination of softness and directionality, but each is in a different axis. That characteristic is what can get you the "fitness beauty" shot, where the definition is good, but so is the skin.)
I should add—and therefore am adding—that if you are already using a bare bulb or other small source, and the softness is coming from room reflections, then flagging off most of the light that isn't headed directly for your subject will help an awful lot. I rarely shoot in my own little hovel because it really is too small to light properly (at under 90 square feet) unless I can jam the light right up to the subject and just barely out of frame.
The same goes for the fill—you want it to just be there, without having much of an effect on the contours. A harder fill close to the lens axis will fill in the shadows without softening the contours appreciably if you keep the levels low. If the fill is both high-level and very soft, it can undo most of the work you've put into the key light.
An approach I've seen work well at times is to use a hard light coaxially with a soft key light. The "straight from the box" version is a beauty dish with its central part (normally a reflector pointing back into the dish) replaced with a grid spot. I've also seen an ordinary reflector head used just in front of and at the center of a large softbox. But now we're starting to talk about "big production" and the haemorrhaging of increasingly large amounts of money—great if you're a pro who can earn it back with a click of the shutter button; not so great for the hobbyist.
Second, you may need to allow for a bit of extra contrast. High key and muscular definition don't always go well together. Letting some of the shadow tones fall lower can help a lot, but only if the shadows are there to begin with. In the case of this photo, monkeying with the levels and curves may increase the overall image contrast, but not in a way that adds significantly to muscular definition—it just looks like a badly processed picture with funny skin tones.
The third thing you can do will radically change the picture, so it's probably not what you're looking for. You can make the body very specular using something like baby oil (for an extreme version) or a fresh coat of some slightly less shiny moisturizer. Cranking that effect up to eleven would involve artificial perspiration (a water/glycerin mix—warm, please—spritzed on your subject). That effect looks great in a very high-contrast, low-key image, but it is a whole lot more athletic than erotic most of the time. It may be the sort of thing that's worth giving a shot at the end of a shoot, though, once you have your main intent captured, since it can have a certain intensity that's missing in the gentler approach, and that intensity may be just the ticket.
