Fitness models simply look and photograph differently from those who do not exersize and work out.  They have superior posture, are better toned and have more photogenic bodies. They have developed a quiet confidence that reflects that they know they look terrific.  Fitness models greatly benefit from different lighting.
   All models should be presented as flatteringly as possible in your images. This means lighting.  After all; the word 'photography' is derived from the Greek word photos; for "painting with light".  Thus, fit models can benefit from non-conventional lighting that emphasizes their toned bodies.  Other than head shots, fitness models should not be lit the same as other models. This is true even if they wear slacks or a dress. Fitness models can move, and pose (yes, I know it is still photography, but the fit person also looks better frozen in an instant in time).  Fitness models can attain and hold poses that others simply can not.    
   Is it merely accident that the Sports Illustrated models have personal trainers and serious exercise regiments?  Those photographers know how to capture the extra edge from fit models. Come learn how to present the fit model at her best.