Well, it just won't gorw as fast. Ideally, your creature should be about 50% fat to help growth. Aneat way to make that rock training thing more effeective is to have repeated access to the "enlarge/shrink creature" miracles (often available in training maps which can be downloaded from our downloads section). Enlarge your creature and have him pick up the very largestrock he can pcik up, then shrink him down and take him for a walk. The fact that he's now so tiny, but still carrying the large rock really speeds up the whole strength-increasing exercise.
It should also be noted that some creatures loose their strength faster than others. For instance, it's almost impossible to keep a Cow's strength at 100% without constant exercise.
Hope that helps.