What activities raise fitness skill but do not make you more muscular or thinner?

Had one of my more geeky sims use a treadmill to build fitness and it made him go thin but not muscular. Strangely a couple times on the basketball court just dribbling around made his muscularity max out. At that point I used the cheat cas.fulleditmode and in CAS moved his fitness/muscularity sliders to where I felt they should be, that might the only way to keep the game from taking your sim to max fat or max fit.