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

Swimming
Fitness skill books

Swimming is helpful. Most of my merfolk are exceptionally fit from all the swimming they do, but I should warn you that your sim will lose weight swimming. There's also the risk of drowning that comes with swimming. You really can't swim forever unless your sim is a merperson, but higher fitness levels help reduce the energy drain that comes with swimming.

Now that I remember, it's a good idea to use a treadmill or jog to level fitness before swimming. Jogging or the treadmill won't increase muscle, but it will reduce weight.

I'm not sure, but won't Yoga build fitness? Perhaps it builds muscles too, but how about Meditation?

I'm fairly sure yoga only builds wellness, unfortunately. I wish it did build fitness.

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.

Try biking.

https://answers.ea.com/t5/Bug-Reports/NEEDS-INPUT-DU-Riding-a-bike-Sim-still-gets-fatter/td-p/9060132

@JustMeAlb

"JustMeAlb;c-17995203" wrote:
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.

I think sims will eventually revert back to wherever the sliders were before they exercise. There is a calorie system, and if I remember correctly, a garden salad will add the weight back.

Something else that came to mind, sims that are consistently physically active will tend to maintain their weight on the slider.

My merfolk usually end up with the slider at the bare minimum no matter what I do, so by default, I create all my merfolk with their weight at the lowest possible on the slider.

"Simmerville;c-17995185" wrote:
I'm not sure, but won't Yoga build fitness? Perhaps it builds muscles too, but how about Meditation?

Yes, Yoga builds muscle.