I’ve also heard something about it being assur to build muscles on Shabbos since it’s boneh. Not sure how reliable that is and I can’t remember the source.
I don’t believe that to be correct. By the same token you shouldn’t be allowed to do anything, since any muscle movement that involves resistence (such as walking, or lifting a book) builds muscles. Likewise, you shouldn’t be able to eat since your body breaks down (“soser”) the molecules of the food.
The Wolf