Physical fitness is not just chemistry. The body is responding to actual physical forces and the response is tightly coupled to how, when and where those forces are applied. Bone density and structure for instance is not just a matter of increasing production of bone cell production. It instead responds to the forces of gravity and other forces on the body as it moves through space. These forces do trigger electrochemical signals in the body, but it wouldn’t be trivial to recreate these signals in such a way that the body is fooled into responding just as it would if the signals were triggered by gravity and other physical forces.

It seems like it could be a more difficult but similar problem to the one experienced by lab-grown meat producers. The meat texture still isn’t right. And these meat “muscles” only require proper taste and texture. They wouldn’t have to actually support a living organism.

Edit: On reread I realize your comment allowed for assisting rather than totally replacing the required physical aspects. I agree with that. Use of chemicals could make it easier to trigger the necessary reactions, lowering thresholds, increasing outputs, etc. This could be applied broadly and the physical forces of everyday life might do the rest.