Great to see people doing this sort of work.
You could try a subtissue in the area that you want to see 'grow'.
To do this, create a mesh that sits inside the upper arm, something like this:
Turn it into a Ziva Tissue.
Select the arm, then your new mesh and run Ziva > Subtissue
On the Subtissue you could try having a zFiber which grows when the arm bends.
To do that, select your subtissue, then run Ziva > Muscle Fiber
Set Enable Muscle Growth and Use Loa For Muscle Growth both to be ON
Also set Muscle Growth Scale to be something higher than 1.
You'll then need to have a 2 point nurbs curve that follows your bone cylinder. (so that it gets shorter when the arm bends)
To do that, make a curve that looks something like this:
Select one CV and then shift-select your bone object and run Ziva > Rivet To Bone
Do the same for the other CV
Lastly you'll need to connect that curve to the zFiber you made before, so:
Select your nurbs curve and then select the subtissue and run:
Ziva > Muscle Line of Action.
That's it! Now that portion of the arm will grow when the arm bends.