khalil66 Hi Khalil!
"Excitation" on the zFiber node models the physical behaviour of muscle flexing.
The muscle contracts and stiffens in the direction of the muscle fibers, while the volume conservation of the material causes the muscle to then bulge.
"Muscle Growth" on the other hand models the effect of either bulking up or wasting away. Animating the "muscleGrowthScale" attribute is equivalent to placing your creature on an exercise regimen (except you can both grow and/or shrink your muscles in a few frames, rather than it taking months, like in the real world).
This feature can additionally be used to achieve cartoon like effects, where the volume of the muscle increases when the muscle flexes (not recommended if the goal is to mimic real life muscle excitation, but potentially a useful effect if exaggeration in needed)