I recently finished a Muscle-rig as my personal project. In this rig I have tried to use couple of techniques to achieve better looking muscle deformations. This rig is a combination of multiple volume preserving helper joints and joint driven blend-shapes.
Bhasmasura - is a character from Hindu Mythology which I modeled, reading the references from the scriptures. I studied muscle anatomy and sculpted the muscles on the model to give it a better muscle definition and more accurate muscle deformations. I have used multiple blend-shapes to define muscle flexing, almost each muscle set has its own blend-shape, there is a separate blend-shape for vein popping. Facial expressions are done by blend-shapes as well.Multiple sets of helper joints which are joint driven too, helps to preserve mesh volume and give accurate deformations. For the rendering part, I have applied Sub-surface scattering shader.