Maya Muscle is a skin deformer that lets you rig characters with underlying muscle objects to create realistic skin deformation. You can also use Muscle’s independent Displacement, Force, Jiggle, Relax, Smooth, and Collision features to create other deformation effects. Any NURBS surface in Maya can be converted to include a Muscle Object shape node and be connected to the Maya Muscle skin deformer.
For character rigging, you can build muscles based on the character’s skeleton so that when the skeleton moves, the muscle objects squash and stretch and in turn drive the surface of the character’s skin.
Muscles basically work as ‘influence objects’ but with special attributes that precisely simulate the physical properties of muscle interacting with skin. Maya Muscle lets you paint weights per-point to define exactly how the muscle attributes (Sticky, Sliding, Relax, Jiggle) affect the skin.
You can find the Maya Muscle guide, which gives you information on setting up a character rig to work with Maya Muscle for skin deformation and on setting up other deformation effects, as a downloadable PDF at the Maya Documentation Archive.
Note: Muscle surfaces use the Blinn shader by default (as opposed to Standard Surface).