The deformers are described in more detail in the Other Muscle deformation section. Typically, you want to use the Muscle Spline deformer, as it is more powerful and flexible. This deformer alters the shape of a simple muscle with a custom spline curve, and any number of controls.
If you select cMuscleSplineDeformer, you can define the number of control points in the Num Controls field. You need at least two, one for start and end, and typically three so there is one in the middle for jiggle. If you want to have more controls and jiggle on the muscle you can increase the number of controls.
A window appears asking you to name the simple muscle, and warning you that future cross section editing will be done. You can still edit the shape after finalizing, but if you do so, you need to reset the base for the muscle. See Reset base pose for selected Muscle Objects. Typically, you should not remodel the muscle shape after it is built.
After a moment the simple muscle is rigged with the deformer you chose and converted to a Muscle Object. The basic rigging creates a "grpMUSCLES" null, and places a completed muscle rig inside of this group for each muscle you make. The muscles are properly constrained to the original joints.
After you have built a simple muscle, you can adjust the parameters.