When creating a rig for a Franken Muscle, you can choose to use an arbitrary mixture of Muscle Rigs and Muscle Pins. Muscle rigs have a number of controls for shaping and jiggling the deformed muscle geometry, whereas a Muscle Pin is designed to give you a single region to either lock down or anchor part of your muscle geometry to your character rig.
While the name "pin" may imply something stationary, the position, orientation, and scale of the pin can be keyframed. This will give you additional flexibility in designing your muscle rigs.
Creating Muscle Pins
Click the Muscle Pin tool on the Muscles shelf tab.
Click the location of the muscle geometry where you want to apply the pin. The pin capture geometry will automatically be sized based on the depth of the geometry you selected.
You can repeat the operation and create another pin by pressing Q.
The capture position for the pin geometry will automatically be set when the pin is dropped into your scene. If you modify the placement of the pin afterward, the capture position will be left unaffected in its worldspace position. If you want to update the capture position, you can modify the position by clicking the Set Capture Pose button on the Capture tab.
For more information, the muscle workflow help page.