Muscle > Skin Setup > Convert Maya Skin to Muscle System

User Guide > Rigging > Maya Muscle > Muscle Menu >

Use this to convert an object that has a skinCluster applied to have a Muscle shape node instead. During the conversion, a dialog box appears asking you to select the axis that points down the length of the selected object.

During the conversion, the following occurs:

Joints are converted to capsules (they have a Muscle Object shape node applied) so they can be connected
The skinCluster is replaced with Sticky weights.
The Muscle deformer is applied and weighting from the skinCluster is transferred to the cMuscleSystem node