This tool creates the control components of a muscle. You would typically want to do this if you intend to build a muscle comprised of custom modeled geometry, and one or more rig systems. This will give you the ability to have multiple attachment points for arbitrarily shaped muscles. It is very similar to the Stroke Muscle tool, since it prompts you to select a skin object and lets you stroke a curve over the skin.
This tool creates a Muscle asset that is identical to one created with the Muscle and Stroke Muscle shelf tools, but will only display visibility for the rig components (the muscle geometry in this asset is ignored and only the inner core and handles are toggled on).
You can control the radius of the rig’s inner core by scrolling the mouse wheel or holding ⇧ Shift while dragging to resize your brush. This inner core will be used as the capture points for deforming muscle geometry.
Creating muscles rigs will leave the endpoints detached from your character rig. They will be drawn in worldspace and consequently should be drawn over your skin in its rest position (ie, T-pose). You can connect the head and tail anchors later using the Attach to Bones shelf tool.
The capture pose for the inner core is automatically set to match the stroked position. If rig handles are further adjusted by repositioning them in the viewport, the capture position will need to be reset by navigating to the Capture/Deform tab and pressing the Set Capture Pose button.
Creating Muscle Rigs
Click the Muscle Rig tool on the Muscles shelf tab.
Select the skin object to draw your stroke on and press Enter.
Stroke a curve onto your skin geometry. You may stroke multiple times until you are satisfied with the placement of your muscle. Each time you draw a stroke, the muscle will be redrawn.
While the stroke state is active, you can adjust the muscle radius using the mouse wheel, or ⇧ Shift + dragging.
After a stroke has been drawn, you may adjust the depth of the drawn muscle rig relative to your skin surface by adjusting the Depth parameter.
Once you are satisfied with the shape and position of the stroked curve, press the Accept Stroke or Accept and Create New Stroked Muscle button to save your stroke as the current pose. You will still be able to modify the position of the rig handles to further refine or shape your muscle.
You can also press Enter to complete the stroke and return to object mode, or press Q to repeat the operation (which will complete the current muscle stroke and begin a new one).
For more information, the muscle workflow help page.