Start by clicking the Stroke Muscle tool on the Muscles shelf, and selecting the skin object of your character. This puts you in a stroke state, where this skin is brought in as your projection geometry.

Draw a stroke on the surface of the skin to create a proxy muscle. You can continue to restroke the same muscle until it is where you want it.

The actual muscle position is projected at a depth, which you can control with the Depth parameter. While in the stroke state, you can also modify the Radius or use the ramp on the Stroke Profile to change the silhouette of your muscle.

Once you are satisfied with the initial placement of your muscle, you can either press Enter to accept the stroke, or press Accept Stroke in the parameter dialog. You also have the option to Accept and Create a New Stroke, which will commit your stroke to the current muscle, but allow you to remain in the muscle stroke tool so you can create another. This allows you too quickly create multiple muscles on your character.

Connect the muscles to the bones using the Attach To Bones shelf tool. Select the muscle you want to attach to your character rig and press Enter, next select the head anchor and press Enter, then select the tail anchor and press Enter.

This interactive way of attaching muscles in the viewport is handy when you have multiple muscles and complex rigs, where locating the nodes in the network tree requires a lot of searching.

Press ⌃ Ctrl + R to activate the pose state, which allows you to modify both translation and orientation of the muscles. You can apply the translation and rotation to multiple muscles at once by holding ⇧ Shift when you select them, and changing the dropdown to Global Transform Handle. This will merge all handles into one handle that will affect all selected objects.

Edit your muscles to push through the surface of the skin, so that the outer surfaces of the muscles bulge the skin out on the character. You can do this using the handles, and also the initial set up controls on the Muscle object. On the Geometry tab there are parameters to define the initial shape of the muscle, before being deformed by they internal muscle rigs. For example, the ramp can change the Profile Curve of your muscle, and the minimum and maximum values will set the range of how the profile is applied. For more information, see the Muscle object.

Once you have your initial muscle positions set up, the parameter values must be saved in this condition as the capture pose. You can do this by selecting the muscles, and pressing the Set Capture Pose button on the Capture Deform tab. Doing this freezes a copy of your muscle parameters so that they can be accessed by the rest of the tools wherever a muscle’s capture pose is required.

Next you can add secondary animation to the muscles automatically by turning on the Use Jiggle checkbox on the Jiggle tab. This will enable jiggle and also allow you to control the behavior of the jiggle qualities. The body of the muscles will automatically have some overshoot and spring-back.

Click the Muscle Displace tool on the Muscles shelf, select the skin object, and then select the muscles to displace with. Right away you should be able to see a bulge effect on the thigh area.

As you play your animation, you will see the skin adaptively deform wherever the muscles are pushing. The muscle movement is independent of the skin surface. The skin slides over and around the muscle volumes as the character moves, since the skin is not bound to the muscles directly.

Since the muscle workflow is entirely procedural, it is relatively simple to add muscles to the displacement setup. Once an additional muscle is built and attached to the rig, you need to incorporate it into the displacement. Simply click the Muscle Displace tool on the Muscles shelf again, select the skin and press Enter. This time you will be prompted to add or remove muscles from the displacement . Holding ⇧ Shift and clicking the new muscle will add it, while holding ⌃ Ctrl and clicking will remove muscles.

Make sure all of the muscles are selected and press Enter. The shelf tool automatically inserts the new muscle into the object merge.

Occasionally, areas of skin are affected by overlapping muscles and the displacement isn’t correct. You can prevent this from happening by using muscle masks. Muscle masks are used to limit the displacement amount of a muscle (or group of muscles) to specific regions of the skin that are painted with a weight value.

Click the Paint Muscle Mask tool on the Muscles shelf, and select the skin geometry of your character. Use the drop down menu to select the muscle to use as the current region. Then, paint the area of skin that the muscle is allowed to affect.

For a more visual approach, you can switch the view to Ghost other objects in the object visibility menu, and use ⌃ Ctrl + to select the muscle you want to paint. This will automatically update the dropdown menu as you change your selection, and allow you to paint confidently on the skin.

Often it makes more sense for the Muscle Deform SOP to treat multiple muscles as a group, rather than individual muscles. You can give clusters of muscles logical names for paint masking. Every Muscle object has a Muscle ID parameter which is meant for this purpose. By default it is set to the expression variable$OS, which just expands to the object node name. This means that every muscle is given a unique ID. By overriding this default, you can take a collection of muscles and give then a common shared ID. Click the Paint Muscle Mask tool on the Muscles shelf, select the 3 upper leg muscles, and change the Muscle ID from $OS to L_thigh.

All muscles being fed into the muscle deform set up get stamped with a point and primitive attribute named Muscle ID. The Muscle Deform SOP uses that value to match up muscles, centers, and weight masks to ensure that the bulging happens in a controlled way. The Muscle Capture SOP is responsible for making the initial link between the muscle IDs and the weight masks.

Paint your weight masks for the clustered regions to get the deformation working properly again. If you turn on the display flag of the Muscle Deform SOP, you should see that the deformation is behaving much better and the overlapping issue should now be resolved.

In typically character setups, you will do most of your muscle work on one side your rig and then use mirroring to copy your work across the symmetry plane.

Click the Mirror tool on the Muscles shelf. This tool is a generic mirroring tool, and does not work exclusively on muscles. Anything you have selected will be mirrored. It may be easier to select your muscles in the network editor instead of the viewport, so you don’t accidentally select other things.

You can also use the New prefix parameter in the operation controls toolbar to replace the old L_ prefix with the new R_ prefix to represent the right side, and press Enter to mirror your symmetrical copies. The mirror function examines the inputs to the nodes and if they are connected to to a parent that also follows the L_ and R_ naming convention, then they are automatically attached to parents on the mirrored side.

If you play your animation, you should see that the mirrored muscles are attached to the correct bones and are animated with the same jiggle settings that the original muscles have.

Even though the muscles are copied, the influence on the skin isn’t. You need to include these new muscles in the muscle displace set up, like you did for the calf muscle. Simply click the Muscle Displace tool on the Muscles shelf, select the skin, and press Enter. You will see which muscles are already included, as they are highlighted. Press ⇧ Shift + the muscles you want to add, and press Enter to accept.

The last step is to mirror the weight masks, so that the skin will be affected by the displacement of the copied muscles.

Click the Mirror Muscle Mask tool on the Muscles shelf, select the points that will receive the mirrored mask weights, and press Enter to confirm your selection.

Select the muscles that you want to have mirrored. You may have to ⇧ Shift + to add any muscles that may not have been automatically detected, and press Enter to confirm your selection. Lastly, select the destination muscles (which should be automatically detected), and press Enter.

Using muscle masks are not only useful for fixing errant deformations, but they also make your simulation more efficient by minimizing the amount of displacement calculations. Using muscle masks means that the deformer only spends time projecting skin points if it finds non-zero values for any muscle IDs it encounters.