UNIVERSAL HUMAN

MUSCULOSKELETAL SYSTEM DOCUMENTATION

CONTENTS

Musculoskeletal System Upgrade Procedure

Use this procedure to append the Musculoskeletal Systems to the Body Rig or Body & Face Rig, and the Musculoskeletal Arms to the Arm Rig. Ensure you've downloaded the most recent version of the assets before starting.

Musculoskeletal System Upgrade Procedure.mp4

With the rig loaded, Append Musculoskeletal System(s) (navigate to Collections folder > Musculoskeletal System)
If you appended more than one, join them (select one object, shift-select the other, Ctrl-J)
Join the Armatures (in the Outliner, select Armature.001 and Armature.002 if present, Ctrl-select Armature, Ctrl-Tab to enter Object Mode, Ctrl-J)
Return to pose mode and delete the Musculoskeletal.001 collection (if present)
Select the "Bones" Bone Collection and click on the Select button, then remove collection
Enter Edit mode and delete the selected bones
Open the Drivers Editor and delete all channels
Select the Musculoskeletal 2 object and deactivate the Shape Key Lock button
In the Armature modifier, set Object to Armature
Repeat steps 7 and 8 for Musculoskeletal 1, if present
Select the Body object, expand the Shrinkwrap 2 modifier and select Musculoskeletal 2 in the Target field (enter "m" in the field to find it quickly)
If both Musculoskeletal systems were appended, expand Shrinkwrap 1 and select Musculoskeletal 1 in the Target field
Musculoskeletal Head only: Disable Corrective Arm Shoulder Forward Clavicle L and R shape keys, and enable Corrective Arm Shoulder Forward L and R shape keys

Usage

The Musculoskeletal 1 object mainly consists of bones (plus some wrist tendons), while Musculoskeletal 2 comprises muscles and tendons. This is to facilitate interactions like tendons sliding over bones or other tendons. The corresponding Shrinkwrap 1 & 2 and Smooth 1 & 2 modifiers on the Body object apply the deformations to the skin. Toggle their visibility on to view the result, or off to increase viewport performance.

You can adjust the definition of the muscles, tendons and bones by changing the Repeat value in the Smooth modifiers. Increasing the Levels in the Subdivision modifier yields higher quality deformations, but also increases the musculoskeletal definition. To retain definition consistency at higher subdivisions, you'll also need to increase the Repeat value in the Smooth modifiers by multiples of four. For example:

Subdivision Level 3: Repeat 8

Subdivision Level 4: Repeat 32

Subdivision Level 5: Repeat 128

You may want to further increase the repeats for a softer appearance (e.g. for younger or female characters).

At low subdivision levels, adjacent surfaces will attract one another (such as where the forearm and upper arm meet when the elbow bends). This can be mitigated by increasing the subdivision level and/or smoothing repeats. If that's not sufficient, you may need to reduce the weights of the Musculoskeletal System Vertex Group in the offending area.

Subdivisions and Repeat values

Customisations

For selective adjustment of the subcutaneous definition, paint or edit the Musculoskeletal Smooth Vertex Group on the Body object. You can add further definition by adding detail to the musculoskeletal mesh (e.g. by replacing its Subdivision modifier with a Multires modifier and sculpting on it).

Adjusting the musculoskeletal system to fit a modified base mesh can be very finnicky, especially if it is to be animated, as the musculoskeletal mesh needs to be closely aligned to the body mesh at all times; if it's too shallow it won't have an effect, and if it protrudes too far it will break the deformation. Therefore you may also need to adjust corrective shape keys to maintain its shallow intersection with the surface.

To help ease the process, make both the base mesh and the musculoskeletal object visible, disable the Shrinkwrap and Smooth modifiers, and poke and prod the musculoskeletal mesh vertices until it protrudes slightly through the skin. To make it animation ready, you will also need to repeat this process for each shape key; first pose the rig until the shape key reaches a value of 1.0 (or thereabouts), reshape the musculoskeletal mesh to align with the skin, then adjust the next shape key.

In its default state, the skull only has a very subtle effect on the skin deformation. For a more pronounced effect, edit or sculpt it so that it protrudes slightly through the skin, and/or sculpt the Body mesh so that it conforms more tightly to the skull.

Smooth Modifiers & Multires

Since smooth modifiers are used to soften the skin deformations, any Multiresolution sculpting will also be smoothed. To retain sculpted detail it will need to be baked to a displacement map and applied as a material (see here for baking tips).

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