Exploring the Muscles and Topology of the Upper Body

Quick update on my human rig project! Following up on my skeleton study, I’ve started to look into the muscular anatomy of the arm and upper torso. This is a very common pain point for rigging a human character. While already having to consider the motion and rhythm of the shoulder girdle, the intricate juggling of the muscles of the shoulder make weight painting a nightmare if you go in blind. By studying the actual function and placement of each muscles, a rig can be better informed in its joint placement and distribution of weights.

I’m looking to strike a balance between having a realistically deforming character and keeping the model/topology clean and optimized. I’ve taken the chest, back, and arm and picked 7 main muscles to focus on getting accurate. This includes the pectoralis, deltoid, trapezius, latissimus dorsi, biceps, triceps, and brachioradialis.

Muscle Tests

My first step was to make a simple low poly approximate model for each of these muscles, and skin them to follow the skeleton mesh I built. By making each muscle individually I can focus on what local topology and skin distribution works best to make the muscle look good in various poses. I can also trial and error behavior and placement for helper joints, representing origins, insertions, and center of mass.

Here is the rig working through various poses:

Low poly muscles isolated in action!

For the general approach I have the expected joints like shoulder, elbow, clavicle acting as parents to the joints that represent muscles. The deltoid for example is a muscle with three major heads, and I used a joint for each heads origin, center of mass, and insertion (the deltoid heads insert to the same spot, so only 7 joints total). This gives me good results for the muscles when represented this way, but could be simplified in the grey model. The lateral deltoid origin is similarly positioned to the humerus joint, so the dedicated joint for the muscle could be excluded. The animation is the same one I used to test the underlying bones, so the motion of the muscles follow nicely.

Topology

I sculpted out a simple upper body in ZBrush, going for an average build male physique. I want this model and rig to be as standardized as possible to work as a base for tool creation and more complex character work.

Current sculpt in ZBrush, hands and lower body will be handled later

I have then been doing some trial and error topology solutions, trying to get something that deforms well in extreme poses while maintaining a reasonable amount of tris. Here are a few variations, most of which trying to solve issues in the armpit area:

Some topology attempts, testing position of poles around the pectoralis

Right now this is an iterative process as I continue to test topology solutions and build out the rest of the body parts. Once I have something solid locked down that’s working for the core joints of the arm I will start to add in the extra joints for specific muscle movements like bicep flexion and forearm twisting.