Anatomical study of the upper torso: Automatic clavicle-scapula rigging system

Francesco Gioia • 29 maggio 2022

Introduction

In this post, I would like to give an introduction on the upper torso anatomy, as well as showing my favourite rigging system for this part of the body. In particular, I will demonstrate the setup on a skeleton model, and the best application is to rig skeletons for muscle simulations, such as in Ziva. Having a good skeleton rig really helps a lot to have stable simulations and proper bony landmarks.

This doesn't mean that you can't use this setup to achieve better deformations as well as some fake-scapula sliding in your skinned models.

Anatomical Study

In this post, I am going to describe the appearance and functionality of three main bone structures: Clavicles, Scapulas, and ribs.


The clavicle (also known as the collarbone) connects the top part of the sternum (also know as manubrium) to a scapula process called acromion.


The scapula connects the humerus with the clavicle. It has no direct bone connection to the ribs, but it rather slides over them, and the connection area is usually referred as scapulothoracic joint.


Rigging these parts, we have to take into account both the sliding behavior of the scapula and the pivot of the clavicle.

Kinesiology

In terms of movements, we can identify 8 main scenarios:

  • Adduction - Clavicles rotate inward, the scapulas get closer.
  • Abduction - Clavicles rotate outward, the scapulas get farther.
  • Elevation - Clavicles rotate upward, the scapulas elevate in a vertical path.
  • Depression - Clavicles rotate downward, the scapulas drop in a vertical path.
  • Upward Rotation - This happens when raising the humerus after a certain angle (usually 70 to 90 degrees depending from person to person), the clavicles rotate upward and the scapula both raises and rotates outward.
  • Downward Rotation - Opposite to the upward rotation.
  • Protraction - Clavicles rotate outward, vertebrae bend forward slightly, the scapulas abduct and there is a tilt in the anterior part.
  • Retraction - Opposite to the protraction.

I won't cover much of the deformation side here, but I will give some quick tips about the main muscles and bony landmarks visible from outside.


In terms of bony landmarks, acromiom is probably the most visible, as well as the spine of the scapula, the inferior margin, and the medial border in adduction. The clavicle itself is all pretty visible even with high fat percentages.


In terms of muscles, it depends on the fat percentage and the level of hypertrophy of the muscle itself. Of course, the two main muscles of the back are the trapezius and the latissimus dorsi, but I would say that on a trained person it is easy to see the posterior delt, the teres major and even the infraspinatus.


Notice that the posterior delt is usually underdeveloped, compared to the other deltoids, this is mainly because most of the training programs privilege front and side delt, as well as press exercises, therefore keep in mind that the posterior delt will be usually a bit smaller then expected on a pretty muscular person.

Base Photo by ShotPot - pexels.com

Rigging Approach

Range of motion test

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Different approaches in creating a custom blendshape deformer in Maya API, Bifrost, Blender and Houdini using VEX, Python and C++

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