Jan 28, 2013

Anatomy Basics: The chest muscle

In this drawing, we can see that the chest muscles drape across the ribcage.
Most artists that draw the figure are familiar with drawing the chest muscles as they fit on the torso. Not everybody is as familiar with how these muscles connect to the arms. This is a critical component to understanding how the shapes of the arms and torso come together as well as how the muscle moves the arms.

Today, we are going to look at how the chest muscle connects to the skeleton and what kind of shapes these muscles make.

A diagram of the chest muscle, the pectoralis major.
The Muscles of the Chest

When we talk about the chest muscles we are usually referring to the pectoralis major as this are the large surface muscle that helps create the shape of the chest.

A muscle usually connects to the skeleton at two points of origin. This is the point where the muscle attaches to the skeleton but does not move that part of the skeleton when the muscle contracts.

At the other end, the muscle attaches to a part of the skeleton that will move when the muscle contracts. This point of connection is called the insertion.

The Connections

The pectoralis major's origin is along the front of the sternum and the clavicle.  The muscle is divided into two groups or heads and the division between the two can be found by looking at which bone the muscle starts at.

The two different heads of the pectoralis major, the clavicular head and the sternocostal head.
The origin of one group of muscles starts at the anterior (front) surface of the clavicle.  It is attached to the medial side (towards the center, closest to the pit of the neck) of the clavicle. This group is then called the clavicular head.

The insertion of the clavicular head is at the intertubercular groove on the humerus (upper arm bone)

The origin of the other group of muscles starts at the anterior (front) surface of the sternum and the upper ribs (on the cartilage near the sternum). This group is then called the sternocostal head.

The insertion of the sternocostal head is also at the intertubercular groove on the humerus.

The Actions

Each head is responsible for performing unique actions, together, they combine to perform several actions.

The chest muscles cause the arm to horizontally adduct at the shoulder or bring the arm in towards the middle of the body of the arms at the shoulder.  This is the action we might be most familiar with as we perform this when doing a pushup or clap.

Another action the pectoralis performs is flexion of the arm at the shoulder. The pectoralis assists in raising the arm, working along with the deltoids.

Lastly, this muscle also rotates the arms medially at the shoulder. This allows the arm to turn in towards the body.

The Shape

In this drawing, we can see that the chest muscles drape across the ribcage.
In general simplified forms, the pectoralis major lays across the torso like a thick bib wrapping around the rib cage and butting up against the shoulder muscles. This will result in a surface change from the chest to the shoulders.

Remember the pectoralis major also connects to the arm.  When the arm is by the side the pectoralis major will often push out and into the arm muscles.

This connection is especially important when the arm is raised. 

These four illustrations demonstrate how the chest muscle fits on the ribcage and changes shape as the arms move.

Notice that the chest muscle changes shape as the body moves. To the right is an illustration in which the figure is raising his arm. Notice how the right pectoral muscle is stretched and no longer retains the rounded shape of a relaxed pectoral muscle.

In the illustrations that include the skeleton and the muscles, you can see how the arm bone pulls on the pectoral muscle, causing the muscle shape to elongate and straighten out slightly. The effect of this is greater the higher the arm is raised.

Hopefully, this gives you a better understanding of the chest muscles and you find it easier to draw the forms of the chest.  This is something that takes practice. If you have the ability, find a life drawing workshop to attend and practice drawing. This is one of the best ways to learn figure drawing.