Anatomical Foundations of the Pectorals

TL;DR

Your pectorals, commonly called pecs, are made of two main muscles: the pectoralis major and the pectoralis minor. They attach to your sternum, clavicle, ribs, and scapula, playing key roles in moving your arm and shoulder. Understanding these attachments and functions helps you effectively train and protect these muscles.

1. The Mental Model

Think of your pectorals as a large, fan-shaped muscle on your chest (pectoralis major) and a smaller, triangular muscle underneath it (pectoralis minor). They work together to move your arm across your body, rotate it, and stabilize your shoulder blade.

2. The Core Material

The pectoral muscles are a group of muscles located on the anterior (front) chest wall. They're crucial for upper body movement and stability.

2.1 Pectoralis Major

Photo by Tima Miroshnichenko on Pexels

This is the larger, more superficial of the two pectoral muscles. It's broadly divided into two heads:

• Clavicular Head: Originates from the medial half of the anterior surface of the clavicle (collarbone).
• Sternal Head: Originates from the anterior surface of the sternum (breastbone) and the costal cartilages of the first six or seven ribs.

Both heads converge and insert into the lateral lip of the bicipital groove of the humerus (upper arm bone).

Key Actions:
* Adduction: Moving the arm towards the midline of the body (e.g., bringing your arm down from out to the side).
* Medial Rotation: Rotating the arm inward (e.g., turning your hand inward while your elbow is bent).
* Flexion: Raising the arm forward (especially the clavicular head).
* Extension: Bringing the arm down from an overhead position (sternal head).

2.2 Pectoralis Minor

Photo by Tima Miroshnichenko on Pexels

This is a smaller, triangular muscle located underneath the pectoralis major.

• Origin: Anterior surfaces of the 3rd, 4th, and 5th ribs.
• Insertion: Medial border and superior surface of the coracoid process of the scapula (shoulder blade).

Key Actions:
* Depression of the Scapula: Pulling the shoulder blade downward.
* Protraction of the Scapula: Pulling the shoulder blade forward and around the rib cage.
* Downward Rotation of the Scapula: Rotating the scapula so the glenoid cavity (socket for the arm bone) faces downward.
* Accessory Muscle of Respiration: Helps to elevate the ribs during forced inhalation.

Here's a diagram showing the relationship and actions:

graph TD
    A["Pectoralis Major"] -->|Origin (Clavicular Head)| B["Medial Clavicle"]
    A -->|Origin (Sternal Head)| C["Sternum & Ribs 1-6/7"]
    A -->|Insertion| D["Lateral Lip of Bicipital Groove (Humerus)"]
    A -->|Action| E["Arm Adduction"]
    A -->|Action| F["Arm Medial Rotation"]
    A -->|Action| G["Arm Flexion (Clavicular)"]
    A -->|Action| H["Arm Extension (Sternal)"]

    I["Pectoralis Minor"] -->|Origin| J["Ribs 3-5 (Anterior Surface)"]
    I -->|Insertion| K["Coracoid Process (Scapula)"]
    I -->|Action| L["Scapular Depression"]
    I -->|Action| M["Scapular Protraction"]
    I -->|Action| N["Scapular Downward Rotation"]
    I -->|Action| O["Assists Forced Inhalation"]

3. Worked Example

Imagine you're doing a push-up.
1. As you lower yourself, your pectoralis major is lengthening while controlling the descent, primarily through eccentric (lengthening) contraction.
2. When you push back up, both heads of the pectoralis major are concentrically (shortening) contracting to powerfully adduct, medially rotate, and flex your arm at the shoulder joint, pushing your body away from the floor.
3. Simultaneously, the pectoralis minor assists by stabilizing and protracting your scapula, allowing for a stable base for the pectoralis major to act upon, and also helping to push your torso upwards relative to your arms.

4. Key Takeaways

• The pectoralis major is a large, superficial muscle with clavicular and sternal heads, primarily responsible for arm movement.
• The pectoralis minor is smaller and deep to the major, primarily influencing scapular movement and stability.
• Both muscles originate from the rib cage/sternum/clavicle and insert on either the humerus (major) or scapula (minor).
• The pectoralis major allows you to push things away from your body and bring your arm across your chest.
• The pectoralis minor helps stabilize your shoulder blade and can assist with breathing.
• Understanding these origins and insertions helps you visualize muscle action during exercises.

Common mistakes you should avoid:
- Forgetting that the pectoralis major has two distinct heads with slightly different functions.
- Neglecting the pectoralis minor's role in shoulder stability and scapular movement.
- Confusing the actions of the pectorals with those of the deltoids or triceps.
- Focusing only on "chest" exercises without considering the range of motion and angles that target different pectoral fibers.

5. Now Try It

Sit or stand comfortably. Place your hand on your upper chest just below your collarbone and then on the middle of your chest. Now, perform a slow "hugging" motion, bringing your arm across your body. Feel how the different parts of your pectoralis major contract under your fingers. Then, try shrugging your shoulders forward and down, feeling for the contraction of your pectoralis minor underneath the larger muscle.

What success looks like: You can clearly identify the contraction of different parts of your pectoralis major and can feel the pectoralis minor working as you move your shoulder blade.