11.5 Muscles of the Pectoral Girdle and Upper Limbs

Muscles of the Pectoral Girdle and Upper Limbs

The muscles of the pectoral girdle and upper limbs control everything from powerful pushing and pulling motions to fine finger movements. These muscles work in coordinated groups to move the shoulder, elbow, wrist, and hand through their full range of motion. Understanding their origins, insertions, and actions is the foundation for making sense of upper body function.

Muscles of the Pectoral Girdle and Upper Limbs

The upper limb muscles can be organized by region and primary action. For each muscle, pay attention to where it sits on the body, because location is your best clue to its function: anterior muscles generally flex, posterior muscles generally extend.

• Pectoralis major spans from the anterior chest wall (clavicle, sternum, and upper ribs) to the intertubercular groove of the humerus. It performs adduction (moving the arm toward the body), medial rotation (rotating the arm inward), and flexion of the humerus. Think of the motion during a push-up or a bear hug.
• Latissimus dorsi originates from the posterior trunk, inferior to the scapula (thoracolumbar fascia, lower thoracic vertebrae, iliac crest), and inserts on the intertubercular groove of the humerus. It performs extension (pulling the arm backward and downward), adduction, and medial rotation. This is the primary muscle used during a pull-up.
• Deltoid covers the lateral aspect of the shoulder, originating from the clavicle, acromion, and spine of the scapula and inserting on the deltoid tuberosity of the humerus. Its anterior fibers assist in flexion, its middle fibers perform abduction (lifting the arm away from the body), and its posterior fibers assist in extension.
• Biceps brachii lies on the anterior upper arm, originating from the scapula (two heads: long head from the supraglenoid tubercle, short head from the coracoid process) and inserting on the radial tuberosity. It flexes the elbow and supinates the forearm (rotates the palm to face upward). It's most powerful when performing both actions simultaneously, like turning a corkscrew.
• Triceps brachii sits on the posterior upper arm with three heads (long head from the infraglenoid tubercle of the scapula; lateral and medial heads from the humerus), inserting on the olecranon process of the ulna. It is the primary extensor of the elbow.
• Brachialis originates from the anterior distal humerus and inserts on the coronoid process and tuberosity of the ulna. It's actually the strongest flexor of the elbow because it pulls directly on the ulna regardless of forearm position, unlike the biceps which loses mechanical advantage during pronation.
• Brachioradialis runs along the lateral forearm, originating from the lateral supracondylar ridge of the humerus and inserting on the styloid process of the radius. It assists in elbow flexion, and it's most active when the forearm is in a neutral (midprone) position, like during a hammer curl.
• Wrist and hand flexors (anterior forearm), including the flexor carpi radialis and flexor digitorum superficialis, flex the wrist and fingers. Most originate from the medial epicondyle of the humerus via the common flexor tendon.
• Wrist and hand extensors (posterior forearm), including the extensor carpi radialis longus and extensor digitorum, extend the wrist and fingers. Most originate from the lateral epicondyle of the humerus via the common extensor tendon.

Movements of Upper Extremity Joints

• Shoulder joint movements:

  1. Flexion: Moving the arm forward and upward
  2. Extension: Moving the arm backward
  3. Abduction: Moving the arm away from the body laterally
  4. Adduction: Moving the arm toward the body
  5. Medial rotation: Rotating the arm inward (e.g., reaching behind your back)
  6. Lateral rotation: Rotating the arm outward (e.g., cocking the arm back to throw)

• Elbow joint movements:

  1. Flexion: Decreasing the angle between the forearm and upper arm (e.g., bringing your hand toward your shoulder)
  2. Extension: Increasing the angle between the forearm and upper arm (e.g., straightening the arm)

• Forearm movements (occurring at the proximal and distal radioulnar joints):

  1. Pronation: Rotating the forearm so the palm faces downward or posteriorly (the radius crosses over the ulna)
  2. Supination: Rotating the forearm so the palm faces upward or anteriorly (the radius and ulna are parallel)

• Wrist joint movements:

  1. Flexion: Moving the palm toward the anterior forearm
  2. Extension: Moving the back of the hand toward the posterior forearm
  3. Radial deviation (abduction): Moving the hand toward the thumb side
  4. Ulnar deviation (adduction): Moving the hand toward the little finger side

• Hand and finger movements:

  1. Flexion: Bending the fingers toward the palm (e.g., making a fist)
  2. Extension: Straightening the fingers
  3. Abduction: Spreading the fingers apart
  4. Adduction: Bringing the fingers together

Rotator Cuff Structure and Function

The rotator cuff is a group of four muscles whose tendons fuse with and reinforce the shoulder joint capsule. Their primary job is to stabilize the head of the humerus in the shallow glenoid fossa during movement. You can remember them with the mnemonic SITS:

• Supraspinatus originates from the supraspinous fossa (superior posterior scapula) and inserts on the greater tubercle of the humerus. It initiates abduction of the arm during the first ~15 degrees and stabilizes the humeral head. This is the most commonly injured rotator cuff muscle.
• Infraspinatus originates from the infraspinous fossa (posterior scapula) and inserts on the greater tubercle. It performs lateral rotation of the arm and stabilizes the humeral head.
• Teres minor originates from the lateral border of the scapula and inserts on the greater tubercle. It also performs lateral rotation and stabilization, working closely with the infraspinatus.
• Subscapularis lies on the anterior surface of the scapula (subscapular fossa) and inserts on the lesser tubercle of the humerus. It is the only rotator cuff muscle that performs medial rotation.

The rotator cuff muscles work together to maintain shoulder stability by:

1. Compressing the humeral head against the glenoid fossa, preventing dislocation
2. Counteracting the upward pull of the deltoid during abduction, which would otherwise cause the humeral head to migrate superiorly and impinge on the subacromial structures
3. Maintaining proper alignment of the humeral head throughout the full range of shoulder movements
4. Preventing excessive translation (sliding) of the humeral head within the glenoid fossa, reducing the risk of impingement and injury

Muscle Interactions and Characteristics

Muscles rarely act alone. Understanding how they work together (and against each other) is critical for predicting movement patterns.

• Antagonist muscles produce opposite actions at the same joint. The biceps brachii (flexor) and triceps brachii (extensor) at the elbow are the classic example. When the biceps contracts to flex the elbow, the triceps must relax and lengthen, and vice versa.
• Synergist muscles assist the prime mover in producing the same action. The brachialis and brachioradialis are synergists to the biceps brachii during elbow flexion. Synergists can also stabilize joints to prevent unwanted movement.
• Innervation of upper limb muscles comes primarily from branches of the brachial plexus (nerve roots C5-T1). Specific nerve injuries produce predictable deficits. For example, damage to the musculocutaneous nerve weakens elbow flexion, while radial nerve damage impairs elbow and wrist extension (resulting in "wrist drop").
• Muscle fiber types in the upper limbs vary by function. Postural muscles and those involved in sustained grip tend to have more slow-twitch (Type I) fibers, while muscles used for quick, powerful movements contain more fast-twitch (Type II) fibers. Most upper limb muscles contain a mix of both types.