11.5 Muscles of the Pectoral Girdle and Upper Limbs

The muscles of the pectoral girdle and upper limbs are key players in arm movement. From the powerful pectoralis major to the versatile deltoid, these muscles work together to enable a wide range of motions, including flexion, extension, and rotation of the arm and shoulder.

The rotator cuff, a group of four muscles surrounding the shoulder joint, plays a crucial role in stabilizing the arm. Meanwhile, muscles like the biceps and triceps control elbow movements, while forearm muscles manage wrist and hand actions. Understanding these muscles is essential for grasping upper body function.

Muscles of the Pectoral Girdle and Upper Limbs

Muscles of pectoral girdle and upper limbs

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• Pectoralis major muscle spans from the anterior chest wall (origin) to the humerus (insertion), enabling adduction (moving arm toward body), medial rotation (rotating arm inward), and flexion (moving arm forward and upward) of the humerus (upper arm bone)
• Latissimus dorsi muscle originates from the posterior trunk, inferior to the scapula (shoulder blade), and inserts on the humerus, facilitating extension (moving arm backward), adduction, and medial rotation of the humerus
• Deltoid muscle covers the lateral aspect of the shoulder and assists in abduction (moving arm away from body laterally), flexion, and extension of the humerus
• Biceps brachii muscle lies on the anterior aspect of the upper arm and enables flexion of the elbow joint and supination (rotating forearm so palm faces upward) of the forearm
• Triceps brachii muscle is located on the posterior aspect of the upper arm and is responsible for extension of the elbow joint
• Brachialis muscle originates from the anterior aspect of the lower humerus and inserts on the ulna (medial forearm bone), contributing to flexion of the elbow joint
• Brachioradialis muscle runs along the lateral aspect of the forearm and assists in flexion of the elbow joint, especially when the forearm is in a neutral position (between pronation and supination)
• Wrist and hand flexor muscles, such as flexor carpi radialis and flexor digitorum superficialis, are situated on the anterior aspect of the forearm and enable flexion (bending) of the wrist and fingers
• Wrist and hand extensor muscles, including extensor carpi radialis longus and extensor digitorum, are located on the posterior aspect of the forearm and facilitate extension (straightening) of the wrist and fingers

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., touching the opposite shoulder)
  6. Lateral rotation: Rotating the arm outward
• Elbow joint movements:
  1. Flexion: Decreasing the angle between the forearm and the upper arm (e.g., bringing hand toward shoulder)
  2. Extension: Increasing the angle between the forearm and the upper arm (e.g., straightening the arm)
• Forearm movements:
  1. Pronation: Rotating the forearm so that the palm faces downward (e.g., turning a doorknob)
  2. Supination: Rotating the forearm so that the palm faces upward (e.g., holding a bowl of soup)
• 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: Moving the hand toward the thumb side (e.g., waving hello)
  4. Ulnar deviation: 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 (e.g., playing piano)
  4. Adduction: Bringing the fingers together

Rotator cuff structure and function

• Rotator cuff is a group of four muscles and their tendons that surround the shoulder joint, providing stability and enabling dynamic movements of the arm
  • Supraspinatus muscle originates from the superior aspect of the scapula and assists in arm abduction (lifting arm away from body) and stabilization of the humeral head (top of upper arm bone) within the shoulder joint
  • Infraspinatus muscle arises from the posterior aspect of the scapula and contributes to lateral rotation (rotating arm outward) and stabilization of the humeral head
  • Teres minor muscle originates from the lateral border of the scapula and aids in lateral rotation and stabilization of the humeral head
  • Subscapularis muscle lies on the anterior surface of the scapula and enables medial rotation (rotating arm inward) and stabilization of the humeral head
• Rotator cuff muscles work together to maintain shoulder stability by:
  1. Compressing the humeral head against the glenoid fossa (socket of the shoulder blade), preventing dislocation
  2. Counteracting the upward pull of the deltoid muscle, which could cause the humeral head to migrate superiorly
  3. Maintaining proper alignment of the humeral head during various arm movements
  4. Preventing excessive translation (sliding) of the humeral head within the glenoid fossa, thus reducing the risk of impingement and injury

Muscle Interactions and Characteristics

• Antagonist muscles work in opposition to each other, such as the biceps brachii and triceps brachii in elbow flexion and extension
• Synergist muscles work together to produce a specific movement, like the biceps brachii and brachialis in elbow flexion
• Innervation of upper limb muscles is primarily through branches of the brachial plexus, which determines their function and control
• Muscle fiber types in the upper limbs vary depending on the muscle's function, with a mix of slow-twitch (Type I) and fast-twitch (Type II) fibers to accommodate different movement needs