Study Notes on Shoulder Muscles

Muscles of the Shoulder

Overview

  • The muscles of the shoulder are categorized based on their anatomical locations and respective functions.

  • Two major categories:

    • Muscles that move the pectoral girdle.

    • Muscles that move the humerus.

  • Rotator cuff muscles also play a significant role in shoulder mobility and stability.

Outline

  • Muscles that Move the Pectoral Girdle:

    • Anterior Muscles

    • Posterior Muscles

  • Muscles that Move the Humerus:

    • Rotator Cuff

    • Anterior Muscles

    • Posterior Muscles

Muscles that Move the Pectoral Girdle (Thoracoappendicular Muscles)

Anterior Muscles

1. Subclavius

  • Origin (O): Junction of the 1st rib and costal cartilage.

  • Insertion (I): Inferior surface of the middle 1/3 of the clavicle.

  • Action (A): Depresses the clavicle.

  • Innervation (N): Nerve to Subclavius.

2. Pectoralis Minor

  • Origin (O): 3rd – 5th ribs near the costal cartilage.

  • Insertion (I): Coracoid process.

  • Action (A): Facilitates scapular depression and protraction.

  • Innervation (N): Medial Pectoral Nerve.

3. Serratus Anterior

  • Origin (O): External surfaces of the angles of ribs 1-8.

  • Insertion (I): Anterior surface of the medial border of the scapula.

  • Action (A): Promotes scapula protraction and upward rotation.

  • Innervation (N): Long Thoracic Nerve.

4. Winging of the Scapula

  • Description: Dysfunction of the serratus anterior may lead to abnormal scapular movement, often due to injury to the long thoracic nerve.

Posterior Muscles

1. Trapezius

  • Origin (O):

    • Upper: Nuchal line and External occipital protuberance.

    • Middle: Spinous processes of T1-T4.

    • Inferior: Spinous processes of T5-T12.

  • Insertion (I):

    • Upper: Clavicle, lateral 1/3.

    • Middle: Acromion.

    • Inferior: Spine of scapula.

  • Action (A):

    • Upper: Elevates scapula.

    • Middle: Retraction of scapula.

    • Inferior: Depression of scapula.

  • Innervation (N): Spinal Accessory Nerve (Cranial Nerve XI).

2. Levator Scapulae

  • Origin (O): Transverse processes of C1-C4.

  • Insertion (I): Medial superior scapula.

  • Action (A): Elevates/scapular upward rotation.

  • Innervation (N): Dorsal Scapular Nerve.

3. Rhomboid Major and Minor

  • Origin (O):

    • Minor: Spinous Processes of C7–T1.

    • Major: Spinous Processes of T2–T5.

  • Insertion (I): Medial border of the scapula (both).

  • Action (A): Retracts scapula and facilitates downward rotation.

  • Innervation (N): Dorsal Scapular Nerve.

Muscles that Move the Humerus

Rotator Cuff Muscles

  • The rotator cuff consists of four muscles:

    • Supraspinatus

    • Infraspinatus

    • Teres Minor

    • Subscapularis

1. Supraspinatus

  • Origin (O): Supraspinous fossa.

  • Insertion (I): Greater tubercle of humerus.

  • Action (A): Initiates glenohumeral (GH) abduction to $30^ ext{o}$ and stabilizes the humeral head in the glenoid cavity.

  • Innervation (N): Suprascapular Nerve.

2. Infraspinatus

  • Origin (O): Infraspinous fossa.

  • Insertion (I): Greater tubercle of humerus.

  • Action (A): Provides GH external rotation and stabilizes the humeral head in the glenoid cavity.

  • Innervation (N): Suprascapular Nerve.

3. Teres Minor

  • Origin (O): Middle 1/3 of the lateral border of the scapula.

  • Insertion (I): Greater tubercle of humerus.

  • Action (A): Allows for GH external rotation and stabilizes the humeral head in the glenoid cavity.

  • Innervation (N): Axillary Nerve.

4. Subscapularis

  • Origin (O): Subscapular fossa.

  • Insertion (I): Lesser tubercle of humerus.

  • Action (A): Allows for GH internal rotation and adduction while stabilizing the humeral head in the glenoid cavity.

  • Innervation (N): Subscapular Nerve.

Muscles That Move the Humerus (Anterior)

1. Pectoralis Major

  • Origin (O): Clavicular head from the anterior medial ½ of the clavicle; sternal head from the anterior sternum, costal cartilages 1-6, and aponeurosis of the external oblique muscle.

  • Insertion (I): Lesser tubercle of humerus.

  • Action (A): Facilitates GH adduction, internal rotation, flexion (from a position of 0 degrees), and extension (from a flexed position).

  • Innervation (N): Lateral and Medial Pectoral Nerves.

2. Deltoid

  • Origin (O):

    • Anterior: Lateral 1/3 of clavicle.

    • Middle: Acromion process.

    • Posterior: Spine of scapula.

  • Insertion (I): Deltoid tuberosity of humerus.

  • Action (A):

    • Anterior fibers: GH flexion, internal rotation, horizontal adduction.

    • Middle fibers: GH abduction.

    • Posterior fibers: GH extension, external rotation, horizontal abduction.

  • Innervation (N): Axillary Nerve.

3. Coracobrachialis

  • Origin (O): Coracoid process of scapula.

  • Insertion (I): Medial 1/3 of humerus.

  • Action (A): Allows for GH flexion and adduction.

  • Innervation (N): Musculocutaneous Nerve.

4. Biceps Brachii

  • Origin (O):

    • Short head: Coracoid process of scapula.

    • Long head: Supraglenoid tubercle.

  • Insertion (I): Radial tuberosity.

  • Action (A): Facilitates elbow flexion, forearm supination, and GH flexion.

  • Innervation (N): Musculocutaneous Nerve.

Muscles That Move the Humerus (Posterior)

1. Latissimus Dorsi

  • Origin (O): Spinous processes of T7 - T12, thoracolumbar fascia, and iliac crest.

  • Insertion (I): Lesser tubercle of humerus.

  • Action (A): Facilitates GH extension, adduction, and internal rotation.

  • Innervation (N): Thoracodorsal Nerve.

2. Teres Major

  • Origin (O): Inferior 1/3 of lateral border of the scapula.

  • Insertion (I): Lesser tubercle of humerus.

  • Action (A): Internal rotation, adduction, and extension of the GH joint.

  • Innervation (N): Subscapular Nerve.

3. Triceps Brachii

  • Origin (O):

    • Long head: Infraglenoid tubercle of scapula.

    • Lateral head: Posterior humerus superior to the radial groove.

    • Medial head: Posterior humerus inferior to radial groove.

  • Insertion (I): Olecranon process of the ulna.

  • Action (A): Provides elbow and shoulder extension.

  • Innervation (N): Radial Nerve.

Force Couples Between Muscles

1. Supraspinatus – Deltoid Force Couple

  • The supraspinatus initiates abduction by elevating the humeral head and pulling it into the glenoid fossa with assistance from other rotator cuff muscles.

  • The deltoid leverages the mechanical advantage to abduct the humerus once the humeral head is stabilized within the glenoid.

2. Upper Trapezius – Serratus Anterior Force Couple

  • The upper trapezius contracts to facilitate upward rotation of the scapula.

  • The serratus anterior contracts, pulling the scapula away from the midline, resulting in a vector that allows the scapula to protract around the rib cage.

Conclusion

  • Understanding the anatomy and functioning of these muscles is crucial for rehabilitation, athletic performance, and overall shoulder health.

  • The coordination between the muscles involved in moving the pectoral girdle and humerus highlights the dynamic nature of shoulder mechanics and its importance in activities ranging from daily tasks to advanced sports performance.