W.6.2- Shoulder Muscles and Anatomy

  • Shoulder muscles categorized into three groups based on their origin and insertion:

    • Axioscapular (Trunk to Girdle)

    • Axiohumeral (Trunk to Limb)

    • Scapulohumeral (Girdle to Limb) including Rotator Cuff

Axioscapular Muscles

  • Key Muscles:

    • Trapezius

    • Serratus Anterior

    • Rhomboids (Major & Minor)

    • Levator Scapulae

    • Pectoralis Minor

    • Subclavius

Axioscapular Muscle Details

  • Pectoralis Minor

    • Joint Crossed: Scapulothoracic

    • Position Relative to Joint: Anterior & inferior

    • Actions 3:

    • Stabilizes the scapula by drawing it anteriorly and inferiorly

    • Assists in scapular protraction

    • Aids in raising the ribs during forced inhalation.

    • Stabilizes scapula

    • Innervation: Medial pectoral nerve

  • Levator Scapulae

    • Joint Crossed: Glenohumeral joint, which allows for a wide range of shoulder movements and plays a crucial role in elevation and rotation of the scapula.

    • Position Relative to Joint: Superior and medial

    • Actions: Elevation of scapula

    • Innervation: Dorsal scapular nerve

  • Rhomboids (Major & Minor)

    • Joint Crossed: Scapulothoracic joint and Glenohumeral joint.

    • Position Relative to Joint: Lateral

    • Actions: Retraction of scapula

    • Innervation: Dorsal scapular nerve

Combined Muscle Actions

  • Mechanics:

    • Two forces that are opposite in direction, applied to the same object at different points, and equal in magnitude will produce rotatory movements of the object.

    • Example: Downward rotation of scapula

  • Trapezius

    • Joint Crossed: Scapulothoracic joint

    • Position Relative to Joint: Depends on which fibers contract

    • Actions 4:

    • Elevation of the scapula (upper fibers)

    • Retraction of the scapula (middle fibers)

    • Depression of the scapula (lower fibers)

    • Rotation of the scapula during arm elevation

    • Elevates, retracts, and rotates scapula

    • Innervation: Accessory nerve (CN XI)

  • Serratus Anterior

    • Joint Crossed: Scapulothoracic

    • Position Relative to Joint: The serratus anterior is located anteriorly to the scapula, allowing it to play a crucial role in shoulder movement and stability.

    • Actions: Protraction and stabilization of scapula

    • Innervation: Long thoracic nerve

Subclavius Muscle:

  • Position Relative to Joint: The subclavius is situated beneath the clavicle, providing support and stability to the shoulder girdle.

  • Actions: Depresses and stabilizes the clavicle.

  • Innervation: Nerve to subclavius.

  • Origin: Costal cartilage of the first rib

  • Insertion: Inferior surface of the clavicle and first rib, allowing for effective movement during shoulder stabilization.

  • Function: Plays a crucial role in maintaining shoulder girdle stability and supporting upper limb movement.

  • Joints crossed: Sternoclavicular joint and acromioclavicular joint, which facilitate a wide range of shoulder motions.

  • Innervation: Primarily innervated by the medial and lateral pectoral nerves, which provide the necessary motor control for muscle contractions.

Axiohumeral Muscles

  • Key Muscles:

    • Pectoralis Major

    • Latissimus Dorsi

  • Pectoralis Major

    • Joint Crossed: Shoulder joint; responsible for flexion, adduction, and medial rotation of the arm. Trapezius

    • The pectoralis major muscle crosses three key joints: the sternoclavicular, acromioclavicular, and glenohumeral joints.Specifically, it acts on the shoulder joint (glenohumeral joint)

    • Position Relative to Joint: Anterior & inferior

    • Actions 4:

    • Flexion of the arm at the shoulder joint

    • Adduction of the arm towards the body's midline

    • Medial rotation of the arm

    • Assist in extension from a flexed position.

      • Flexion (clavicular head)

      • Extension from flexed position (sternal head)

    • Inervation: The shoulder muscles are primarily innervated by the brachial plexus, specifically including the suprascapular nerve and the axillary nerve for optimal function.

  • Latissimus Dorsi

    • Joint Crossed: Glenohumeral

    • Position Relative to Joint: The latissimus dorsi is positioned posteriorly and inferiorly to the glenohumeral joint, playing a crucial role in adduction, extension, and internal rotation of the arm.

    • Actions 4:

    • Adduction: Draws the arm towards the body.

    • Extension: Moves the arm backward, increasing the angle at the shoulder.

    • Internal Rotation: Rotates the arm inward towards the body.

    • Depression of the scapula: Aids in lowering the shoulder blade.

      • Medial rotation (attaches to anterior of proximal humerus)

      • Raises body towards arms when climbing

    • Innervation: Thoracodorsal nerve

Scapulohumeral Muscles

  • Key Muscles:

    • Deltoid

    • Teres Major

    • Rotator Cuff Muscles (Supraspinatus, Infraspinatus, Teres Minor, Subscapularis)

  • Teres Major,

    • Joint Crossed: Glenohumeral joint
      • Position Relative to Joint: medial and inferiorly to the shoulder joint
      • Actions: Adduction and medial rotation of the arm
      • Innervation: Lower subscapular nerve (C5, C6)

  • Deltoid

    • Joint Crossed: Glenohumeral

    • Position Relative to Joint: Originates on the lateral third of the clavicle, acromion, and spine of the scapula, inserting on the deltoid tuberosity of the humerus.

    • Actions:

      • Clavicular part: Flexion and medial rotation

      • Acromial part: Abduction

      • Spinal part: Extension and lateral rotation

    • Innervation: Axillary nerve

Rotator Cuff Summary

  • Function: Dynamic ligaments that stabilize the glenohumeral joint, allowing for movement unlike normal ligaments.

  • Supraspinatus

    • Joint Crossed: Glenohumeral

    • Position Relative to Joint: Superior

    • Actions:

      • Stabilizes GHJ

      • Prevents superior dislocation

      • Initiates abduction

    • Innervation: Axillary nerve for the deltoid muscle and suprascapular nerve for the supraspinatus.

  • Infraspinatus

    • Joint Crossed: Glenohumeral

    • Position Relative to Joint: The infraspinatus muscle is located posteriorly to the glenohumeral joint, playing a crucial role in external rotation and stabilization of the shoulder.

    • Actions 3

    • External rotation of the humerus

    • Stabilization of the shoulder joint during movements

    • Contribution to abduction when the shoulder is in certain positions.

    • :

      • Stabilizes GHJ

      • Prevents inferior dislocation

    • Innervation: Suprascapular nerve

  • Teres Minor

    • Joint Crossed: Glenohumeral

    • Position Relative to Joint: Located posteriorly and laterally to the glenohumeral joint, the teres minor plays a crucial role in shoulder stability and movement.

    • Actions:

      • Stabilizes GHJ

      • Prevents inferior dislocation

      • Lateral rotation

    • Innervation: Axillary nerve

  • Subscapularis

    • Joint Crossed: Glenohumeral

    • Position Relative to Joint: The subscapularis muscle is positioned anteriorly to the glenohumeral joint, allowing it to play a crucial role in internal rotation and stabilization of the shoulder.

    • Actions:

      • Stabilizes GHJ

      • Prevents anterior dislocation

      • Medial rotation

      • Helps hold head in glenoid fossa

    • Innervation: Upper and lower subscapular nerves