Anterior View Shoulder Muscles

Anterior View Shoulder Muscles

Muscles of the Shoulder

Bones of the Shoulder:

• The scapula and clavicle, which together form the pectoral girdle, and the proximal end of the humerus.
• Three joints of the shoulder:
  • The sternoclavicular joint is where the sternum and clavicle meet.
  • The acromioclavicular is where the acromion of the scapula and clavicle meet.
  • The glenohumeral joint is where the glenoid cavity of the scapula and the head of the humerus meet. This is usually the joint we're referring to when we talk about "the shoulder joint."
    The muscles of the rotator cuff cross the glenohumeral joint and contribute to its movements and stability.
    The glenohumeral joint is a highly moveable ball-and-socket joint that allows flexion, extension, abduction, adduction, medial and lateral rotation, and circumduction of the arm.
    Protraction and retraction of the scapula move the position of the glenohumeral joint, which provides even greater range of mobility of the arm (try it yourself – hold your scapula in place and move your arm – then allow your scapula to move, too – your reach is extended when the scapula rotates!).
• Many of the muscles of the shoulder are innervated by the brachial plexus.
• Shoulder separation occurs when the acromioclavicular joint is dislocated – this occurs when the ligaments that hold the acromion and clavicle are injured.
• Dislocation of the glenohumeral joint occurs when the ligaments that secure the humerus to the scapula are injured – this can damage the axillary and radial nerves, or tear the muscles of the rotator cuff.

Shoulder in Anterior View

We show the ribs, sternum, vertebral column of the axial skeleton, and the clavicle and scapula of the pectoral girdle; and, the humerus.

• Fibers of pectoralis major arise from the medial clavicle, the body and manubrium of the sternum, and the costal cartilages of ribs 1-6; they form a collective insertion on the humerus – specifically, on the lateral lip of the intertubercular groove (aka, intertubercular sulcus).
  • If you activate your pectoralis major, you'll activate pectoralis major – notice that your arm flexes, adducts, and rotates medially (think of hugging).
• The deltoid forms a cup-like cap over the shoulder; its fibers arise from the lateral 1/3rd of the clavicle, the acromion, and the spine of the scapula, and insert on the deltoid tuberosity of the humerus.
  • Notice the small space between the origins of pectoralis major and deltoid on the clavicle – this is the clavipectoral triangle: the cephalic vein, which drains the lateral side of the superficial arm, dives deep via this triangle on its way to drain into the axillary vein.
• Deltoid in lateral view: we sketch a small scapula, clavicle, and proximal humerus in lateral view – label the spine and acromion of the scapula.
  • The anterior fibers of the deltoid originate on the lateral 1/3rd of the clavicle; thus, these fibers flex and medially rotate the arm.
  • The middle fibers arise from the acromion of the scapula; these fibers abduct the humerus (after supraspinatus initiates the movement).
  • The posterior fibers originate along the spine of the scapula; these fibers extend and laterally rotate the arm (the opposite of the anterior fibers).
• Subscapularis lies in the subscapular fossa of the scapula; it inserts on the lesser tubercle of the humerus, and medially rotates the arm. It contributes to the rotator cuff.