Pectoral (Shoulder) Girdle

Definition

Pectoral or shoulder girdle consists of both shoulder blades (scapulas) and collar bones (clavicles). There are 5 joints and about 20 muscles involved in the pectoral girdle on each side.

Function

The pectoral girdle connects the upper limbs to the axial skeleton and serves as the attachment site for the muscles of the upper back, chest and neck.

 

The Bones Attached to the Pectoral Girdle

• The upper arm bone (humerus) articulates with the scapula on either side.
• The breastbone (sternum) articulates with the clavicles.

The scapula is not attached to the axial skeleton (the spine) or ribs. When the scapula is in the neutral position, its medial border is about 2 inches (5 cm) away from the spine ^[5].

The Joints of the Pectoral Girdle

1. Scapulohumeral Joint

The scapulohumeral, glenohumeral or shoulder joint connects the glenoid cavity of the scapula and the head of the humerus. The range of motion of the upper arm in the shoulder joint ^[6]:

• 90-95° abduction (sideways)
• 75° adduction anterior to the trunk
• 40-60° extension (backward)
• 90-100° flexion (forward)
• 70-90° internal and external rotation
• 45° horizontal abduction (moving the horizontally extended arm from sideways to backward)
• 135° horizontal adduction (moving the horizontally extended arm from sideways to forward)

2. Acromioclavicular Joint

The acromioclavicular joint connects the acromion of the scapula and the clavicle. The total range of motion is 20-30° ^[6].

3. Sternoclavicular Joint

The sternoclavicular joint connects the sternum and the clavicle articulate. The range of motion ^[6]:

• Anteriorly by 15° (by protraction–forward movements–of the shoulders)
• Posteriorly by 15° (by retraction–backward movement–of the shoulders)
• Upward by 45° (by raising the shoulders)
• Downward by 5° (by depressing the shoulders)

4. Scapulothoracic Joint

The scapulothoracic or scapulocostal joint is not a true joint but a sliding or functional joint between the scapula and the ribs 2 to 7 ^[5]. The scapula and the ribs are not in the direct contact but rather the muscles of the chest wall and the muscles under the scapula slide against each other. The scapulothoracic joint enables movements of the scapula:

• Elevation (by 10 cm), depression (by 2 cm) ^[7-p.176]
• Other movements ^[6]:
  • Adduction or retraction (medially, toward the spine), which is accompanied with internal rotation (medial scapular border moves toward the trunk)
  • Abduction or protraction (laterally, toward the side, by up to 15 cm ^[8]), which is accompanied with external rotation (lateral scapular border moves forward)
  • Upward rotation (the inferior scapular angle moves sideways and upward)
  • Downward rotation (the scapula returns to its neutral anatomical position)
  • Tilting (the upper scapular angle moves forward and the lower backward, and vice versa)

5. Suprahumeral Joint

The suprahumeral or subacromial joint is also not a true but a functional joint between the head of the humerus and the coracoacromial ligament (Picture 1).

The Ligaments of the Pectoral Girdle

Ligaments are bands of the fibrous tissue that connect bones (Picture 1).

• The shoulder capsule consists of several ligaments that connect the upper arm bone (humerus) and the scapula.
• The coracoclavicular ligament connects the coracoid process of the scapula and the clavicle.
• The acromioclavicular ligament connects the acromion of the scapula and the clavicle.
• The superior transverse scapular ligament runs above the suprascapular notch.

Muscles and the Movements of the Pectoral Girdle

Muscles that FLEX (BEND FORWARD) THE NECK:

Sternocleidomastoideus ^[3]

• Origin: the mastoid process on the base of the skull behind the ear
• Insertion: clavicle and sternum
• Sternocleidomastoideus also moves the head in the way that the face turns right or left

Muscles that ELEVATE THE SHOULDERS:

Levator scapulae ^[2]

• Origin: 1st to 4th cervical spinal vertebra
• Insertion: Upper part of medial border of the scapula

Trapezius ^[2]

• Origin: Back of the skull, 7th cervical to 12th thoracic spinal vertebra
• Insertion: The spine and acromion of the scapula, lateral third of clavicle
• Trapezius also extends the neck and thus raises the head

Muscles that DEPRESS THE SHOULDERS:

Trapezius ^[2]

Pectoralis major ^[1]

Muscles that BRING TOGETHER (RETRACT, ADDUCT) THE SHOULDER BLADES:

Trapezius ^[2]

Rhomboid major ^[2]

• Origin: 2nd to 5th thoracic spinal vertebra
• Insertion: Lower two-thirds of medial border of the scapula

Rhomboid minor ^[2]

• Origin: 7th cervical to 1st thoracic spinal vertebra
• Insertion: Upper third of medial border of the scapula

Muscles that ABDUCT THE ARM (MOVE IT SIDEWAYS):

Deltoid (middle head) ^[1]

• Origin: Acromion and the spine of the scapula
• Insertion: Deltoid tuberosity of the upper arm bone

Biceps brachii (long head) ^[8]

• Origin: Supraglenoid tubercle (lateral angle) of the scapula
• Insertion: Tuberosity of the radius (the thinner bone in the forearm)

Supraspinatus ^[1]

• • Origin: Supraspinous fossa of the scapula
  • Insertion: Superior aspect of greater tubercle of the upper arm bone

Muscles that ADDUCT THE ARM (BRING IT TO THE BODY):

Pectoralis major ^[1]

• Origin: a) clavicular head: medial half of the clavicle;  b) sternal head: sternum, upper costal cartilages (1-6)
• Insertion: upper humerus
• Pectoralis major also flexes and extends the arm and rotates it medially.

Teres major ^[1]

• Origin: Lower lateral border and inferior angle of the scapula
• Insertion: Medial lip of the intertubercular (bicipital) groove of the anterior upper arm bone

Teres minor ^[1]

• Origin: Middle part of lateral border of the scapula
• Insertion: Inferior aspect of greater tubercle of humerus

Triceps brachii ^[8]

Deltoid (posterior head) ^[8]

Latissimus dorsi

• Origin: spine from Th7 to L5, sacrum, inferior angle of the scapula, lower three or four ribs
• Insertion: Intertubercular groove of the humerus

Muscles that FLEX THE ARM (BRING IT FORWARD):

Deltoid (anterior head) ^[8]

Pectoralis major ^[8]

Biceps brachii ^[8]

• Origin of the long head: Supraglenoid tubercle (lateral angle) of the scapula
• Origin of the short head: Coracoid process of the scapula
• Insertion: Tuberosity of the radius (the thinner bone in the forearm)

Coracobrachialis ^[8]

• Origin: Coracoid process of the scapula
• Insertion: Mid-medial surface of the upper arm bone

Muscles that EXTEND THE ARM BACKWARD:

Triceps brachii ^[1]

• Origin: Infraglenoid tubercle (lateral angle) of the scapula
• Insertion: Posterior olecranon of the ulna (the thicker bone of the forearm)

Deltoid (posterior head) ^[1]

• Origin: Spine of the scapula
• Insertion: Deltoid tuberosity of the upper arm bone

Teres major ^[8]

Latissimus dorsi ^[8]

Muscles that ROTATE THE ARM OUTWARD (EXTERNALLY):

Infraspinatus ^[1]

• Origin: Infraspinous fossa of the scapula
• Insertion: Middle part of the greater tubercle of the upper arm bone

Deltoid (posterior head) ^[1]

• Origin: Spine of the scapula
• Insertion: Deltoid tuberosity of the upper arm bone

Teres minor

Muscles that ROTATE THE ARM MEDIALLY (INTERNALLY):

Subscapularis ^[1]

• Origin: Front side (subscapular fossa) of the scapula
• Insertion: Upper, medial part of the upper arm bone (lesser tubercle of humerus)

Teres major ^[8]

Pectoralis major ^[8]

Deltoid (anterior head) ^[8]

Latissimus dorsi ^[8]

Muscles that ELEVATE RIBS and thus ASSIST IN BREATHING:

Pectoralis minor ^[1]

• Origin: Anterior surfaces of the 3rd, 4th and 5th rib
• Insertion: Coracoid of scapulae

Serratus anterior ^[2]

• Origin: anterior surfaces of first 8 or 9 ribs
• Insertion: anterior surface of medial border of the scapula
• Serratus anterior also abducts or protracts (moves to the side) shoulder blades

Subclavius ^[4]

• Origin: 1st rib
• Insertion: the bottom surface of the clavicle

A muscle that DEPRESSES the HYOID BONE:

Omohyoid muscle

• Origin: Suprascapular ligament and superior border of the scapula
• Insertion: Inferior border of the hyoid bone

Rotator Cuff

The rotator cuff consists of the tendons of 4 muscles that rotate the upper limb in the shoulder joint: supraspinatus, infraspinatus, subscapularis and teres minor ^[9].

• References

  1. Muscles that act on arm  GetBodySmart
  2. Muscles that act on the shoulder (scapula)  GetBodySmart
  3. Clavicular head of sternocleidomastoid muscle  InnerBody
  4. Subclavius  Healthline
  5. Frank RM et al, 2013, Scapulothoracic Anatomy and Snapping Scapula Syndrome  Hindawi
  6. Glenohumeral Joint  Kean University
  7. Houglum PA, Dolores B Bertoti DB, 2012  Brunnstrom’s Clinical Kinesiology, 6th Edition
  8. The Shoulder Complex  College of Medicine, National Taiwan University
  9. Rotator Cuff Biomechanics  Shoulder.co.uk