Flexing muscles of the shoulder and back on lady.
By roonb – IMG_1547small, CC BY 2.0

Learning Objectives

By the end of this section, you will be able to:

Identify, locate, and describe the origin, insertion and function of four muscles of the shoulder and back.

We will consider the following four muscle groups:

  • pectoralis major
  • deltoid
  • serratus anterior
  • latissimus dorsi

Pectoralis major

         The pectoral major may colloquially be referred to as “pecs” due to it being the largest and most superficial muscle in the chest area. The pectoralis major (from Latin pectus ‘breast’) is a thick, triangular and fan-shaped muscle that covers both sides of the upper region of the chest (only one side is shown in the figure above).  

inferior, medial half of clavicle, anterior body of the sternum and costal cartilages of ribs 1-6

intertubercular groove (sulcus) of the humerus.

flexion, adduction and rotation of the arm (humerus) at the glenohumeral joint (shoulder).

Try to visualize why this movement occurs based on this muscle’s origin and insertions sites.

Deltoid

         This is the major muscle of the lateral region of the glenohumeral joint (shoulder joint).  It is named after the Greek letter delta, which means triangular. Its three fibres can contract independently or simultaneously.  

clavicle, acromion process of scapula and from the scapular spine

Helpful mnemonic (KenHub):

Deltoid helps you carry SACS:

  • Clavicle
  • Acromion
  • Spine of Scapula

deltoid tuberosity of humerus

Its primary functions is abduction of the arm along with other muscles. Due to its multiple origin sites, the deltoid can also rotate the arm.

As shown below, contraction of the deltoid pulls the head of the humerus into the glenoid cavity.

http://Shoulder_motion_with_rotator_cuff_(supraspinatus).gif

Serratus anterior

         This broad muscle can be seen underlying the pectoralis muscles. It forms much of the lateral musculature of the axillary (underarm) region in the lateral wall of the thorax. Try to identify its origin and insertion sites on the figure. After identifying this muscle, can you think of the reason that it’s called ‘serratus’?

Its fibers attach the upper eight ribs (origin).

It inserts on the anterior surface of the scapula thus stabilizing the scapula.

This muscle abducts and rotates the scapula. It rotates the scapula upward and laterally (shrug shoulders). The serratus anterior is often called the “boxer’s muscle” as it is responsible for protraction of the scapula – a movement which occurs when throwing a punch (AnatomyZone).

Latissimus dorsi

         This large, flat muscle on the back stretches to the sides, behind the arm, and is partly covered by the trapezius on the back near the midline. The latissimus dorsi (AKA: ‘the lats muscle’ or ‘the lats’) is the widest muscle in the human body (KenHub). This muscle is named for the latin word that means “broadest” and  dorsi for back.

Visit this AnatomyZone site. Click on ‘View Interactive 3D model’ below the video. Rotate the image in 3D using your mouse to view the origins and insertion. You can also click on the bones for identification.

spinous processes of thoracic vertebrae T7–T12, thoracolumbar fascia (indirectly connecting this muscle to lumbar vertebrae), iliac crest of the hips, sacrum and inferior 3 or 4 ribs (ribs 9-12)

The Latissimus dorsi run across the back and under the armpit to insert on the intertubercular groove of the humerus.

Primary functions: arm/shoulder adduction (pull downs, pull ups); arm/shoulder extension (swinging the arm toward the back); medial rotation of the arm/shoulder. Also, extension or lateral flexion of the spine. It can influence the movement of the scapula, such as their downward rotation during a pull up. It also has a synergistic role in extension and lateral flexion of the lumbar spine.

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