Shoulder Abduction Muscles: Medical Anatomy and USMLE

Shoulder Abduction Muscles in the Upper Limb

Ever wonder how Super Man lifts his arms above his head and flies in the sky? He uses his shoulder abduction muscles! In this quick and easy lesson, we explain the shoulder muscles responsible for abducting the shoulder, the anatomy, the degrees of arm abduction for each shoulder muscle, and the nerves that innervate the shoulder abduction muscles.

Super Man flying! Question: Which arm of Super Man is doing shoulder abduction?
Answer: His left arm! Read more below to learn why. Image Source: Pxfuel

What is Arm Abduction? What does Arm Abduction Mean?

Arm abduction, also known as shoulder abduction, is the movement of the arm away from the body in the same plane of your chest. The best example is to have your arms straight by your sides and then lifting them up in the same plane as your chest, like doing a jumping jack. Do not lift your arms in front of you or behind you – that is not arm abduction but rather arm flexion or arm extension, respectively.

Diagram of Arm abduction, arm adduction, and arm circumduction. Notice the arm movement of the upper limb/shoulder for arm aBduction.
Diagram of Arm abduction, arm adduction, and arm circumduction. Notice the arm movement of the upper limb/shoulder for arm aBduction. Image Source: Wikimedia Commons

What are the muscles that abduct the shoulder?

The shoulder abduction muscles are Supraspinatus, Deltoid, Trapezius, and Serratus Anterior. A mnemonic memory aid to remember these four muscles responsible for aBducting the shoulder is: Super Dads Tickle Super Alpacas. Imagine our super dads reaching up with their arms stretched out above their heads, like Super Man, and tickling a really tall super alpaca. Yep.

Mnemonic to Remember the Muscles that Abduct the Shoulder

Super Dads Tickle Super Alpacas

Supraspinatus -> Deltoid ->Trapezius ->Serratus Anterior

Which of the following muscles below does NOT help with abducting the shoulder?
Serratus Anterior

What are the Degrees of Arm Abduction for Each Shoulder Muscle?

Supraspinatus motion. (Image Source: Wikipedia)

Supraspinatus is the first shoulder muscle to initiate arm shoulder abduction. The supraspinatus can abduct the shoulder for the first 15 degrees (0-15 degrees). The supraspinatus is located on the greater tubercle of the humerus.

Can you name those shoulder abduction muscles in our ortho bro ❤ lift lift lift Image Source: us at

Deltoid is the second shoulder muscle responsible for arm abduction. The deltoid can abduct the shoulder arm from 15 degrees to 90 degrees (15-90 degrees).

Trapezius animation. (Image Credit: Wikipedia)

Trapezius is the third shoulder muscle responsible for arm abduction. The trapezius abducts the arm from 90 degrees to 160 degrees (90-160 degrees) (Penn State), or also just greater than >90 degrees (USMLE First Aid) (Lam et al 2019).

Serratus Anterior Animation (Image Credit: Wikipedia)

Serratus Anterior is the last shoulder muscle responsible for arm abduction. This muscle can abduct the arm greater than > 100 degrees (USMLE First Aid), or from 160 degrees to 180 degrees (160-180 degrees) (Penn State).

What degrees can the supraspinatus abduct the arm?
0-15 degrees
15-90 degrees
90-160 degrees
>100 degrees
What degrees can the trapezius abduct the arm?
>90 degrees
0-15 degrees
15-90 degrees

What are the nerves that innervate the shoulder arm abduction muscles?

Supraspinatus is innervated by the Suprascapular Nerve.

Deltoid is innervated by the Axillary Nerve.

Trapezius is innervated by the Accessory Nerve.

Serratus Anterior is innervated by the Long Thoracic Nerve.

22-year-old Division I football athlete presents to the orthopedic clinic with difficulty initiating abduction in his right arm. He recently was in a traumatic car accident last night. Which nerve is mostly affected?
Suprascapular nerve
Axillary nerve
Accessory nerve
Long thoracic nerve
68-year-old female with history of breast cancer presents to the clinic after her breast surgery and sentinel node biopsy. She has difficulty abducting her right arm past 100 degrees after surgery removing a tumor in her right breast. Which nerve is most likely impacted?
Long thoracic nerve
Suprascapular nerve
Axillary nerve
Accessory nerve
~~~~~~Explanation The long thoracic nerve is a common nerve affected by breast cancer surgery and sentinel node biopsy. While a student may not know about this fact, we can deduct from the patient vignette. Patient has difficulty abducting past 100 degrees which is the function of the serratus anterior, innervated by the Long Thoracic Nerve. A mnemonic to remember the innervation of the serratus anterior is SALT! serratus anterior=long thoracic Explanation~~~~~~

What are the Blood Supply Arteries/Vasculature to the Shoulder Arm Abduction Muscles?

Blood supply to the Supraspinatus is the suprascapular artery.

Blood supply to the Deltoid is the posterior circumflex humeral artery.

Blood supply to the Trapezius is the Transverse Cervical Artery

Blood supply to the Serratus Anterior is the circumflex scapular artery.

32-year-old male presents to the emergency room with burns, multiple lacerations, and a dislocated left shoulder, after a factory explosion at work. The physician successfully reduced his dislocation, but patient continues to present with weakness abducting his shoulder greater than 15 degrees. Most notably on physical exam, there appears a deformity in the top of his shoulder. What artery supplies the blood to this affected abduction muscle?
Posterior cirumflex humeral artery
Transverse cervical artery
Circumflex scapular artery
Suprascauplar artery
Shoulder Abduction Muscles with Ortho Bro – Medical meme; Image Source: 🙂
shoulder muscle abducts at 15-90 degrees?

Works Cited

Fanelli, Matthew. Upper Limb: Radiology Tutorial of Two-Dimensional Anatomy. Penn State Hershey College of Medicine.

Kalluri AG, Miao KH, Bordoni B. Anatomy, Shoulder and Upper Limb, Supraclavicular Fossa. 2021 Feb 8. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan–. PMID: 30725950.

Lam, Jonathan; Burdoni, Bruno. Anatomy, Shoulder and Upper Limb, Arm Abductor MusclesLam, Jonathan; Burdoni, Bruno. NCBI. 2019.

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