Biology

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.

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.
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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

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

Supraspinatus motion. (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.

Deltoid Muscle (green, blue, and red) (Credit: WIkimedia Commons)

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. (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 (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).

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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.

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.

Quiz Yourself!

 
Which
shoulder muscle abducts at 15-90 degrees?
   
Deltoid
   
Supraspinatus
 
Serratus
Anterior
Trapezius
 

Works Cited

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

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

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