Supraspinatus: Origin, Insertion, Action & The "Empty Can" Test

The Supraspinatus is a small but vital muscle located in the depression above the spine of the scapula. It is the "S" in the SITS acronym for the Rotator Cuff. It is famous for being the most commonly torn rotator cuff muscle and for its role in starting the motion of lifting the arm.

[Image of Infraspinatus muscle anatomy]

Quick Anatomy Snapshot

Group	Rotator Cuff (Superior Aspect).
Origin (Proximal)	Medial two-thirds of the Supraspinous Fossa of the Scapula.
Insertion (Distal)	Superior facet of the Greater Tubercle of the Humerus.
Nerve Supply	Suprascapular Nerve (C5, C6).
Blood Supply	Suprascapular Artery.
Primary Actions	Initiation of Abduction: Lifts the arm for the first 0-15 degrees. Stabilization: Compresses the humeral head into the glenoid to prevent superior migration.

Deep Dive: The "Starter" Muscle

The Supraspinatus is architecturally designed to generate torque when the arm is at the side, a position where the Deltoid is mechanically disadvantaged.

1. The First 15 Degrees

When your arm is hanging by your side, the Deltoid's line of pull is mostly vertical (pulling the bone up into the socket). The Supraspinatus pulls horizontally, initiating the first 15 degrees of abduction. After 15 degrees, the Deltoid takes over as the prime mover.

2. The Critical Zone

The distal tendon of the Supraspinatus has a region of poor blood supply known as the "Critical Zone" (approx. 1 cm from insertion). This hypovascularity makes it difficult for the tendon to heal after micro-trauma, leading to degeneration and tears.

3. Under the Arch

The muscle passes underneath the Coracoacromial Arch (Acromion process + Coracoacromial ligament). If this space narrows (due to bone spurs or inflammation), the Supraspinatus tendon gets pinched, causing Impingement Syndrome.

Physio Corner: Clinical Relevance

💪 Functional Fact: Think of the Supraspinatus as the "Suitcase Muscle." While it is famous for abduction, it also works isometrically to prevent your arm from being pulled out of the socket when carrying a heavy load at your side.

Palpation

Locate the Spine of the Scapula. Move your fingers superiorly into the fossa above the spine. The muscle is deep to the Upper Trapezius. It is difficult to feel distinct contraction, but tenderness here is a hallmark of rotator cuff pathology.

⚠️ Clinical Pathology: Painful Arc
Patients with Supraspinatus tendinitis often exhibit a "Painful Arc."
• 0-60°: Minimal pain.
• 60-120°: Sharp pain as the inflamed tendon passes under the acromion.
• 120-180°: Pain decreases as the tendon clears the arch.

Manual Muscle Testing (MMT)

We use the "Empty Can" position to bias the Supraspinatus.

Testing Tip: Internal rotation places the greater tubercle (and the tendon) in a position where it must work against gravity without as much Deltoid assistance.

Step-by-Step Procedure (Jobe's Test)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Standing or Sitting. Arms abducted to 90°, horizontally adducted 30° (in the scapular plane). Action: Patient internally rotates shoulders (thumbs pointing down - "pouring out a can"). Resistance: Therapist applies downward pressure on the distal forearms while patient resists. Positive Sign: Pain or weakness indicates Supraspinatus pathology.
Grade 0, 1 (Palpation)	Action: Palpate superior to the scapular spine. Cue: "Try to lift your arm out to the side." Grade 1: Contraction felt deep to Trapezius. Grade 0: No activity.

Frequently Asked Questions

Does it rotate the arm?

Debated. Some texts say it assists weakly in lateral rotation, others say medial. However, its primary function is Abduction and Stabilization (compressing the joint). Rotation is negligible compared to Infraspinatus/Subscapularis.

What is the "Drop Arm Test"?

The examiner passively lifts the patient's arm to 90° abduction and asks them to lower it slowly. If the Supraspinatus is fully torn, the arm will drop suddenly once the Deltoid loses its mechanical advantage (usually around 90-60°).

Which nerve entrapment affects it?

Entrapment of the Suprascapular Nerve at the Suprascapular Notch (often by the transverse scapular ligament) can cause atrophy and weakness of both the Supraspinatus and Infraspinatus.

Test Your Knowledge: Supraspinatus Quiz

1. Which nerve supplies the Supraspinatus?

2. Where does the Supraspinatus insert?

3. What is the primary action of the Supraspinatus?

4. Which test is specific for Supraspinatus pathology?

5. The Supraspinatus tendon passes under which structure?

6. Where does the Supraspinatus originate?

7. The "Critical Zone" of the tendon refers to:

8. Which muscle takes over abduction after the first 15 degrees?

9. Which muscle lies superficial to the Supraspinatus?

10. True or False: Supraspinatus is the most commonly torn rotator cuff muscle.

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Magee, D. J. (2014). Orthopedic Physical Assessment. 6th ed. Elsevier.