Triceps Brachii: Origin, Insertion, Nerve, Action & The "Three-Headed" Extensor

The Triceps Brachii is the sole muscle of the posterior compartment of the arm. As the name implies (Tri = three, Cephal = head), it has three heads: Long, Lateral, and Medial. It is the primary extensor of the elbow, opposing the Biceps Brachii.

[Image of Brachialis muscle anatomy]

Quick Anatomy Snapshot

Head	Origin (Proximal)
Long Head	Infraglenoid Tubercle of the Scapula.
Lateral Head	Posterior surface of the Humerus, Superior to the radial groove.
Medial Head	Posterior surface of the Humerus, Inferior to the radial groove (Deepest head).
Insertion (Distal)	Posterior surface of the Olecranon Process of the Ulna and fascia of the forearm.
Nerve Supply	Radial Nerve (C6, C7, C8).
Primary Actions	Elbow Extension: All three heads. Shoulder Extension/Adduction: Long Head only (because it crosses the shoulder joint).

Deep Dive: The Radial Groove

The relationship between the Triceps and the humerus is defined by the Radial Groove (Spiral Groove).

1. The "Sandwich"

The Radial Nerve and Deep Brachial Artery run in the radial groove.
• The Lateral Head bridges over the groove.
• The Medial Head lies below the groove.
This arrangement protects the nerve, but a mid-shaft fracture of the humerus can trap the nerve between the bone and the muscle, causing "Wrist Drop."

2. The Workhorse

The Medial Head is the deep "workhorse" of extension. It is active in all forms of extension. The Long Head is recruited for power and when the shoulder is extended. The Lateral Head is the strongest but is recruited primarily against resistance.

Physio Corner: Clinical Relevance

💪 Functional Fact: The Triceps is essential for "closed chain" movements like pushing yourself out of a chair, using crutches, or doing a push-up. Weakness here makes it very difficult to use walking aids.

Palpation

Have the patient extend their elbow against resistance.
• Long Head: Medial proximal arm.
• Lateral Head: Lateral proximal arm (creates the "horseshoe" shape).
• Medial Head: Palpated distally on the medial side, just above the olecranon, deep to the tendon.

⚠️ Clinical Pathology: Triceps Tendonitis
Pain at the tip of the elbow (Olecranon) is often insertional tendonitis. It is common in weightlifters or laborers who perform repetitive forceful extension. Sudden forceful flexion against a contracted triceps can cause an avulsion fracture of the olecranon.

Manual Muscle Testing (MMT)

We test extension against gravity.

Testing Tip: Ensure the shoulder is abducted to 90 degrees to eliminate the effects of gravity on the shoulder, focusing on the elbow.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Gravity)	Position: Prone (face down). Arm abducted to 90°, forearm hanging off the edge of the table (flexed). Action: Patient extends the elbow until the arm is straight. Resistance: Applied at the distal forearm (wrist), pushing down into flexion. Grade 3: Full range against gravity. Grade 4/5: Holds against strong resistance.
Grade 2 (Gravity Eliminated)	Position: Sitting. Arm abducted to 90° and supported on a table (in the horizontal plane). Action: Patient slides hand away, straightening the elbow. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate the tendon just proximal to the olecranon. Cue: "Try to straighten your arm." Grade 1: Tendon tightens. Grade 0: No activity.

Frequently Asked Questions

Why does the Long Head act on the shoulder?

Because it originates from the Infraglenoid Tubercle of the scapula. It crosses the shoulder joint inferiorly, allowing it to extend and adduct the arm. The other two heads originate on the humerus and do not cross the shoulder.

What is the antagonist?

The primary antagonists are the Biceps Brachii and the Brachialis, which flex the elbow.

Which head is the deepest?

The Medial Head is the deepest. It is covered by the Long and Lateral heads for most of its length.

Test Your Knowledge: Triceps Brachii Quiz

1. What is the origin of the Long Head of the Triceps?

2. Which nerve supplies the Triceps?

3. Where does the Triceps insert?

4. Which head crosses the shoulder joint?

5. The Radial Nerve passes between which two heads?

6. Which head is the deepest?

7. The Lateral Head originates ________ to the radial groove.

8. What is the primary antagonist to the Triceps?

9. The Triceps reflex tests which nerve root?

10. Which exercise best isolates the Triceps?

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.
• Kendall, F. P. (2005). Muscles: Testing and Function, with Posture and Pain. 5th ed. Lippincott Williams & Wilkins.