The triceps brachii is the muscle that extends the forearm. Located on the back of your upper arm, it straightens the elbow by pulling the forearm backward from a bent position. A smaller helper muscle called the anconeus assists with this movement, but the triceps does the vast majority of the work.
How the Triceps Brachii Is Built
The name “triceps brachii” literally means “three-headed muscle of the arm,” and each head has a slightly different starting point. The long head originates from the shoulder blade, just below the shoulder socket. The lateral head starts on the back of the upper arm bone (humerus), above a groove where the radial nerve passes. The medial head begins on the back of the humerus below that same groove. All three heads merge into a single tendon that attaches to the bony point of the elbow called the olecranon process, which is the hard bump you can feel at the tip of your elbow.
Because the long head connects to the shoulder blade while the other two heads attach only to the upper arm bone, the long head plays a dual role. It extends the forearm at the elbow and also contributes to shoulder stability, particularly when your arm is raised overhead.
The Anconeus: A Small but Useful Assistant
Sitting just behind the elbow is the anconeus, a small triangular muscle that runs from the outer lower end of the humerus to the upper back surface of the ulna (the forearm bone on the pinky side). It’s a weak extensor on its own, contributing roughly 15% of the force during isometric elbow extension. Its more important job may be stabilizing the elbow joint during low-force movements and helping maintain the joint’s position when you rotate your forearm.
What Controls the Movement
The radial nerve powers forearm extension. It branches off the posterior cord of the brachial plexus, a network of nerves that originates from spinal nerve roots C5 through T1 in your neck and upper back. Different branches of the radial nerve fire at different points along its path: branches in the armpit supply the long head and part of the medial head, while branches in the spiral groove of the humerus supply the lateral head, the rest of the medial head, and the anconeus.
This is why injuries to the radial nerve, whether from a fracture of the humerus or prolonged pressure on the back of the arm (sometimes called “Saturday night palsy”), can weaken or eliminate your ability to straighten the elbow.
How Extension Works With Flexion
The triceps and the biceps brachii are classic antagonists. When the triceps contracts to extend your forearm, the biceps and another flexor called the brachioradialis must relax and lengthen to allow the movement. The reverse happens when you curl your arm: the biceps contracts while the triceps lengthens. Interestingly, both the biceps’s antagonist (the triceps) and the brachioradialis are innervated by the radial nerve, while the biceps itself is innervated by a completely different nerve, the musculocutaneous nerve. This separate wiring allows your nervous system to fine-tune the balance between bending and straightening.
During controlled movements like slowly lowering a weight, the flexors don’t just go limp. They co-contract at low levels to stabilize the joint and keep the motion smooth, preventing the elbow from snapping into full extension.
Where the Triceps Is Strongest
The triceps doesn’t produce the same force at every elbow angle. Research measuring maximal voluntary contractions at angles between 10° and 120° of flexion (with a straight arm defined as 0°) found that extension force is lowest near full extension and increases in a roughly linear fashion, peaking at around 80° of flexion. Beyond 80°, force begins to decline again. For practical purposes, the most mechanically favorable position for the triceps to extend the forearm is around 60° of elbow flexion when the shoulder is in a neutral position. This is why exercises like skull crushers or overhead triceps extensions feel hardest at certain points in the range and easier at others.
Normal Range of Elbow Extension
A fully straight arm is considered 0° of extension. Many people, especially younger women, naturally extend a few degrees past straight into mild hyperextension. CDC reference data from over 2,000 healthy individuals shows that females aged 2 to 8 average about 6.8° of hyperextension, while adult males aged 45 to 69 average close to 0° or even slightly short of full extension. These small differences are normal and don’t indicate a problem. Losing the ability to fully extend, however, can signal joint swelling, muscle tightness, or tendon injury.
Triceps Tendonitis and Weakness
Pain at the back of the elbow during extension is the hallmark of triceps tendonitis, an overuse condition common in people who do repetitive pushing or pressing movements. The discomfort is typically localized right where the tendon inserts into the olecranon and gets worse with resisted extension, like pushing yourself up from a chair or locking out a bench press. Swelling around the back of the elbow is possible, and some people notice weakness or a sense of instability when straightening the arm under load.
A physical exam that reproduces pain with resisted elbow extension is usually enough to make the diagnosis. Imaging with ultrasound or MRI is reserved for cases where the severity is unclear or a partial tear is suspected.
Clinicians assess triceps strength on a 0 to 5 scale. A score of 5 means you can hold a fully extended position against strong resistance. A score of 3 means you can straighten the arm against gravity but can’t resist any added pressure. A score of 1 means a faint contraction can be felt in the muscle, but the forearm doesn’t visibly move. Significant drops on this scale point to nerve damage or a tendon disruption rather than simple tendonitis.