The triceps brachii is the primary muscle that extends the elbow. It sits along the back of your upper arm and is the only major muscle responsible for straightening your arm from a bent position. A smaller muscle called the anconeus assists with this movement but contributes relatively little force on its own.
The Three Heads of the Triceps
The name “triceps brachii” literally means “three-headed muscle of the arm.” Each head has a different starting point on the bone but all three merge into a single tendon that attaches to the olecranon, the bony point of your elbow.
- Long head: The only head that crosses two joints. It originates on the shoulder blade (scapula) and runs down the inside of the upper arm. Because it crosses the shoulder, its contribution to elbow extension changes depending on your arm position.
- Lateral head: Starts on the outer, upper portion of the humerus (the upper arm bone). It forms the visible outer bulk of the triceps and is easy to see on a muscular arm.
- Medial head: Sits deeper, originating on the back surface of the middle portion of the humerus. Despite being the least visible head, it plays a surprisingly large role in generating force.
All three heads funnel into a shared tendon that anchors to the olecranon process of the ulna, which is the forearm bone on the pinky side. When the triceps contracts, it pulls on this attachment point and straightens the elbow.
How Each Head Contributes to Force
The three heads don’t share the workload equally, and which head does the most depends on your shoulder position. Research measuring muscle force and electrical activation found that with your arm down at your side (0° shoulder elevation), the long head generates significantly more force than the other two. But when your arm is raised to shoulder height or overhead (90° to 180°), the medial head takes over as the dominant force producer.
At overhead positions, the lateral head also ramps up its force output, producing significantly more than the long head, though still less than the medial head. This shift happens because the long head becomes slack when the shoulder is flexed, reducing its mechanical advantage. All three heads peak in force production when the elbow is bent between 85° and 110°, and force drops off as the arm approaches full extension.
This has a practical takeaway: if you’re doing exercises for the back of your arm, overhead movements will emphasize the medial and lateral heads, while movements with your arm at your side will load the long head more heavily.
The Anconeus: A Small Helper
The anconeus is a small, triangular muscle that sits just behind the elbow joint, covering the outer side. Electromyographic studies confirm that its electrical activity increases during extension, so it does function as an extensor. However, it’s a weak one. When researchers blocked the anconeus in a study, the triceps easily compensated for the lost contribution.
The anconeus has a more important role as a stabilizer. Its strong attachment to the lateral joint capsule makes it an active stabilizer of the elbow, particularly on the outer (posterolateral) side. Surgeons and physical therapists use the anconeus in rehabilitation of elbow instability for this reason.
Nerve Supply and Reflexes
The radial nerve controls the triceps. This nerve branches from the posterior cord of the brachial plexus and carries fibers from spinal nerve roots C5 through T1. The triceps reflex, which doctors test by tapping just above the olecranon with a reflex hammer, specifically assesses the C7 and C8 spinal segments. If the forearm extends in response to the tap, those nerve pathways are intact.
Damage to the radial nerve, whether from a fracture of the mid-humerus, prolonged pressure (sometimes called “Saturday night palsy” from draping an arm over a chair), or other injury, can weaken or eliminate the ability to extend the elbow.
How the Biceps and Triceps Coordinate
The biceps and triceps work as antagonists. When you extend your elbow, your nervous system simultaneously activates the triceps and inhibits the biceps through a process called reciprocal inhibition. Sensory fibers in the triceps send a signal through a single relay neuron in the spinal cord that suppresses the motor neurons of the biceps. This inhibition is fast and automatic, allowing smooth, efficient straightening of the arm without the biceps fighting against the movement. The same mechanism works in reverse when you bend the elbow: the biceps fires while the triceps is inhibited.
Triceps Tendon Pain at the Elbow
Pain at the very back of your elbow, right where the triceps attaches, can indicate triceps tendinopathy. This condition is less common than tennis elbow or golfer’s elbow, but it follows the same pattern: activity-related pain that worsens with pushing or straightening movements. You may notice swelling and tenderness when pressing on the tip of the olecranon. Resisted extension (pushing against resistance while straightening your arm) typically reproduces the pain, though strength is usually preserved.
Imaging can help confirm the diagnosis. X-rays occasionally reveal a small bone fragment pulled away from the olecranon where the tendon attaches. Ultrasound, performed with the elbow bent, shows changes in tissue density and sometimes calcification. MRI provides the most detailed picture and can rule out other causes of posterior elbow pain.
Exercise Selection for the Triceps
A study comparing overhead dumbbell extensions with lying (flat) dumbbell extensions in 21 trained men found that both exercises produced similar activation patterns in the long and lateral heads. The concentric (lifting) phase generated more muscle activity than the eccentric (lowering) phase in both exercises, and the initial and middle portions of the range of motion produced higher activation than the final portion near full extension.
Because both exercises activated the triceps in nearly identical patterns, the researchers concluded that programming both in the same workout is redundant. If you want to target different heads, varying your shoulder position (arm overhead versus arm at your side versus arm behind you) is the key variable, since shoulder angle shifts which head contributes the most force.