The tricep, formally called the triceps brachii, is the large muscle running along the back of your upper arm. It makes up roughly two-thirds of your upper arm’s total muscle volume, making it considerably larger than the biceps on the front side. Its primary job is straightening your elbow, a motion you use every time you push a door open, throw a ball, or press yourself up from a chair.
Three Heads, One Muscle
The name “triceps” literally means “three heads,” and each head is a distinct bundle of muscle fibers that merges into a single tendon at the elbow. The long head starts at the shoulder blade, just below the shoulder socket. The lateral head originates from the back of the upper arm bone (the humerus), on the outer side. The medial head also starts on the back of the humerus but sits deeper and lower than the lateral head.
All three heads converge into one shared tendon that attaches to the pointed bony tip of the elbow, called the olecranon. This is the hard bump you feel when you rest your elbow on a table. Because the tendon pulls on this single point, all three heads work together to produce the same motion: extending (straightening) the elbow.
How Each Head Contributes to Movement
Even though all three heads straighten the elbow, they don’t contribute equally in every position. When your arm is down at your side, the long head generates the most force and shows the highest activation. But as you raise your arm overhead, the long head gets stretched across the shoulder joint and loses mechanical advantage. At that point, the medial head takes over as the primary driver of elbow extension.
This shift happens because the long head is the only part of the triceps that crosses two joints. It connects to the shoulder blade, so shoulder position directly affects how much it can contract. The lateral and medial heads only cross the elbow, so their length stays the same no matter where your shoulder is positioned. This two-joint design also gives the long head a secondary role: it helps pull the arm backward and inward toward the body, assisting with shoulder extension and adduction.
Muscle Fiber Composition
The triceps is a predominantly fast-twitch muscle, with roughly 60 to 70 percent of its fibers classified as Type II (fast-twitch). Fast-twitch fibers generate force quickly and powerfully but fatigue faster than slow-twitch fibers. This composition makes the triceps well suited for explosive pushing and throwing movements. It also means the muscle tends to respond well to heavier resistance training and lower-to-moderate rep ranges, though training variety still matters for complete development.
Nerve and Blood Supply
The radial nerve controls all three heads of the triceps. This nerve branches from the posterior cord of the brachial plexus, a network of nerves running from the neck through the armpit and into the arm. The nerve fibers originate from spinal segments C6, C7, and C8, with each segment primarily serving a different head. If you’ve ever hit your arm in just the right spot and temporarily lost the ability to push or straighten your elbow, radial nerve compression was likely the cause.
Blood reaches the triceps mainly through the deep brachial artery (profunda brachii), which travels along the back of the humerus between the bone and the lateral head. Branches of this artery thread through the medial head as well, creating a network that keeps all three heads supplied during activity.
Training the Triceps Effectively
Because shoulder position changes which head does the most work, varying your arm angle during exercises is one of the simplest ways to train the full muscle. Overhead extensions (arms raised above the head) place the long head in a stretched position, forcing the medial head to work harder. Pushdowns and pressing movements with arms at your sides let the long head contribute more. Both positions activate the lateral head to a similar degree.
One practical note from exercise research: overhead dumbbell extensions and lying dumbbell extensions produce very similar activation patterns across all three heads. Doing both in the same workout adds volume without adding variety, so picking one or the other is more efficient. Pairing an overhead movement with a pushdown or close-grip press covers the full range of head activation more effectively.
Compound pressing movements like bench presses, overhead presses, and dips already load the triceps heavily. Isolation exercises are useful for additional volume but aren’t strictly necessary if your pressing volume is already high.
Common Triceps Injuries
The most frequent triceps injury is tendonitis at the distal tendon, where the muscle attaches to the elbow. It shows up as pain and tenderness right at the bony tip of the elbow, and the discomfort gets noticeably worse when you try to straighten your arm against resistance. You might also notice swelling around the elbow and a general feeling of weakness during pushing movements.
Triceps tendonitis is common in people who do a lot of repetitive pressing or throwing, including weightlifters, baseball pitchers, and manual laborers. A physical therapist can usually diagnose it through a hands-on examination without imaging, though an X-ray or MRI may be ordered if the severity is unclear or if initial treatment doesn’t help. Most cases respond to rest, load management, and gradual rehabilitation. If symptoms persist beyond six months or significant strength loss remains, surgical repair becomes an option.
Full tendon tears are rarer but more serious. They typically happen during a sudden heavy load, like catching yourself during a fall or attempting a max-effort lift. A complete tear usually means you can’t straighten your elbow at all against gravity, and the injury generally requires surgical repair followed by several months of rehabilitation.