Guide

Biceps, brachialis and brachioradialis: the elbow flexors in the pull-up

Which arm muscle leads with each grip, what happens in every phase of the rep, and why the inside of your elbow sometimes hurts·8 min read

Three muscles bend your elbow in a pull-up: the biceps brachii, the brachialis and the brachioradialis. Which one leads depends on how your palms face: a supinated chin-up grip favors the biceps, an overhand pull-up grip shifts work toward the brachialis and brachioradialis, and a neutral grip sits in between. Here is how each one is built, what it does in every phase of the rep, and why high-volume chin-ups sometimes leave the inside of your elbow sore.

Anatomy at a glance: the three elbow flexors

Three muscles bend your elbow when you pull. They sit in layers on the front of the upper arm and the top of the forearm, and each one behaves differently depending on how your hand is turned.^[1]^[2]^[3]

Muscle	Origin	Insertion	Nerve	Action
Biceps brachii (long and short head)	Long head: supraglenoid tubercle of the scapula. Short head: coracoid process	Radial tuberosity, plus the bicipital aponeurosis into the forearm fascia	Musculocutaneous nerve (C5–C6)	Flexes the elbow, supinates the forearm (its most powerful action once the elbow is bent), weak shoulder flexion^[1]
Brachialis	Distal half of the anterior humerus	Ulnar tuberosity and coronoid process of the ulna	Musculocutaneous nerve; the lateral part usually also gets a branch from the radial nerve	Pure elbow flexion in every forearm position; the strongest of the three flexors^[2]
Brachioradialis	Lateral supracondylar ridge of the humerus	Distal radius, near the styloid process	Radial nerve (C5–C6)	Flexes the elbow, strongest in mid-pronation (neutral grip); brings the forearm back to neutral from either extreme^[3]

You can find all three on yourself. Bend your elbow to 90 degrees, turn your palm up and squeeze: the belly that rises in the middle of the upper arm is the biceps. Now turn your palm down and keep squeezing. The biceps softens, and on either side of its lower tendon, just above the elbow crease, you can press into a firmer, deeper muscle. That is the brachialis.

For the brachioradialis, rest your forearm on a table with the thumb pointing up, as if holding a hammer. Push the back of your wrist up against your other hand and the long ridge on the thumb side of the forearm hardens from the elbow halfway to the wrist.

Which flexor leads with which grip?

The biceps is the only one of the three that also supinates: it turns the palm up by pulling the radius around the ulna. When the forearm is already supinated, the biceps tendon is unwrapped from the radius and pulls in a straight line. That is why a chin-up (supinated, palms facing you) puts the biceps in its strongest position.^[1]

Turn the palms down for a pull-up and the biceps tendon wraps around the radius. The muscle is now at a mechanical disadvantage for bending the elbow, so more of the work shifts to the muscles that do not care about forearm rotation: the brachialis, which flexes the same way in every position, and the brachioradialis, which is at its best near mid-pronation.^[2]^[3] The biceps still works hard in a pull-up, just not as the lead flexor.

A neutral grip (palms facing each other) is the middle ground. The brachioradialis is close to its strongest line of pull, the biceps is partly unwrapped, and most people find it the most elbow-friendly of the three.

What happens phase by phase?

Dead hang. With the elbows straight, the flexors are nearly silent. The long head of the biceps still matters: its tendon runs over the top of the humeral head inside the shoulder joint and helps keep the joint centered under load.^[1] That is one reason a long hang feels like shoulder work, not arm work.
Initiation. The first few centimeters come from the shoulder blades and the lats. In EMG recordings the lower trapezius and the pectoralis major reach their peak early, before the elbow flexors.^[4]
Mid-pull. Between roughly 90 degrees of elbow bend and the chin reaching bar height, elbow flexor demand peaks. The biceps and brachialis are both near their best working length and the lats are shortening fast.
Top. At the bar the elbow is fully bent. The biceps is short and weak here, the brachialis still produces force, and the shoulder extensors finish the job.
Lowering. On the way down the same muscles lengthen under load. Average activation is lower in the eccentric phase than in the concentric one,^[5] but the tendons at the elbow still carry large forces, and this is the phase that tends to irritate them when volume climbs.

What does the research say?

Surface EMG studies agree on the big picture. Youdas and colleagues recorded 25 trained adults doing pull-ups and chin-ups and found the latissimus dorsi at roughly 117 to 130 percent of its maximum voluntary isometric contraction, with the biceps brachii at 78 to 96 percent. The biceps and the pectoralis major were significantly more active in the chin-up than in the pull-up, while the lower trapezius was more active in the pull-up.^[4]

Dickie and colleagues compared supinated, pronated, neutral and rope grips in 19 strength-trained men. Across a full repetition, peak and average activation of the arm and shoulder muscles were similar between hand positions. The only clear grip effect was higher middle trapezius activity with the pronated grip than with the neutral grip. In every variation the concentric phase produced more brachioradialis, biceps and pectoralis activation than the eccentric phase.^[5]

Read these results with two caveats. First, the differences between grips are modest: the lats dominate every version, and the biceps works hard in all of them. Second, none of these studies measured the brachialis directly. It lies under the biceps, so surface electrodes pick up mostly biceps signal, and fine-wire recordings are rare in pull-up research.

Grip width is barely studied for the elbow flexors. Mechanically, a narrower grip lets the elbow bend further and asks more of the flexors; a wider grip shortens the elbow range and shifts emphasis to the shoulder. Treat that as a model, not a measured result.

"All arms, no back" or "arms give out first": what to do

You feel it only in your arms. The usual reason is that the shoulder blades never move. You hang passively, bend the elbows first and curl yourself up. The fix is sequencing, not more arm strength: pull the shoulder blades down and together before the elbows bend, and drive the elbows toward your ribs rather than toward the floor. Scapular pull-ups and assisted reps with a slow two-second start from the bottom teach this well. The common form mistakes guide covers the pattern in detail.

Your back feels fine but your arms fail first. Then the flexors really are the weak link. Train them directly:

Chin-up negatives: jump or step to the top, lower in 4 to 6 seconds, 3 to 5 reps, twice a week.
Hammer curls and reverse curls: the neutral and pronated grips bias the brachialis and brachioradialis, which do most of the flexing in an overhand pull-up. 2 to 3 sets of 8 to 12.
Slow eccentric curls with a supinated grip for the biceps, and band-assisted chin-ups to practice the full range under lighter load.

If your reps are stuck and the arms give out before anything else, the plateau guide walks through how to tell a strength limit from a fatigue limit. If the back is the limit instead, shift the volume to rows and pull-downs for a block and keep arm work at maintenance level. The full muscle map is in the muscles a pull-up trains.

Elbow problems in pull-up trainees: what to watch

Three tendon complaints come up again and again in high-volume pull-up trainees. None of this is a diagnosis; it is a map of what clinicians look at.

Inside of the elbow: medial epicondylalgia ("golfer's elbow"). This is a tendinopathy of the common flexor-pronator origin on the medial epicondyle, the bony bump on the inside of the elbow. The classic signs are local tenderness over that bump and pain with resisted wrist flexion or forearm pronation. It is linked to repetitive, forceful gripping.^[6] High-volume chin-ups, where you grip hard with the wrist bent and the forearm loaded in supination, are a typical trigger.

Front of the elbow: distal biceps tendinopathy. Pain sits deep in the elbow crease where the biceps tendon reaches the radial tuberosity, often worse when you lower slowly from a chin-up or resist supination.

Front of the elbow, deeper: "climber's elbow" (brachialis tendinopathy). Common in climbers who hang on bent arms for long periods, and in pull-up trainees who do a lot of top holds and lock-offs. Pain is at the front of the elbow, just above the joint, and gets worse with loaded flexion in a pronated or neutral grip.

Load management is the part of treatment every version shares. Cut the volume that provokes it rather than stopping entirely, change the grip so the sore tendon is unloaded for a few weeks, slow the eccentric, and add light isometric and slow-loaded work for the painful tendon as it calms down.

See a clinician if pain lasts more than a few weeks despite reduced load, if there is swelling or a loss of elbow motion, if you have numbness or tingling in the ring and little finger (the ulnar nerve runs just behind the medial epicondyle), or, urgently, if you felt a sudden pop at the elbow followed by bruising and weakness turning the palm up. That last pattern can mean a distal biceps tendon rupture, which has a time-sensitive repair window.

How to program for your elbow flexors

Two to three pulling sessions a week is enough for most people. Rotate grips across the week so no single tendon takes all the volume: one overhand day, one neutral or underhand day, and a third session that is lighter or assisted. Ramp total weekly reps by no more than roughly 10 to 20 percent at a time, and hold a level for a week before adding again. If your elbows complain, drop the underhand volume first, keep the neutral grip, and slow your negatives. The weekly pull-up plan shows how that looks on paper. PullUp Master applies the same rules automatically: it adjusts volume from how the last session felt, so a sore elbow or a failed set lowers the next day's load instead of pushing it up.

Frequently asked questions

Do pull-ups build biceps?

Yes. Surface EMG shows the biceps working at roughly 78 to 96 percent of its maximum in pull-ups and chin-ups, more in the chin-up. They do not isolate it like a curl does, but they load it heavily.

Are chin-ups or curls better for biceps?

Chin-ups load the biceps with more weight and train the back at the same time; curls isolate it and are easier to progress in small steps. For arm size use both; for pull-up performance prioritize chin-ups and add curls as accessory work.

Why does the inside of my elbow hurt after pull-ups?

Most often it is irritation of the common flexor-pronator tendon at the medial epicondyle, known as golfer's elbow, which is linked to hard, repetitive gripping. Reduce the volume that provokes it, change grip for a few weeks, and see a clinician if it lasts or comes with numbness.

What is the brachialis?

A thick muscle under the biceps that runs from the front of the humerus to the ulna. It flexes the elbow in every forearm position and is the strongest elbow flexor, which is why it does much of the work in an overhand pull-up.

Should I train biceps separately?

Only if your arms fail before your back does, or you want more arm size. Two or three sets of hammer curls and slow chin-up negatives twice a week are usually enough; keep it small so it does not eat into recovery for your pulling sessions.

Sources

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