Chin-up muscles worked

Every role here is for this technique: underhand grip about shoulder-width, from a dead hang to the chin over the bar.

What muscles does the chin-up work?

The prime movers of the chin-up are latissimus dorsi, biceps brachii and brachialis. They produce shoulder extension and elbow flexion. Six synergists assist, among them brachioradialis, teres major, the sternal part of pectoralis major and the posterior deltoid. Five stabilizers hold joints and segments still, among them the wrist and finger flexors, infraspinatus, and teres minor.

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Joint actions and their prime movers

The joint actions of the chin-up and the prime movers that produce each
Joint action	Prime movers
Shoulder extension	Latissimus dorsi
Elbow flexion	Biceps brachii and brachialis

Every muscle by role

A prime mover produces the lift’s main joint action against the load; a synergist assists that action, or adds a secondary action the lift needs; a stabilizer contracts mostly isometrically to hold a joint or a segment still. A role describes what the muscle does in the technique above, not how hard it works or how much it grows.

Prime movers 3

Prime movers in the chin-up, with what each does
Muscle	What it does here
Latissimus dorsi	Pulling the upper arms down and back
Biceps brachii	Elbow flexion with the forearm turned up More active in the chin-up than the pull-up (Youdas et al. 2010).
Brachialis	Elbow flexion

Synergists 6

Synergists in the chin-up, with what each does
Muscle	What it does here
Brachioradialis	Elbow flexion
Teres major	Shoulder extension
Pectoralis major, sternal part	Pulling the upper arm down from overhead More active in the chin-up than the pull-up (Youdas et al. 2010).
Posterior deltoid	Shoulder extension
Rhomboids	Drawing the shoulder blades in
Lower trapezius	Pulling the shoulder blades down

Stabilizers 5

Stabilizers in the chin-up, with what each does
Muscle	What it does here
Wrist and finger flexors	Gripping the bar
Infraspinatus	Holding the head of the humerus in its socket
Teres minor	Holding the head of the humerus in its socket
Rectus abdominis	Keeping the body from swinging
External oblique	Keeping the body from swinging

Lowering and lifting

What the prime movers of the chin-up do in each half of a rep
Phase	What the prime movers do
Lowering	Latissimus dorsi and biceps lengthen as the body drops
Pulling	They shorten to lift the chin over the bar

Where these prime movers also work

Latissimus dorsi: prime mover in the bent-over barbell row, pull-up and lat pulldown; synergist in the parallel-bar dip.
Biceps brachii: prime mover in the bent-over barbell row, pull-up, lat pulldown and barbell biceps curl.
Brachialis: prime mover in the bent-over barbell row, pull-up, lat pulldown and barbell biceps curl.

Compared with the other vertical pull lifts

Pull-up: the same prime movers, synergists and stabilizers. Biceps brachii here: more active in the chin-up than the pull-up (Youdas et al. 2010). The sternal part of pectoralis major here: more active in the chin-up than the pull-up (Youdas et al. 2010). The lower trapezius in the pull-up: more active in the pull-up than the chin-up (Youdas et al. 2010).
Lat pulldown: prime movers in both, latissimus dorsi, biceps brachii and brachialis; prime movers only in the lat pulldown, teres major.

Sources

Youdas et al. (2010), J Strength Cond Res 24(12):3404
NSCA, Essentials of Strength Training and Conditioning, 4th ed. (2016)

The cited studies that measured muscle activity used electromyography, and a larger signal need not mean more force or more growth (Vigotsky et al. (2018), Front Physiol 8:985). The roles draw on those studies for which muscles a lift involves and how two lifts differ, never to rank one muscle against another.

See these muscles in 3D

Visualiser · Biology Muscles Worked by Exercise 3D Explorer Choose a lift and a real body lights its prime movers, synergists and stabilisers, front and back, with the same muscles in 3D. Visualiser · Biology Muscle Origin, Insertion and Action Explorer Paint a muscle’s origin and insertion on real 3D bones, or click a bone to see what attaches to it.