Back squat muscles worked

Every role here is for this technique: barbell high on the upper back, shoulder-width stance, lowered until the thighs are parallel to the floor.

What muscles does the back squat work?

The prime movers of the back squat are the quadriceps, gluteus maximus and adductor magnus. They produce knee extension and hip extension. The synergists are the hamstrings and soleus. Twelve stabilizers hold joints and segments still, among them the erector spinae, multifidus and rectus abdominis. The roles follow Escamilla (2001), Schoenfeld (2010), Németh and Ohlsén (1985), and NSCA (2016).

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Joint actions and their prime movers

The joint actions of the back squat and the prime movers that produce each
Joint action	Prime movers
Knee extension	The quadriceps
Hip extension	Gluteus maximus and adductor magnus

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 back squat, with what each does
Muscle	What it does here
Quadriceps	Knee extension, all four heads
Gluteus maximus	Hip extension
Adductor magnus	Hip extension Its hamstring part extends the hip, with more leverage low in the squat, where the hip is bent, than near the top (Németh and Ohlsén 1985).

Synergists 2

Synergists in the back squat, with what each does
Muscle	What it does here
Hamstrings	Hip extension, while also crossing the knee They lengthen at the hip as they shorten at the knee, so their length changes much less than that of gluteus maximus.
Soleus	Controlling the forward lean of the shin

Stabilizers 12

Stabilizers in the back squat, with what each does
Muscle	What it does here
Erector spinae	Holding the spine neutral under the bar
Multifidus	Stiffening each lumbar segment
Rectus abdominis	Bracing the trunk against the load
External oblique	Bracing the trunk and resisting rotation
Internal oblique	Bracing the trunk and resisting rotation
Transversus abdominis	Raising abdominal pressure to stiffen the trunk
Quadratus lumborum	Holding the pelvis level against the spine
Gluteus medius	Keeping the pelvis level and the knees over the feet
Gluteus minimus	Keeping the pelvis level and the knees over the feet
Upper trapezius	Making the shelf the bar sits on
Middle trapezius	Holding the shoulder blades together under the bar
Rhomboids	Holding the shoulder blades together under the bar

Lowering and lifting

What the prime movers of the back squat do in each half of a rep
Phase	What the prime movers do
Lowering	The quadriceps and gluteus maximus lengthen under load
Rising	They shorten to extend the knees and hips

Where these prime movers also work

The quadriceps: prime mover in the front squat, leg press, conventional deadlift, sumo deadlift, Bulgarian split squat, walking lunge and leg extension; synergist in the barbell hip thrust and front plank.
Gluteus maximus: prime mover in the front squat, leg press, conventional deadlift, sumo deadlift, Romanian deadlift, barbell hip thrust, Bulgarian split squat and walking lunge.
Adductor magnus: prime mover in the front squat, leg press, conventional deadlift, sumo deadlift, Romanian deadlift, Bulgarian split squat and walking lunge; synergist in the barbell hip thrust.

Compared with the other squat lifts

Front squat: the same prime movers; stabilizers only here, quadratus lumborum, the middle trapezius and the rhomboids; stabilizers only in the front squat, the anterior deltoid.
Leg press: the same prime movers; synergists only here, soleus; stabilizers only here, the erector spinae, multifidus, quadratus lumborum, gluteus minimus, the upper trapezius, the middle trapezius and the rhomboids; stabilizers only in the leg press, soleus, gastrocnemius, and the wrist and finger flexors.

Sources

Escamilla (2001), Med Sci Sports Exerc 33(1):127
Schoenfeld (2010), J Strength Cond Res 24(12):3497
Németh and Ohlsén (1985), J Biomech 18(2):129
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.