What Muscles Do Squats Work? (Complete Breakdown)

Ask what squats work and most people say "legs." Training studies that measured muscle growth with MRI give a sharper answer: squats reliably build the quads, glutes, and adductors (inner thighs), while the hamstrings, which work on every rep, barely grow at all. Here's a muscle-by-muscle breakdown of the squat, what the research says about each muscle, and how depth, stance, and bar position move the work around.

Quick Answer

The squat trains the quadriceps, gluteus maximus, and adductors as prime movers; those are the muscles that grow in squat training studies. The hamstrings, erector spinae (lower back), calves, glute medius, and core work as stabilizers; in two MRI training studies, the hamstrings showed little to no growth. Squatting deeper shifts more of the growth stimulus to the glutes and adductors. A wider stance adds glute activity without changing how hard the quads work.

Squat Muscles Worked at a Glance

Primary Movers

These three muscle groups drive you out of the bottom of the squat, and they're the ones that grow when you train it.

1. Quadriceps (Front of the Thigh)

The quads are four muscles: the vastus lateralis (outer thigh), vastus medialis (the teardrop above the knee), vastus intermedius (beneath the rectus femoris), and rectus femoris (down the middle). Together they straighten the knee, which is much of the work of standing up out of a squat.

The three vasti respond well. In a 10-week MRI study by Kubo and colleagues (European Journal of Applied Physiology, 2019), squat training increased knee-extensor muscle volume by 4.9% with full squats and 4.6% with half squats, nearly the same at either depth. Don't cut depth too short, though: in Bloomquist et al. (2013), deep squats (to 120° of knee bend) built more front-thigh muscle over 12 weeks than shallow squats (to 60°). And Bryanton et al. (2012) found that the quads' relative effort rose with squat depth, not with heavier loads.

The exception is the rectus femoris, the only quad muscle that crosses the hip as well as the knee. As you squat down, it stretches at the knee but slackens at the hip, so its overall length barely changes. In Kubo's study, rectus femoris volume didn't change at either depth. In Zabaleta-Korta et al. (2021), leg extensions grew all three measured regions of the rectus femoris, while Smith-machine squats grew only the middle of the vastus lateralis.

Loading bias: depth, an upright torso (high-bar or front squat), and letting your knees travel forward all put more of the work on the quads. To cover the rectus femoris, add leg extensions.

2. Gluteus Maximus

The glute max is the main hip extensor. It works hardest coming out of the bottom of a deep squat, when the hips are most bent.

Depth is the glutes' biggest lever. In Caterisano et al. (2002) (Journal of Strength and Conditioning Research), the glutes' share of activity among four hip and thigh muscles rose from 16.9% in partial squats to 28.0% at parallel and 35.4% in full squats, while the shares of the quads and hamstrings didn't change. Kubo's MRI study showed the same pattern in actual growth: glute volume rose 6.7% with full squats versus 2.2% with half squats.

Squats also match the hip thrust for glute growth. In Plotkin et al. (2023) (Frontiers in Physiology), 9 weeks of back squats grew the glutes about as much as 9 weeks of hip thrusts in previously untrained young adults, even though the hip thrust produced higher glute EMG. More EMG didn't mean more growth.

Loading bias: squat deeper, take a wider stance, or use a low-bar position (more on each below).

3. Adductors (Inner Thigh)

The adductors, mostly the adductor magnus, the largest of them, run along the inside of the thigh. Their rear fibers extend the hip much like a hamstring does, so they help drive you out of the bottom, especially in deep squats.

They're the squat's least-known prime mover. In Kubo's study, adductor volume increased 6.2% with full squats versus 2.7% with half squats. In Plotkin's study, squats grew the adductors (and the quads) more than hip thrusts did.

Loading bias: depth matters most. For direct work, the hip adduction machine trains them without a barbell.

Stabilizers and Secondary Movers

These muscles hold your position under the bar. They work hard, but where squat training studies measured them (the hamstrings and glute medius), they barely grew.

4. Hamstrings (Back of the Thigh)

The hamstrings help extend the hip; in McCaw and Melrose (1999), biceps femoris activity was highest on the way up. But squats barely grow them: hamstring volume didn't change in Kubo's study, and Plotkin's study found little to no hamstring growth in either the squat or the hip-thrust group.

The usual explanation is length. The hamstrings cross both the hip and the knee, so as you squat down they lengthen at the hip and shorten at the knee, and their overall length barely changes.

What to do instead: train them directly with leg curls and Romanian deadlifts. See Best Hamstring Exercises Ranked.

5. Erector Spinae (Lower Back)

The bar tries to fold your torso forward, and the erector spinae, the muscles running along your spine, stop it. They work isometrically, holding the spine neutral rather than moving it.

That job is bigger than it looks. In Hamlyn et al. (2007), activity in the lowest part of the erectors during squats at 80% of one-rep max (1RM) was higher than in a bodyweight squat, the superman, and the side bridge, and even higher than in a deadlift at the same relative load. Nuzzo et al. (2008) found that squats and deadlifts at 90–100% of 1RM produced more lower-back muscle activity than stability-ball exercises, and recommended them for building the back extensors.

Loading bias: the more you lean forward, the more the lower back works. Low-bar squats lean further forward than high-bar squats.

6. Glute Medius and Minimus

These smaller hip muscles keep your pelvis level and help your knees track over your toes instead of caving in. They work as stabilizers; Plotkin's MRI data showed little to no growth in them from either squats or hip thrusts.

7. Calves (Soleus and Gastrocnemius)

The calves steady the ankle and help keep the bar balanced over the middle of your foot. In Bryanton's analysis, their relative effort rose with heavier loads, not with deeper squats. If you want bigger calves, train them directly with calf raises.

8. Core (Abs and Obliques)

Your abs and obliques brace to keep your torso rigid. But in both Hamlyn's and Nuzzo's studies, abdominal and oblique activity showed no significant difference between heavy squats and the lighter or non-squat exercises they were compared with, even as lower-back activity climbed. Squats train bracing; they don't replace direct ab work. See Best Ab Exercises Ranked.

9. Upper Back

In a back squat, the traps, rhomboids, and lats hold the bar tight against your back and keep your chest up. In a front squat, the upper back works to keep your elbows high and your torso upright so the bar stays on your shoulders.

How Depth Changes the Work

In these studies, the deeper squat matched or beat the shallower one for every muscle measured. Squat as deep as you can while keeping a neutral spine and full control.

How Stance Width Changes the Work

Two EMG studies tested stance width directly:

• Paoli et al. (2009) measured eight thigh and hip muscles at three stance widths. Only the gluteus maximus changed: it was more active at the widest stance.
• McCaw and Melrose (1999) compared a narrow stance (75% of shoulder width), shoulder width, and a wide stance (140%). Quad activity changed with the load but not with the stance; stance width did affect the glutes and the inner thigh (adductor longus).

You can't isolate the quads with foot position. Pick the stance that lets you reach depth comfortably, and go wider if you want more glute involvement; in Paoli's study, the glute increase showed up only at the widest stance.

How Bar Position Changes the Work

• Front squat vs back squat: Gullett et al. (2009) found that bar position didn't change muscle activity, while the back squat produced higher compressive forces at the knee. Contreras et al. (2016) found no difference in glute, hamstring, or quad EMG between the front squat, parallel squat, and full squat, even though the front squat used lighter loads. At maximum loads, Yavuz et al. (2015) saw slightly more vastus medialis activity in the front squat and more hamstring (semitendinosus) activity in the back squat.
• High-bar vs low-bar: a 2017 review by Glassbrook and colleagues found that the high-bar squat keeps the torso more upright, with more knee bend and more quad activity, while the low-bar squat leans further forward, with more lower-back, adductor, and glute activity.
• Knees past the toes: in Fry et al. (2003), blocking the knees from moving past the toes cut torque at the knee by about a fifth but raised torque at the hip more than tenfold, and made lifters lean further forward. The authors concluded that loading the joints appropriately may require the knees to move slightly past the toes.

Bar position matters less than most lifters think; depth matters more. To choose between variations, see Squat: Back vs Front vs Goblet vs Bulgarian Split.

How to Bias Each Muscle

For quads: squat deep with an upright torso (high-bar or front squat) and let your knees travel forward. Add leg extensions for the rectus femoris, which squats barely grow. See Best Quad Exercises Ranked.

For glutes: squat to full depth and try a wider stance. You don't need hip thrusts if you squat deep (in Plotkin's study the two grew the glutes about equally), but hip thrusts or glute bridges, the floor version, add glute work without loading your quads as much. See Best Glute Exercises Ranked.

For adductors: full depth does most of the work. The hip adduction machine adds direct volume.

For hamstrings: squats won't do it. Use leg curls and Romanian deadlifts; see What Muscles Does the Romanian Deadlift Work.

For the lower back: heavy squats already load it hard. If it's your limiting factor, lean less: try the high-bar or front squat, or move some leg volume to the leg press, where the seat supports your back.

For sets, reps, and weekly volume, see the best rep range for hypertrophy and sets per week for muscle growth.

Common Mistakes That Change Which Muscles Work

• Cutting depth short: half squats gave about a third of the glute growth of full squats (Kubo), and shallow squats built less quad muscle than deep ones (Bloomquist). Cue: sit down between your heels to the deepest position you can control.
• Keeping the knees behind the toes at all costs: this shifts load from the knees to the hips, and likely the lower back (Fry). Cue: let your knees travel forward over your toes.
• Folding forward out of the bottom: when your hips rise faster than your chest, your lower back takes over. Cue: brace before each rep and drive your upper back into the bar.
• Expecting squats to build your hamstrings: they barely do (Kubo, Plotkin). Add leg curls or Romanian deadlifts.

The Bottom Line

Squats build the quads, glutes, and adductors, the three muscle groups that grow in squat training studies, while the hamstrings, lower back, calves, and core work hard as stabilizers. Depth is the biggest lever: full squats grew the glutes and adductors two to three times more than half squats, with nearly the same quad growth. A wider stance adds glute activity, and bar position matters less than most lifters think. Squat deep, then fill the gaps squats leave: leg curls or Romanian deadlifts for the hamstrings, and leg extensions for the rectus femoris.

For more leg training, see Best Quad Exercises Ranked, Best Glute Exercises Ranked, and What Muscles Does the Deadlift Work.