Squats work more muscles than almost any other single exercise. The primary drivers are the quadriceps on the front of your thighs and the gluteus maximus in your hips, but the movement also demands significant effort from your adductors (inner thighs), erector spinae (lower back), core, hamstrings, and calves. That’s why squats are often called a “full lower body” exercise, though they also place real demands on your trunk.
Quadriceps: The Dominant Force
Your quadriceps, the four muscles spanning the front of your thigh, do the heaviest lifting during a squat. Their job is to straighten your knee as you stand back up, and electrical muscle activity measurements consistently show they produce more force than any other muscle group in the movement. One study measuring muscle activation found that quadriceps activity was substantially higher than hamstring and calf activity at every knee angle tested, from shallow bends all the way to 90 degrees of knee flexion.
Within the quadriceps group, the vastus lateralis (outer thigh) and vastus medialis (inner thigh, just above the knee) show particularly strong activation during the standing-up phase. In competitive bodybuilders performing back squats to full depth, vastus lateralis activation was roughly 22 to 30 percent higher during the ascent compared to the descent, and vastus medialis followed a similar pattern. This makes sense: the hardest moment of a squat is pushing out of the bottom, and your quads are doing most of that work.
Glutes: Your Hip Powerhouse
The gluteus maximus, the largest muscle in your body, extends your hip as you rise from the bottom of the squat. It fires throughout the movement but ramps up considerably during the ascending phase, with research on back squats showing that activation during the concentric (standing up) portion exceeded the eccentric (lowering) portion across all squat variations tested.
The gluteus medius, a smaller muscle on the side of your hip responsible for stabilizing your pelvis, also works hard. During full-depth back squats, gluteus medius activation during the ascent was about 12 percent higher than during the descent. You’ll feel this muscle working especially if you focus on pushing your knees outward as you squat.
How Depth Changes Glute Activation
Squat depth has a meaningful impact on how much your glutes contribute. Gluteus maximus activity increases by about 65 percent when you move from a shallow squat to a medium-depth squat (roughly thighs parallel to the floor). Going deeper adds another potential 25 percent on top of that, though findings here are mixed. Some studies report similar glute activity between parallel and deep squats, while others find modest increases. If your goal is maximum glute involvement, squatting to at least parallel is clearly beneficial. Going deeper may help further, but the biggest jump in activation happens between a quarter squat and a parallel squat.
Adductors and Inner Thigh Muscles
The adductor muscles along your inner thigh play a larger role in squats than most people realize. The adductor longus showed greater activation during the ascending phase than the descending phase across all squat variations in EMG studies, with effect sizes that were large enough to be practically meaningful. These muscles help extend the hip when it’s in a flexed position, which is exactly the demand at the bottom of a squat.
Interestingly, widening your stance doesn’t necessarily increase the demand on your adductors the way many people assume. A study examining narrow, medium, and wide stance squats found that the adductor moment at the hip was statistically the same across all three widths. What does change with a wider stance is the rotational demand at the hip: wider stances require significantly more effort from the hip extensors (glutes) and the muscles that rotate your thigh outward.
Lower Back and Erector Spinae
Your erector spinae, the muscles running along both sides of your spine, work hard to keep your torso upright against the load. During full-depth back squats, the iliocostalis (the outermost erector spinae muscle) was about 9 percent more active during the ascent than the descent. The longissimus, its deeper neighbor, follows a similar pattern.
How much your lower back works depends heavily on your torso angle. Research shows that trunk angle is the single strongest predictor of where the load falls between your hips and knees, explaining 35 percent of the variance on its own. Every additional degree of forward lean increases the hip-to-knee moment ratio by about 2 percent, meaning more work shifts to your glutes and lower back. This is why a more upright squat style (like a front squat) feels easier on the lower back, while a hip-dominant squat with more forward lean loads the posterior chain more heavily.
Core Muscles as Stabilizers
Your abdominal muscles don’t move your body during a squat, but they’re essential for keeping it stable. The rectus abdominis contracts against the erector spinae in a co-contraction that builds intra-abdominal pressure, essentially creating a rigid cylinder around your spine. This pressure is critical for protecting the lower back, especially under heavy loads.
The abs play a particularly important role at the very bottom of the squat, where you reverse direction from lowering to standing. At this turning point, the lever arm of the spine changes rapidly, and your nervous system has to coordinate contraction on both sides of the trunk to stiffen and stabilize it. This is one reason heavy squats develop functional core strength in a way that isolated ab exercises don’t replicate: the core has to react dynamically to shifting forces rather than just hold a static position.
Hamstrings: Active but Secondary
The hamstrings contribute to the squat, but their role is smaller than you might expect. EMG studies consistently show that hamstring activation (both the biceps femoris on the outer thigh and the semitendinosus on the inner thigh) is much lower than quadriceps activation throughout the movement.
Part of the reason is biomechanical. The hamstrings cross both the hip and the knee, and during a squat these two joints create opposing demands: the hip needs the hamstrings to extend it, while the knee needs them to shorten. This “canceling out” effect limits how forcefully they can contract. Additionally, at the top of the squat when the knee is nearly straight, the hamstring tendons sit very close to the knee’s axis of rotation, giving them a poor leverage point for generating force. Hamstring activity does increase as the knee bends deeper, rising proportionally from 0 to 90 degrees of flexion, but it never approaches quadriceps levels. If hamstring development is a priority, exercises like Romanian deadlifts or Nordic curls will be more effective than squats alone.
Calves: Ankle Stabilizers
Your calf muscles, the gastrocnemius and the deeper soleus, work during squats primarily to stabilize the ankle joint. They don’t generate the large forces that your quads and glutes do, but they’re active throughout the movement, especially as your shin angle increases at the bottom of the squat.
The soleus is particularly relevant to squat performance because its flexibility determines how far your ankle can bend. When you squat deep with your heels on the ground, your ankle needs significant dorsiflexion (the shin moving toward the toes). If your soleus is tight, your heels will lift or your torso will pitch excessively forward to compensate. This is why many lifters use raised-heel squat shoes or place small plates under their heels: it reduces the ankle mobility demand and allows a more upright torso position.
Front Squats vs. Back Squats
One of the most common questions about squat variations is whether front squats and back squats target different muscles. The answer is less dramatic than you might think. A biomechanical comparison of both lifts found that overall muscle recruitment was similar between them, with no significant difference in activation levels for the muscles measured. The ascending phase produced greater muscle activation than the descending phase in both variations.
The real difference is in joint loading, not muscle activation. Front squats produced significantly less compressive force on the knee and lower extensor moments compared to back squats. This makes front squats a useful alternative for anyone managing knee sensitivity, while back squats allow heavier loads because the bar position is more mechanically favorable. Both variations train the same muscles effectively.
How Stance Width Shifts the Demand
Widening your squat stance increases the rotational demands at the hip. Compared to a narrow stance, a wide stance produces significantly greater hip extensor moments and lateral rotator moments. In practical terms, this means wider squats place more demand on your glutes and the deep external rotators of the hip. The effect is progressive: medium stance requires more hip rotation effort than narrow, and wide requires more than medium.
Narrower stances, by contrast, tend to increase the demand on the quadriceps because the knees travel further forward over the toes, increasing the knee extensor moment. Neither stance width is inherently better. Your choice depends on your anatomy, mobility, and which muscles you want to emphasize.