Back squat

Movement

What the hips and knees have to resist

In a squat, the push of the ground back up on the body passes in front of the hip and behind the knee¹. That creates a flexion moment, a turning effect that bends each joint¹. The hip and knee extensors, muscles that straighten a joint, have to work against it¹. A 2024 review calls these two groups the primary muscle groups of the lower body¹.

The trunk works too. The same review states that the squat calls for a high level of trunk-muscle work to hold the spine and torso steady¹.

How bar position and stance change the shape

A 2017 review compared the high-bar and low-bar back squat². It describes how the two differ²:

• The high-bar back squat has a more upright torso and more knee bend².
• The low-bar back squat has more hip bend and a greater forward lean².

When the trunk leans further forward in a squat, the flexion moment at the hip grows and the one at the knee shrinks¹. In general, a greater forward lean also asks more of the back extensors to keep the trunk steady¹.

One study tested bar position and stance width together⁹. Nine men and nine women squatted their three-repetition maximum, the heaviest weight they could lift three times, in four set-ups⁹. A low bar with a wide stance gave the hip a larger share of the total turning effect than the other set-ups⁹. A high bar with a narrow stance gave the knee a larger share⁹.

What it loads

The muscles that straighten the hip and knee

The quadriceps is a group of four muscles on the front of the thigh⁸. Acting through the patella (kneecap), it straightens the leg at the knee⁸. The gluteus maximus is set up to extend the hip, moving the thigh backward¹⁰. The hamstrings, the three long muscles at the back of the thigh, extend the hip and bend the knee^8,11.

A 2001 review of the knee in the squat reports moderate to high activity in the quadriceps, hamstrings and gastrocnemius (the large calf muscle)¹². That activity generally rose as the knees bent further¹².

The erector spinae is the main muscle that extends the spine⁸. It is one of the back extensors that generally have to work harder to keep the trunk steady as it leans further forward^1,8.

How bar position shifts muscle activity

Three sources compared the high-bar and low-bar versions^2,9,13:

• A 2017 review. The low-bar squat can be characterised by more erector spinae, adductor and gluteal activity, and the high-bar squat by more quadriceps activity².
• 28 experienced men at 50% to 70% of their one-repetition maximum (the heaviest weight a person can lift once). The high-bar squat gave higher leg muscle activity, and the low-bar squat higher trunk muscle activity¹³.
• Nine men and nine women at a three-repetition maximum. A high bar with a narrow stance gave less gluteus maximus activity⁹. It gave more activity in the vastus lateralis, the quadriceps muscle on the outer thigh^8,9.

What stance width and depth change

Two reviews reported on stance width, and one on depth^1,5:

Variation	What a review reported
Medium or wide stance	Higher gluteus maximus activity and lower gastrocnemius activity, in a 2024 review1
Stance width	Did not appear to change hamstring, quadriceps or gluteus medius (a buttock muscle beneath the gluteus maximus) activity, in the same review1
Stance width, hip rotation, front squat	Did not significantly affect muscle activation, in a 2012 review of back squat studies5
Greater depth	Conflicting findings for the quadriceps and hamstrings; gluteus maximus activity rose from shallow to medium depth, and deeper than that is disputed1

How load changes muscle activity

A 2012 review of 18 studies reports that the prime movers of the back squat become more active as the external load rises⁵. In the study of 28 experienced men, activity rose with load in each muscle measured¹³.

The patellar tendon

The patellar tendon runs from the kneecap to the front of the tibia (shinbone), just below the knee⁸. The four quadriceps muscles share one tendon, which continues below the kneecap as this band, also called the patellar ligament⁸. Studies of squatting have estimated or measured the load on it^7,14:

• Speed. In 10 resistance-trained men, squats went to parallel with 60% of the one-repetition maximum⁷. Estimated peak tendon force was greater in a fast jump than at a slow, fixed pace⁷.
• Weight. In 10 strength-trained athletes, squats went to about 60 degrees of knee bend¹⁴. Tendon strain (stretch, as a share of resting length) rose from 4.7% to 8.2%¹⁴. The load across that range went from 40% to 80% of the one-repetition maximum¹⁴.
• The person. In the same athletes at 80% of the one-repetition maximum, strain varied widely, from 5.1% to 10.2%¹⁴.
• Depth. In 20 healthy adults, a two-leg full squat scored higher on a tendon loading index than a two-leg squat to 60 degrees of knee bend¹⁵.

The knee ligaments

A 2001 review reports shear forces at the knee during the squat, forces that push two surfaces to slide across each other¹². Low to moderate backward shear occurred throughout the squat¹². The main restraint against it was the posterior cruciate ligament, a ligament inside the knee¹². Low forward shear occurred between 0 and 60 degrees of knee bend¹². The main restraint against that was the anterior cruciate ligament, another ligament inside the knee¹².

A 2013 review concluded that deep squats do not add to injury risk for passive tissues, the parts that do not contract¹⁶. It set this against a commonly voiced concern¹⁶.

How the body adapts

Full squats compared with half squats

Two systematic reviews, which search for the research on a question in a set, checkable way, compared full and partial range of motion^3,17. The first review pooled 16 studies in a meta-analysis³. Full range of motion produced significantly greater growth in the lower limbs than partial range³. It found no significant advantage for either range in muscle thickness, pennation angle or fascicle length³. Pennation angle is the angle of a muscle’s fibres to its line of pull, and a fascicle is a bundle of fibres.

The second review, of 11 studies, calculated effect sizes, a standard measure of how large a difference is¹⁷. It found greater gluteus maximus and adductor growth with full range than with partial range in the final part of the range¹⁷. It concluded that the evidence suggests some muscles, such as the quadriceps, tend to grow best when trained at a longer muscle length¹⁷. That held through either a partial or a full range¹⁷.

One trial compared squat depths directly, following 17 men through 10 weeks of squat training, twice a week⁴. Some trained with full squats and some with half squats⁴. The volume of the knee extensors grew in both groups⁴. The adductors and gluteus maximus grew more with full squats⁴:

Training group	Gain in gluteus maximus volume over 10 weeks
Full squat	About 6.7%4
Half squat	About 2.2%4

The rectus femoris, the quadriceps muscle at the front of the thigh, and the hamstrings showed no change in volume in either group^4,8.

Depth also came up in a trial of 17 male students over 12 weeks¹⁸. They were randomised, assigned by chance, to deep squats, to 120 degrees of knee bend, or shallow squats, to 60 degrees¹⁸. Deep squats produced larger growth in the front thigh muscles¹⁸. The study observed no differences in patellar tendon cross-sectional area, the size of a slice cut straight across it¹⁸.

The back squat compared with the hip thrust

In untrained college-aged adults, one trial compared 9 weeks of back squat or hip thrust training¹⁹. Gains in gluteal muscle cross-sectional area were similar with the two exercises¹⁹. Strength gains favoured the exercise each group had trained¹⁹. Deadlift strength improved similarly in both groups¹⁹.

What tendon studies used instead

The two studies here that tracked patellar tendon stiffness with training used knee extensions, not the back squat^20,21. In 12 healthy young men, 12 weeks of knee extensions raised tendon stiffness in the leg trained heavy, not the leg trained light²⁰. Stiffness is how much force it takes to stretch something by a given amount. In 15 healthy young men, tendon changes after 9 weeks of knee extensions were not related to strength gains²¹. Larger quadriceps growth went with larger gains in tendon stiffness²¹.

Progressive overload

A 2026 position stand describes progressive overload as the need to increase the stimulus placed on the muscle throughout a training programme²². The back squat studies below set and raised the load in different ways.

Setting the load by percentage or by effort

One 8-week study of resistance-trained men who back squatted and bench pressed compared two ways of setting the load²³:

• By percentage. The load was set from each man’s pre-test one-repetition maximum and rose in a linear fashion²³. In weeks 3, 5 and 7, it rose by 2.5% only if every set and repetition was done the week before²³.
• By effort. From week 2, the men chose loads to land in a target RPE (rating of perceived exertion), a score for how hard effort feels²³. For every 0.5 RPE outside the target range, the next set’s load moved by 2%²³.

In 21 of these men, back squat one-repetition maximum rose about 13.9 kg with percentage loading and 17.1 kg with RPE²³. The difference between the groups was not significant²³.

Setting the load by bar speed

Velocity-based training sets loads according to how fast the bar moves. The listening to your body page covers it more widely. Three sources compared it with loads set as a percentage of maximum, and they disagree^6,24,25:

• 20 male collegiate sprinters, 6 weeks. One group aimed for an average lifting speed of 0.54 m/s²⁴. Its load moved 5% when speed was 0.06 m/s off, and 10% when 0.12 m/s or more off²⁴. One-repetition maximum rose 16.4% this way and 11.5% at a fixed 80% of the starting maximum, a significantly greater gain²⁴.
• 16 trained men, 6 weeks. One-repetition maximum rose 9% with loads set from bar speed and 8% with loads set from the pre-test maximum²⁵. The total back squat volume lifted differed significantly between the two groups, by 9%²⁵.
• A 2021 meta-analysis of six studies and 124 people aged 16 to 30. It found no significant difference in back squat one-repetition maximum between the two approaches⁶.

Sets, sessions and set structure

• Adding sets. In untrained college-aged adults over 9 weeks, back squat sets per session rose from 3 in week 1 to 6 in weeks 7 to 9¹⁹. Each set was 8 to 12 repetitions, and the load changed when someone did fewer than 8 or more than 12¹⁹.
• Fewer repetitions, higher loads. In 21 experienced resistance-trained men over 8 weeks, back squat loads ran from 65% to 90% of maximum, with 11 sets a week²⁶. The programme shifted gradually from more repetitions at lower intensity to fewer at higher intensity²⁶.
• Two or four sessions a week. The same men were randomly assigned to train each muscle group two or four times a week, with the same weekly volume²⁶. Their back squat maximum rose by about 13.3 kg and 14.3 kg, and training frequency did not affect the gain²⁶.
• Cluster sets. In 36 resistance-trained women and men, 6 weeks of linear periodised back squat training used either cluster sets or traditional sets²⁷. Periodised means the training followed planned changes. A cluster set is split into smaller groups of repetitions with a short rest between them. Both groups made comparable gains in maximum, and the cluster-set group showed lower acute, or short-term, fatigue²⁷.

Connections

The muscles it works

• The quadriceps, the knee extensors at the front of the thigh^1,8.
• The gluteus maximus, which extends the hip^4,10.
• The hamstrings, active in the squat, though their volume did not change in one 10-week trial^4,12.
• The adductors, which grew more with full squats in that trial⁴.
• The gastrocnemius, active in the squat and less active with a medium or wide stance^1,12.
• The erector spinae, a back extensor, which generally has to work harder as the trunk leans forward^1,8.

The tendon and ligaments it loads

The back squat loads the patellar tendon, with greater estimated force in a fast jump and more strain with heavier weight^7,14. The posterior cruciate ligament is the main restraint against backward shear at the knee throughout the squat¹². The anterior cruciate ligament is the main restraint against forward shear between 0 and 60 degrees of knee bend¹².

The sport it belongs to

The squat is one of the three events of powerlifting²⁸. A 2024 systematic review found 1.0 to 4.4 injuries per 1000 hours of powerlifting training²⁸. The most common sites were the lower back and pelvis, the shoulder, and the elbow and upper arm²⁸. A systematic review searches for the research on a question in a set, checkable way. That review covered the sport as a whole, not the back squat on its own²⁸.

Sources

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Every source the library cites

Changes

• October 23, 2026 Added a Progressive overload section.
• September 29, 2026 Added a figure showing where it sits, links to the exercise pages that load it, and tighter wording in a few places.