The Muscles Used in Squats - Squat Biomechanics Explained

The squat is arguably the most popular exercise used by athletes and fitness enthusiasts alike, and for good reason. There is ample evidence describing its use for improving lower body muscular endurance, strength, muscle size, and power. 

This article discusses the traditional barbell back squat from a fitness perspective. It does not discuss the squat as it relates to performance such as competing in powerlifting or Olympic Weightlifting.

Rather, the purpose of this article is to provide an overview of the movement requirements, involved musculature, common technique errors, and recommendations to maximize safety and performance.

A lot of these principles are covered in the Corrective Exercise Course. 

Learning Objectives:

1. Comprehend the movement requirements, joint actions, and involved musculature of the squat exercise.

2. Identify common faulty movement patterns during the squat exercise.

3. Provide exercise technique recommendations for fitness enthusiasts wishing to perform the squat exercise.

Introduction

The squat is one of the most debated exercises in the fitness and sports community, but it is hard to argue its effectiveness. There is ample evidence describing its use for improving lower body muscular endurance, strength and power (Clark, Lambert & Hunter, 2012; Folland & Williams, 2007; Marques et al., 2015; Soriano, Jiménez-Reyes, Rhea, & Marín, 2015). There are several variations of the squat exercise including the bodyweight squat, barbell back squat, barbell front squat, dumbbell squat, sumo squat, split squat, box squat, plie squat, squat jump, overhead squat, and single-leg squat, to name a few.

For the sake of this article we will discuss the barbell back squat from a fitness perspective. We will not discuss the squat as it relates to performance, such as powerlifting. Instead, we will discuss the safest variation of the squat exercise for a fitness enthusiast seeking to improve technique and minimize faulty movement patterns and potential injury.

It is important to note variations of the squat exercise exist to maximize 1 repetition maximum (1RM) potential, such as using an excessively wide stance with a toe out posture. This posture reduces the amount of hip and knee flexion and ankle dorsiflexion needed to reach full depth. While from a biomechanical perspective this variation enables the lifter to complete the exercise with higher loads because range of motion is reduced, it may not be the safest variation on articulating joint surfaces for beginning exercisers who have no desire for improving their 1RM. Experienced individuals or athletes seeking to improve 1RM can use these variations once properly instructed and have displayed adequate physical capabilities.

Hip Flexion: Decreasing the angle between the femur (thigh) and pelvis. This occurs from a standing position when a person elevates their knee toward their abdomen (femoral-on-pelvic hip rotation) or when bending forward from the trunk, as if touching their toes (pelvic-on-femoral rotation).

Knee Flexion: Decreasing the angle between the lower leg (tibia, fibula) and femur. This occurs when a person bends their knee, bringing their heel closer to their thigh or butt.

Ankle Dorsiflexion: Flexion at the ankle in which the top of the foot (dorsal) is brought closer towards the shin.

Overview

The barbell squat is a compound, multi-joint exercise designed to target many muscles of the lower body and lumbo-pelvic-hip complex (pelvis, low-back, and abdominals). The primary joint actions that occur during the squat include:

Eccentric (lowering) Phase

> Hip flexion
> Knee flexion
> Ankle dorsiflexion

Concentric (lifting) Phase

> Hip extension
> Knee extension
> Ankle plantarflexion

Table 1 provides a list of involved musculature. This is not an exhaustive list as the nervous system activates muscles in synergies (groups) rather than in isolation. Many muscles are involved in the joint actions listed above.

Technique

Starting Position

> Stand with feet approximately shoulder-width apart, toes pointing straight ahead, and knees aligned over second and third toes.

• As much as 5-8° of external foot rotation is allowed in the starting position as some consider this normal anatomical position (Schoenfeld, 2010).

> To perform the high-bar back squat, rest the barbell on the shoulders, behind the neck, with hands grasping the bar wider than shoulder-width apart.

> To perform the low-bar back squat, rest the barbell on the middle trapezius region with hands grasping the bar wider than shoulder-width apart.

• It is important to note adequate shoulder mobility (external rotation) is required to hold the bar securely.

Movement Pattern

> Slowly begin to squat down by hinging at the hips and then flexing at the knees.

> Allow glutes to “stick” out behind the body as if sitting into a chair.

>Keep the chest up and the cervical spine in a neutral position. Avoid excessive cervical flexion, extension, or anterior translation (jutting the head forward).

> Squat to a depth that can be safely controlled with no movement compensations.

• Common movement compensations include knee valgus (knock knees), rounding or arching of the low-back, an excessive forward lean of the torso, and overly externally rotating or pronating the feet.

> To rise back up, contract the gluteals and place pressure through the heels as the knees and hips are extended.

> Stand up straight until hips and legs are fully extended. Fully contract the gluteals in the standing position for maximal muscle recruitment.

See Squat Stances Reviewed for more.

What is The Right Squat Depth?

A blanket statement regarding squat depth for all individuals is inappropriate at best. Depending the person’s physical capabilities and goals, some should squat to a depth in which the thighs are always parallel to the floor. Others can do full squats (below parallel). Conversely, some experts recommend a limited range of motion (i.e., ¼ squat) to avoid stress on an individual’s knees. 

Due to individual differences in shape, size, and overall fitness, people inherently display differences in joint mobility, joint stability, and neuromuscular control (coordinated muscle activation). Individuals who display ample mobility and joint range of motion, combined with optimal joint stability, may be able to safely perform squats using a full or near full range of motion.

This typically requires at least 15-20° of ankle dorsiflexion and 120° of hip flexion (Greene, 1994). Research suggests if an individual possesses less than adequate ankle dorsiflexion, they may be at greater risk of injury to the knees, hips, or low-back during functional movement patterns (Lun, Meeuwisse, Stergiou, & Stefanyshyn, 2004; Powers, 2003). In other words, each joint must exhibit proper range of motion for the efficient transference of forces throughout the body to produce ideal movement.

According to Schoenfeld (2010), individuals with a history of patellofemoral injury should limit the depth of their squat. This recommendation is due to the fact that peak compressive forces at the knee occur at near maximum knee flexion angles. In addition, those with existing knee injury or previous reconstruction of the posterior collateral ligament (PCL- ligament on the back of the knee) should restrict knee flexion to 50-60° to minimize posterior shear forces. Moreover, muscular development of the quadriceps is maximized while performing squats to a depth with thighs parallel to the floor.

There appears to be no benefit to quadriceps development if a person performs squats to a full depth (below parallel). Yet, Schoenfeld explains, hip development is maximized when performing below parallel squats and may be important for individuals needing to perform this movement pattern (such as powerlifters or Olympic weightlifters). However, for a fitness client seeking to improve general fitness, below parallel squats are not recommended until adequate levels of stability and mobility are attained.

Identifying Ideal Squat Depth

An easy test can be performed to identify ideal squat depth. A person should perform a barefoot squat using a mirror or a partner to evaluate his or her mechanics. During the eccentric phase of the squat, an individual’s torso and shin angle should remain parallel (see Figure 1). In addition, there should be no excessive arching or rounding of the low back. Lastly, look for any faulty movement patterns at the foot/ankle. The feet should not excessively pronate (arches collapse) or externally rotate during the eccentric phase.

Once any of these movement compensations have been observed, the squat is at a depth no longer suited for the individual. He or she will need to stop just before any these faulty movements occur. By maintaining ideal posture and technique throughout the movement, he or she will develop ideal motor skills needed for this exercise. As mobility and stability improve, the individual will be able to successfully squat to deeper depths.

Movement Compensations

Performing a squat with ideal technique is needed to maximize muscle recruitment and minimize risk of injury. Nonetheless, individuals lacking ideal joint mobility, joint stability, or neuromuscular control often display movement compensations. A movement compensation is the body’s way of seeking the path of least resistance to perform a particular movement pattern.

For example, if an individual performs an overhead lift with excessive lumbar extension (arched low-back), this is a sign the person lacks shoulder flexion range of motion. In order to perform the movement pattern the person “borrows” range of motion from the spine and pelvis to compensate for lack of mobility through the shoulder complex (most notably tightness through the latissimus dorsi). The following section describes common movement compensations that occur during a squat.

Feet Turn Out Excessively / Feet Flatten / Heels Raise

If an individual lacks adequate mobility of the ankle complex (limited ankle dorsiflexion), he or she will likely gain additional range of motion by altering foot mechanics. This usually comes in the form of excessively turning the feet outward, pronation at the foot/ankle complex, or raising the heels off the floor.

When someone lacks ankle dorsiflexion, which occurs in the sagittal plane, the range of motion must then take place in another plane (frontal or transverse). Excessive external rotation of the feet (beyond 8°) enables a person to squat to a lower depth because motion is occurring primarily in the transverse plane (Figure 2). In other words, due to limited ankle mobility, the knees are not able to track over the toes in the sagittal plane, so motion is borrowed from another plane. This may be caused by tightness in the calf complex (gastrocnemius, soleus) and/or restriction in the talocrual (ankle) joint.

Some research indicates restriction in ankle mobility may cause knee valgus (knock knees), which is often a recipe for patellofemoral pain or even ACL injury (Bell, Oates, Clark, & Padua, 2013; Dill, Begalle, Frank, Zinder, & Padua, 2014; Macrum, Bell, Boling, Lewek, & Padua, 2012). In these instances, flexibility exercises for the calves and possibly joint mobilization for the ankle may be required to regain 15-20° of ankle dorsiflexion.

Sagittal Plane: An imaginary plane that bisects the body into right and left sides. Movements in the sagittal plane include flexion and extension, such as knee flexion/extension, hip extension/flexion and shoulder extension/flexion.

Frontal Plane: An imaginary plane that bisects the body into front and back halves. Movements in the frontal plane include abduction and adduction, such as hip adduction/abduction and lateral trunk flexion (side bending).

Transverse Plane: An imaginary plane that bisects the body into top and bottom halves. Movements in the transverse plane include rotational movements, such as trunk rotation, hip internal/external rotation and shoulder internal/external rotation.

Read also: The Body Planes of Motion - Saggital, Frontal, and Transverse

Pronation at the foot is also likely to occur if an individual lacks adequate ankle dorsiflexion. Slight pronation is allowed but the individual should be able to perform the movement pattern by primarily flexing at the ankle versus complete collapse of the arch. By looking at the natural posture of the foot from a side view, one can see a space between the floor and the bottom of the foot. When the arch collapses, this space is no longer visible (the foot appears to roll inward) (Figure 3).

When observing from the posterior view it’s easy to see the Achilles tendon is now bowed versus straight up and down in a vertical position. A useful cue is to imagine a small grape underneath the arch of the foot. During the squat, a person’s foot should not smash this grape, but rather, keep the foot’s natural arch position. A collapse of the arch may alter mechanics up the body affecting alignment at the knees and hips, including knee valgus.

An individual’s heels rising off the floor is a not a common movement compensation, but it does occur from time to time (Figure 4). In many cases this movement compensation is not observed simply because individuals wear shoes with an elevated heel. A shoe with an elevated heel places the foot into plantarflexion. As such, the person can complete the squat exercise with less degree of ankle dorsiflexion (Macrum et al., 2012). However, performing a barefoot squat can bring attention to this movement impairment. Muscular tightness of the calf complex or joint restriction in the ankle itself are the primary causes of this movement compensation.

While each of these movement compensations was described individually, it is common to see a combination of these foot impairments occurring simultaneously, most notably a combination of foot pronation and external rotation.

Knee Valgus

Arguably the most significant movement compensation to observe during the squat exercise is knee valgus, also known as medial knee displacement, or “knock knees.” Knee valgus is a primary predictor of knee injury including patellofemoral pain (pain in the front of the knee) and ACL injury. Knee valgus is a combination of femoral adduction and internal rotation in relation to the tibia. In other words, the shin is pointing outward and the thigh is collapsing and rotating inward (Figure 5).

Knee valgus can occur due to impairments occurring at the ankle and/or hip (Bell, Padua, & Clark, 2008; Padua, Bell, & Clark, 2012). Since the body is a kinetic chain, any impairment at one joint can affect adjacent joints up and down the chain. The knee is caught between the hip and ankle, and as a result any faulty movement pattern occurring at one of these joints can affect the knee. Consequently, knee valgus has been associated with limited ankle mobility and weakness of the hip abductors and external rotators, most notably the gluteus medius. Corrective exercise interventions to regain ankle mobility and hip/core stability seem to be an effective measure to correct knee valgus (Bell et al., 2013; Padua, & DiStefano, 2009).

Squat Progressions

Helping beginner exercisers learn how to squat properly is imperative and can be best achieved using a systematic and progressive approach. After all, we need to learn how to walk before we can run. Below is a recommended list of squat progressions to help individuals learn and perfect their squat technique. Over time movement patterns and motor skills become engrained requiring little conscious thought and effort. In addition, the individual will gain the ideal mobility and stability needed to perform the squat exercise with optimal posture.

1. Stability Ball Wall Squat
2. Assisted Bodyweight Squat (holding suspension straps or cables)
3. Bodyweight Squat
4. Dumbbell Squat
5. Dumbbell Front Squat
6. Barbell Back Squat
7. Advanced Versions
8. Single-leg Squat
9. Squat Jump

Conclusion

The squat is an effective exercise for improving lower body muscular endurance, strength and power. It is a compound movement involving many joint actions and associated musculature. Individuals performing the squat exercise should be aware of common faulty movement patterns that occur at the foot/ankle, knees, and hips. By becoming aware and consequently correcting these faulty movements novice exercisers will be able to avoid unnecessary and preventable injuries during exercise.

References

Bell, D., Padua, D., & Clark, M. (2008). Muscle Strength and Flexibility Characteristics of People Displaying Excessive Medial Knee Displacement. Archives of Physical Medicine and Rehabilitation, 89(7), 1323-1328. doi:10.1016/j.apmr.2007.11.048

Bell, D., Oates, D., Clark, M., & Padua, D. (2013). Two- and 3-Dimensional Knee Valgus Are Reduced After an Exercise Intervention in Young Adults With Demonstrable Valgus During Squatting. Journal of Athletic Training, 48(4), 442-449. doi:10.4085/1062-6050-48.3.16

Clark, D., Lambert, M., & Hunter, A. (2012). Muscle Activation in the Loaded Free Barbell Squat. Journal of Strength and Conditioning Research, 1169-1178. doi:10.1519/JSC.0b013e31822d533d

Dill, K., Begalle, R., Frank, B., Zinder, S., & Padua, D. (2014). Altered Knee and Ankle Kinematics During Squatting in Those With Limited Weight-Bearing–Lunge Ankle-Dorsiflexion Range of Motion. Journal of Athletic Training, 49(6), 723-732. doi:10.4085/1062-6050-49.3.29

Folland, J., & Williams, A. (2007). The Adaptations to Strength Training. Sports Medicine, 37(2), 145-168.

Greene WB, Heckman JD. (1994) American Academy of Orthopedic Surgeons. The Clinical Measurement of Joint Motion. Chicago, IL.

Lun, V. (2004). Relation Between Running Injury And Static Lower Limb Alignment In Recreational Runners. British Journal of Sports Medicine, 38(5), 576-580.

Macrum E, Bell DR, Boling M, Lewek M, Padua D (2012). Effect of limiting ankle-dorsiflexion range of motion on lower extremity kinematics and muscle-activation patterns during a squat. Journal of Sport Rehabilitation. 21(2), 144-50.

Marques, M., Gabbett, T., Marinho, D., Blazevich, A., Sousa, A., Tillaar, R., & Izquierdo, M. (2015). Influence of Strength, Sprint Running, and Combined Strength and Sprint Running Training on Short Sprint Performance in Young Adults. International Journal of Sports Medicine, 789-795.

Padua DA, Bell DR, Clark MA. (2012). Neuromuscular characteristics of individuals displaying excessive medial knee displacement. Journal of Athletic Training. 47(5), 525-36. doi: 10.4085/1062-6050-47.5.10

Padua, D., & Distefano, L. (2009). Sagittal Plane Knee Biomechanics and Vertical Ground Reaction Forces Are Modified Following ACL Injury Prevention Programs: A Systematic Review. Sports Health: A Multidisciplinary Approach, 1(2), 165-173.

Powers, C. (2003). The Influence of Altered Lower-Extremity Kinematics on Patellofemoral Joint Dysfunction: A Theoretical Perspective. Journal of Orthopaedic & Sports Physical Therapy, 33(11), 639-646.

Schoenfeld, B. (2010). Squatting Kinematics and Kinetics and Their Application to Exercise Performance. Journal of Strength and Conditioning Research, 24(12), 3497-3506. doi:10.1519/JSC.0b013e3181bac2d7

Soriano, M., Jiménez-Reyes, P., Rhea, M., & Marín, P. (2015). The Optimal Load for Maximal Power Production During Lower-Body Resistance Exercises: A Meta-Analysis. Sports Medicine, 1191-1205. doi:10.1007/s40279-015-0341-8