The Muscles Worked in the Squat

The Squat: Muscles Worked

Force is a quantity characterized by magnitude, direction, and a point of application. In any interaction between two bodies, force is the instantaneous measurement of the quantity that causes movement or deformity. When we are discussing forces in barbell training, there are internal forces acting within your body—bones, muscles, and tendons acting on each other. And, there are external forces, measured by the magnitude and direction of the barbell. When you interact with the environment through movement, internal forces produce external forces. Muscles transmit force through tendons to act on your skeletal system, using leverage around joints to cause movement of the body and (in our case) the barbell. Through movement, internal connects to external. Intent becomes action through a cascade of voluntary and involuntary reactions, and something in your environment can be changed. Every meaningful action you take, on some level, comes from the crashing of your body into something else and the controlled application of force.

One of the amazing things about physical training is that we have learned how to use those same external connections with the environment to affect our internal structures. Regularly requiring your body to produce force against an external resistance will make you stronger. Whether your goal is strength, endurance, speed, or power, doing activities that require these traits tends to improve them as well. It is as though we were built to fill our world to the fullest extent possible, adapting to meet its challenges, with the capacity for almost constant change. You might think of physical training as the creation of micro-environments, ones that set up challenges that force our bodies to change in predictable and intentional ways, according to our goals.

The exercises that make up these challenges and the structure of one’s training are, in a sense, artificial. Concerning strength training, we’ve learned that certain challenges—certain exercises—will make a person stronger. Because our goal is the useful application of force for general purposes, these exercises have to be natural in the sense that they use muscles, bones, and joints in ways that those structures are meant to be used. And, most of our main lifts seem like natural movements: The bench press and overhead press use the hands and arms in ways that mimic or collate normal usage. The deadlift—picking something up from the ground—represents perhaps the most basic interaction with our environment. The squat, however, is less obviously a natural movement.

It’s difficult to find a crucial evolutionary action that mimics the barbell back squat. Where in our history did people regularly place something on their backs, squat down, and stand up with it? Instead, the squat seems to be the result of someone brainstorming the biggest possible movement a human can perform while maintaining balance and constantly applying force to an external object (i.e., no jumping) with the heaviest loads possible. The barbell back squat is a contrived movement, taking skill and practice to perform well. But, the squat has proved to be the best lift for the development of size and strength of which our human bodies are capable. Here, we will talk through some of the muscle mass involved in this king of lifts.

What is a Squat?

In the most basic form, the squat is a lift in which the bar is carried on the lifter’s back—there are other kinds of squats in which the lifter uses arms, hands, harnesses, etc. to carry the bar but those all require qualifications. The squat is performed when the lifter flexes at the knees, hips, and ankles (dorsiflexion) to a position that is “bellow parallel,” which we define as the position in which the crease of the hip joint is lower than the top of the lifter’s patella. The lifter then returns to the standing position. In a correctly performed squat, the lifter’s back will be held rigid throughout the movement, and the lifter will remain in balance for the entirety of the lift.

A Note on Balance

Balance is a state in which the center of mass of an object is located vertically over its base of support. Every object that exists within the gravitational framework of the earth has a center of mass—the mathematical average of gravity acting on your body. Because gravity pulls straight down, we are concerned with maintaining that average vertically over your base of support, an area created by your feet. Front to back, your base of support is represented by your toes to your heels, and from side to side by the width of your feet. When you carry the bar on your back in a squat, the bar’s mass and your mass combine to form one system with a new center of mass. You suddenly become much more top-heavy, and the average of gravity acting on you plus the barbell moves closer to the barbell the heavier it gets.

The center of mass and base of support isn’t the whole story of balance, however. Your ability to maintain balance is a combination of sensory input, how you process that input (your sensory perception), and muscular output. A lot happens for you to simply not fall down. For a lift like the squat, which challenges your normal spatial orientation by adding a bunch of weight to your upper body and forcing you to move it through a large range of motion, a lot of the muscle activity involved in the lift comes from unconscious micro-adjustments, designed to keep you in balance and in a position to produce force against the barbell.

There is a lot going on during the squat, making it helpful to our discussion to examine different phases of the movement.

The Starting Position

You’ve just unracked the bar and are standing ready to complete the first rep of your first work set for the day. Your stance will be about shoulder-width apart, toes pointing out slightly. Just standing there at the top is nowhere near as fatiguing as the rest of the movement, but if you stand there long enough, you will begin to tire. In this position, with the bar on your back and you standing tall underneath it, your muscles are exerting enough force to organize your skeleton to intercept as much of the downward force of the bar, between it and the floor, as possible. The bar is attempting to squash you flat, and you are resisting that compressive force with your bones. Your back is held rigid by the spinal erectors: Thoracic extension, created by a chest-up position, helps maintain your posture. The squeeze of your lower back muscles and a hard contraction of your abdominal muscles —supported by a big held breath—supports your lumbar spine. You do not need to further extend your lower back, but rather to brace and hold tight. The rigid spine keeps you upright and will allow you to move the bar later in the lift.

The Descent

As you start the descent, you switch from complete resistance of the barbell—an equal and opposite force to its downward velocity—to a controlled resistance. The flexing of the hips and knees is not an act of pulling the bar down to the bottom position, which would involve the concentric use of the hip and knee flexor muscle groups. Instead, you are allowing the bar to descend, checked only by the extensors of those joints. These are the same muscle groups that will do the lifting on the upward portion of the lift. During the descent, however, they are engaged to keep the bar from free-falling, with you placed squarely underneath it.

The knee extensors are the muscles of the quadriceps group. “Quad-” indicates four muscles: the vastus medialis, vastus lateralis, vastus intermedius, and the rectus femoris. The rectus femoris starts on the hip at the anterior inferior iliac spine, running down toward your knee. It and the three vastus muscles—all of which begin on the femur—cross the knee and have primary functions as knee extensors. The big rectus femoris is the muscle that gives your thigh its “V” shape when you’ve been doing your squats with dedication.

Hip extension is primarily the domain of the gluteus maximus, which recruits your hamstring muscles (the biceps femoris, semitendinosus, and the semimembranosus), and the adductor magnus of the inner thigh.

The gluteus maximus is the largest muscle in the human body, which should clue us in that hip extension is an important and powerful movement. The gluteus maximus extends from the pelvis (originating on the posterior ilium, sacrum, and coccyx) to the iliotibial tract (IT band) running down the lateral thigh. Its primary action is to extend the hip with supporting roles to rotate the femur laterally and abduct the thigh. Nearly all big, powerful movements require some extension or rotation of the hip muscles, all of which are resisting the weight on the descent of the squat, preparing for a powerful concentric contraction and upward movement.

Crossing two joints, your hamstrings serve two primary functions. The hamstrings are a group of three muscles the biceps femoris (the name bi- indicates a muscle with two heads at the origin), the semimembranosus, and the semitendinosus. Each of the muscles originates at the ischial tuberosity (the short head of the biceps femoris originates on the femur and does not cross the hip joint). The muscles cross the hip and the knee joint as well, inserting on the lateral head of the fibula (biceps femoris), the front and inside aspect of the tibia (semitendinosus), and posterior medial aspect of the tibia (semimembranosus). The hamstring group extends the hip and flexes the knee.

When an action occurs around a joint that is the same as the function of the muscle, the muscle gets shorter. So, as the knees flex on the descent of the squat, even though it is the weight of the bar creating the movement, the hamstrings are getting shorter. They do not actively contract to flex the knees, but they are getting shorter as the knees flex. Conversely, when movement occurs that is opposite to the muscle’s function, the muscle lengthens. During the descent of the squat, the hamstrings and other hip extensors are being lengthened under load as the hips flex. This simultaneous lengthening (proximately, at the hip) and shortening (distally, at the knee) means that the hamstrings are not changing length in a one-to-one ratio to your body’s movement as you descend. They are, however, acting as antagonist muscles to the flexing of your hip, resisting the stretch and preparing for a “stretch reflex” on the way up.

A proper squat will set up the external rotators of your femurs to keep the knees traveling in line with the toes. As they do this, their antagonist muscles—the adductors—are lengthening eccentrically. All muscles that are lengthened and loaded on the way down will contribute to the powerful contraction on the way up to move the bar. Switching from down to up, eccentric to concentric action, involves a reflexive action that we call the stretch reflex.

The Stretch Reflex

Muscles are sensory organs. They react to changes in your body’s position and their own length, sending signals to your brain for certain regulatory actions. When muscles stretch, either by the action of other muscles or by some external force, receptors automatically cause the muscles to contract in order to maintain stability. In this way, your body prevents one muscle from pulling a bone clear out of its joint by activating antagonist muscles. This same reflexive excitement helps contribute to a forceful concentric action following an eccentric movement. When you are on the descent of a squat (eccentric), the extensors of your hips and knees are using the feedback from the muscles lengthening under load to start to generate force. They are, in a sense, already spooled up or primed for a shortening contraction. When you switch from down to up, they capable of a quick and forceful shortening, extending the hips and knees and helping to lift the bar back to the starting position. (See AB Schwartz, “Movement: how the brain communicates with the world,” Cell (2016).)

The stretch reflex essentially hacks the body’s force-producing process by cutting down your reaction time. One of the primary variables for force production of a muscle is the time available for force development. (Zatsiorsky and Kraemer, “Science and Practice of Strength Training” (2d ed. 1995).) Peak force production does not occur instantaneously. It takes time for your muscles to spool up to their maximum contractile capacity. This means that if there is only a short window to develop force production—say while performing a throw or a power clean—the athlete who is more naturally explosive is going to have an advantage. Whereas a lift that occurs more slowly—a deadlift, for example—allows the lifter enough time to generate maximum force, at least for those lifters who have the grit and guts to do so. The stretch reflex cuts down the time required to generate maximum or near-maximum force, as long as the lifter does not pause too long at the bottom or the muscles do not become unloaded. (Pin squats, as an example, can unload the muscles at the bottom, interrupting the stretch reflex, making the lift much more difficult than a regular, down-and-up squat.)

You must balance the benefits of the stretch reflex, resisting the weight on the way down and lifting in a controlled manner, with the potential buildup of fatigue during the descent. Accordingly, slow, tempo squats utilize a stretch reflex but garner much more fatigue on the eccentric portion of the lift.

At the Bottom

The image of the bottom position of the squat presents a snapshot of the lift’s challenges and efficacy for training a large portion of the body’s overall muscle mass. The back is held rigid, knees out over your toes, hips back and just below parallel, and slight dorsiflexion of the ankles. On the journey down from the standing position, you’ve created rotational force at each of these major joints, as if the bar is a hand on a wrench, and your hips and knees are bolts resisting its turning force. At the top, you and the bar were on neutral terms, all the weight of the bar going straight down, and your body aligned to resist it directly. At the bottom, you are at a mechanical disadvantage. Using leverage, you’ve created a situation in which resisting and overcoming the downward trajectory to which the bar and gravity conspire is most difficult. And, in doing so, you’ve created the environment of change. This is the point of greatest muscular force. It’s the “oh! S#%!” moment that your body will interpret physically though the signals to build muscle and get stronger as you eat, rest, and recover from this training session. You’ve chosen to put yourself here for very good reasons, and now you must stand up.

Your hips must extend, an action performed by the glutes, hamstrings, and adductors primarily. Your knees must also extend, giving full work to the quadriceps muscle group. Your spine allows for the transmission of force, requiring it to remain rigid. Thus, your spinal erectors continue to hold the vertebral column extension against the turning force of the barbell. Your trunk muscles help keep the spine stable under load—the rectus abdominis, the transverse abdominis, and the interior and exterior obliques hard at work. All these muscles are working in their normal functions, but under conditions that require more concentration, balance, and force than you are likely to meet anywhere else in your environment. And, as a result, they become stronger for the infinite jobs you might give to them every day.