A few weeks ago I was having a conversation with one of our interns at Boost Physical Therapy & Sport Performance on the topic of squat mechanics. His question is one that I hear all the time, “Do front squats train the quads more than the back squat?” I looked at him dead in the face and said “no.”

Now most of you may be thinking in your head, “Aaron what are you talking about? Everyone knows front squats are more ‘quad dominant’ than back squats and therefore work the quads more!” However, before you completely dismiss my answer, let me explain the why behind my response.

A year ago I wrote a blog article entitled “The Real Science of the Squat” where I broke down the biomechanics of the different squat techniques. In the article I explained how torque is generated and manipulated at the hip, knee and low back through the use of a static mathematical model.1,2

Here’s a quick summary.

The Real Science of the Squat

Imagine you’re holding a 10 lb dumbbell in front of yourself at shoulder height. Do you feel the weight of the dumbbell trying to pull your arm down? What you’re feeling is the force of gravity. It always pulls straight down. As gravity pulls down on the dumbbell, it causes a rotational force at the shoulder joint. This force is called torque. The muscles of the shoulder must then be activated to overcome this force in order to hold the weight from moving.

The distance from the force of gravity and the point of rotation (your shoulder joint in this case) is called a lever arm. Much like a wrench turning a bolt, the longer the lever arm the more rotational force (torque) can be applied at the joint.

Imagine now someone tied a rope around the same 10 lb. dumbbell and attached it to your elbow. It would now be much easier to hold your arm in front of you. This is because the length of the lever arm decreased, which meant there was less force (torque) required by the shoulder to keep the arm extended. Make sense?

But how does this relate to the squat?

If we take a snapshot of a squat at its parallel position, we can calculate the amount of torque that is generated at each joint complex (hips, knees and low back). Like we mentioned earlier, gravity always pulls straight down and during the squat it is often represented as a vertical line drawn through the middle of the barbell. This vertical line then runs through the body and divides the thigh.

The distance from this vertical line to the center of a joint becomes a lever (just like a wrench turning the bolt). Often sport scientists will analyze the squat at a parallel squat position (hip crease in line with the knee).1,4 The longer the lever arm in this position, the more torque that will be generated at that joint during the squat.

The Torque Differences Between Front & Back Squats

When we analyze the different squat techniques, we find a few interesting things.

The back squat has a longer lever arm compared to the front squat, making it more efficient for lifting big weight (this is known as leverage). Mechanically, our bodies can squat more weight when the lever arm is longest at the hips. 5

The front squat (with the more upright trunk position) creates a longer lever arm at the knee and therefore places more torque on the knee joint.

Many will take these findings and claim the back squat is therefore more “hip dominant” compared to the “quad dominant” front squat. They will also think that because there is more torque placed on the hips or knees in a certain squat technique, the musculature that surrounds that specific joint must work harder to overcome that force.

Unfortunately, that’s not how the body works during multi-joint movements like the squat. Let me explain.

Forget What You Learned in Anatomy 101

A lot of the misunderstanding on this topic comes from the way we learned about the body in the first place…our anatomy textbooks. Think about how muscles and their function are described in school. You have muscles that cross only one joint (called mono-articular) and muscles that cross two joints (called bi-articular).

In the front of your thigh for example, you have the vastus medialis and vastus lateralis. Both muscles cross only the knee joint and are therefore considered to be mono-articular muscles that extend the knee when they contract. The rectus femoris is another quad muscle that helps with knee extension. However because it also crosses the hip joint (making it a bi-articulate muscle) it is capable of flexing the hip joint as well when it contracts.

On the opposite side of the thigh we find the hamstrings and glute max. The glute max is one of the most powerful muscles of the entire body. It is a mono-articulate muscle as it only crosses the hip joint. The semitendinosis (one of your hamstrings) is a bi-articulate muscle as it crosses both the hip and the knee joints. This muscle can therefore be used to extend the hip or flex the knee, making it a direct antagonist of the quads. According to your anatomy textbook, if the quads are contracting hard to try and extend the knee the hamstrings have to relax to allow the movement to occur. Sound familiar?

This is where things get interesting.

In the early 1900’s there was a biomechanist named W.P. Lombard that looked into how the body really works, outside of our textbook understanding. The “paradox” he described occurs when muscles that should technically oppose each other are both active during certain movements. For example, when we perform a squat, the hamstrings and the quads are both activated.



If the hamstrings (which flex the knee) are activated at the same time as the quads (which extend the knee), shouldn’t the body just freeze up and not move? According to how you learned about muscles in school, this doesn’t make sense.

Our body uses bi-articulate muscles (like the hamstrings) to transport energy from one joint to another. Many experts in the field of biomechanics stress that when a bi-articulate muscle is activated during a motion like the squat, movement at one joint instantly creates an opposite reaction at the other joint in which the muscle spans.22

For example, when the glute max (a mono-articulate muscle) contracts to extend the hip during the squat, the rectus femoris (a bi-articulate muscle) on the other side of the leg is activated as well to aid knee extension.

When you stand up from the bottom of the squat, the hips and knees extend at the same time. During this process the bi-articulate muscles of the hamstrings and rectus femoris are activated BUT do not change in length. The hamstrings are shortening at the hip joint and lengthening at the knee joint while the rectus femoris is lengthening at the hip joint and shortening at the knee joint.

For this reason, bi-articulate muscles have been regarded as “tendons” as they allow mono-articulate muscles to have an indirect action on a joint they do not pass. This means the activation of the hamstrings and rectus femoris do NOT cancel each other out and lead to cramping or freezing of movement!

We can therefore come to the conclusion that during the squat:

The glutes can indirectly extend the knee joint

The quads can indirectly extend the hip joint

Basically, the glutes and vastus muscles of the quads function as force and work generators while the hamstring and rectus muscles distribute torque and power between the two joints.

Did I lose you yet?

We can now use this understanding to help answer the question “are the quads activated more during a front squat than a back squat?”

While there is clearly more torque comparatively placed on the knee joint during a front squat, the body does NOT activate the quads more to overcome this force. The quads are already “turned on” during the squat in order to lift the weight. Instead the body redistributes power from the hips to the knees to help the body complete the lift.

Therefore the quads are not activated more in a front squat than a back squat.

What does the research say?

There is a multitude of studies that have compared the front and back squat and found no significant differences between the lifts in terms of muscular activation.

Recently in 2016, a group of researchers led by Dr. Jonathan Sinclair compared the front and back squat at the same weight (70% of the 1 RM front squat). While there was more forward lean seen in the back squat (qualifying it as a more “hip dominant” lift) they found no significant difference in muscle forces in the lower body.13

Other experts in the field of sport science Dr. Bret Contreras and Dr. Brad Schoenfeld, had a group of 13 women with experience in resistance training perform their estimated 10-rep maximum in the front squat along with the back squat to parallel and full depth. They found no significant difference between any of the lifts and variations.12

In fact, only 1 scientific study to our knowledge has ever shown any difference between the two lifts in terms of muscular activation. The researchers found the vastus medialis was activated to a greater degree during the front squat while the semitendinosis was activated more during the back squat.23 These differences however did not appear until maximal weights in each lift were attempted and they also found no significant difference in any other muscles between the two lifts (including the other vastus muscle or hamstring muscles).

Practical Application

Busting Through a Sticking Point

In a recent article Greg Nuckols (an experienced coach, author and powerlifter) explained how this concept can help us learn to grind through a sticking point of a squat.6 When most athletes miss a squat, they end up getting stuck about half way up (when the hips are above parallel). By applying what we learned today, we can conclude the lift stops ascending because the body is unable to create enough force to overcome the torque created at the hips (which are still left in a flexed position).

Greg theorized that by driving your upper back into the bar and pushing your knees forward (shifting your hips under the bar) you can effectively grind through a sticking point. This movement decreases the length of the lever arm length at the hip joint and therefore the amount of torque the hips need to overcome. This means less force is needed at the hips to overcome the torque, leading to an easier ability to stand up and finish the lift!

Understanding The “Good Morning” Squat

We can also use our new understanding of Lombard’s paradox to explain one of the most common technique faults, the “good morning squat” or “stripper squat”. This describes the problem of the hips rising faster than the chest on the ascent of the lift.

There are 2 main theories as to why this problem exists.

Quad Weakness Coordination Problems due to Fatigue

Many are under the impression that quad weakness is the main culprit behind this technique flaw. However, when you look at the research, there are a number of flaws to this idea. First, the majority of research to support this theory centers on studies performed on subjects lifting boxes or sandbags from the ground (not a barbell on their back).

In 2012 a group of researchers from Cairo University analyzed the technique and muscular activation of a group of participants while lifting a sand bag weighing roughly 30% of their bodyweight. After fatiguing their quad muscles in isolation (using an expensive isokinetic machine) they reanalyzed their movement pattern and found they tended to lift more with their back!20 A number of similar studies have found these same results.19,21

However, what these studies failed to demonstrate is how the body responds to fatigue when lifting a weight on the back. This means their results cannot be generalized to a movement like barbell squatting. The movement of picking up a box from the floor is different than when squatting a barbell on the back.

This is where the second theory comes into play.

During the ascent of a squat we ideally want the chest and hips to rise at the same. This keeps the bar centered over the mid-foot, which means the body is in balance and capable of producing efficient power.

However as athletes fatigue while squatting (near the end of a high rep training session or when attempting a near maximum weight), they often lose their ability to stay balanced and maintain perfect coordination, allowing their chest to fall forward.14,16,17 Excessive forward trunk lean leads to the hips rising faster than the chest on the ascent of the squat, which lengthens the hamstrings. Because the bi-articulate hamstrings are now lengthened, the body loses the ability to transfer force from the quads to aid the glutes in hip extension.

Dr. Gregory Myer and Dr. Brad Schoenfeld have explained this fault as a problem with suboptimal motor recruitment patterns (i.e. the ability for the body to turn on the right muscles at the right time to maintain technique).18

What this means is the “stripper squat” isn’t caused solely by a lack quad strength but rather due to problems in coordination and motor control that leads to a disruption in “lombards paradox”.

I find that instead quad weakness, the more likely cause of this technique fault is the ability to “turn on” the glutes at the right time due to fatigue.

In a recent study, researchers looked at the muscular recruitment changes the body takes when lifting near maximum and maximal weights. They found that when experienced athletes jump from 90% to 100% of their 1RM, they actually recruit their glutes more in an effort to keep their chest upright on the ascent.17 The researchers commented that this action transfers force from the hips to the knees, allowing the body to complete the squat ascent with the more ideal upright chest position.

If the athlete has pushed into such a fatigued state that it has effected their coordination and allowed their hips to rise too quickly, have them drop the weight and perform their remaining sets at a weight they can show better technique.

Final Thoughts

So the title of today’s article is really a trick question. The real answer is neither! While a squat may look more “hip dominant” or “quad dominant” the squat does not exclusively train the quads, hamstrings or glutes. All of the muscles of the lower body are activated together to extend the hips and knees regardless of how the squat looks or the amount of torque that is placed on the joints.

If there is less torque on one joint, the body redistributes energy from the vastus quad muscles and glutes via the hamstrings and rectus muscles to help overcome the increased torque at the other joint.7

I once heard someone say, “Nature never works against herself.” I hope that this article was able to help you take a step back from the traditional textbook way of look at the body and expand your understanding for how movement really happens.

Until next time,

With

*A special thank you to Greg Nuckols and his writing for being an inspiration for this article.

References

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