The Real Science of the Squat Pt 2

Welcome back to Squat University! Last week we entered into Biomechanics 101, an introduction into the mechanics of the human body. We learned what torque is and how it is generated during the squat.

While the analysis from last week was a great starting point to understanding the squat, we can’t stop there. We need to take a deeper look at the 3 squat techniques and compare them realistically.

Math Pt 2 (1)Biomechanics 101

Last week we compared torque as the rotational force of a wrench turning a bolt. This is what you feel at the shoulder when you try to hold a weight out in front of your body. This is a common illustration used by many professors in physics classes across the world. It is also common in strength & conditioning texts such as Mark Rippitoe’s book Starting Strength (2).

Lever Arm

In order to calculate torque at a joint we need to know two things. First we need to know the length of the lever arm. In this illustration, this is the length between the point of rotation (the shoulder joint) and the line of force acting upon that joint (gravity pulling down the dumbbell). The length of the shoulder will therefore be the lever arm.

Moment Arm

If we know the lever arm length and the angle of the joint we can find the moment arm (3). A longer moment arm will create more torque at a joint compared to a shorter moment arm (given the pull on the end of the lever is the same).

However, what we didn’t discuss was what happens when the pull on the lever changes. Torque can be manipulated by not only changing the length of the moment arm but also by changing the amount of force pulling down on the lever. When holding a 10 lb dumbbell out in front of your shoulder, there is roughly 44.5 Newton’s of force pulling down on your joint. This value represents the force of gravity’s acceleration acting upon the weight. In our illustration this created 33.4 Nm (Newton Meters) of torque at the shoulder joint. We came to this number by plugging in the length of the moment arm (.75meters or roughly 30 inches), the angle of the arm and the weight of the dumbbell.

The equation for torque at the shoulder looked something like this.

Math (2)

On the other hand, what if we now picked up a 20 lb dumbbell and tried to raise and hold it at the same extended position? This weight would then be converted to ~89 Newton’s of force. To get 89 Newton’s you must convert 20 lbs to 9.1 kg. This is then multiplied by 9.8 m/s2 (standard gravity acceleration) to end up with 89 Newton’s. If we assume the length of our arm didn’t change, the mathematical equation to calculate the new torque value would be:

Math Pt 2 (1)

Squat Analysis

Now that we know how torque can be manipulated by changing either the moment arm length and/or the amount of force pulling down on the lever, let’s now analyze the squat with weights that are more natural to each lift. A conservative estimate would be that an athlete can squat 15% more weight using a low-bar technique when compared to the high-bar technique. Most powerlifters use the low-bar variation over a high-bar back squat in competition for this reason. We could also make an educated guess and say most athletes could squat 15% more in the high-bar back squat compared to the front squat.

Parallel Diagnostic

If we assume a 1 repetition maximum in the low-bar back squat to be 500 lbs, this would mean this individual could theoretically high-bar back squat 435 lbs and front squat around 378 lbs. Let’s see how the change in weight on the barbell changes the torque placed on the various joint complexes of the body.

Low-Bar Back Squat (500 lbs)

If we assume a lifter is capable of a 500 lbs low-bar back squat, this means there will be 2224.11 Newton’s of force now pulling down on the bar. This is a much larger value than we saw with the previous 225 lb loaded barbell.

For this analysis we will use the exact same lever arm lengths and joint positions from the previous illustration. We’ll again “freeze frame” the squat at the parallel position (hip crease in line with the knee) (4). The only thing that we’ll change will be the weight on the bar.

Low Bar Diagnostic

Math Pt 2 (2)

Math Pt 2 (3)

High-Bar Back Squat Analysis (435 lb)

Lets now see what happens when this athlete lifts 425 lb (1934.98 Newton’s) with the high-bar back squat. With this technique there is a more closed angle at the knee joint (now at 125° compared to the previous 120° with the low-bar technique). The angle at the hip joint will be at 55° which is more open when compared to the low-bar back squat hip angle of 40°.This is a normal change due to the more upright trunk position of this squat variation (4).

High Bar Back Squat Diagnostic

Math Pt 2 (4)

Math Pt 2 (5)

Front Squat Analysis (360 lbs)

Lastly, lets assume the same athlete now attempts to lift 378 lb (1681.43 Newton’s) with the front squat technique. The angles during the “freeze frame” at the parallel squat position will change again from the previous two techniques. The front squat uses a more closed angle of the knee joint (now at 130°). It also employs a more vertical trunk in order to keep the bar balanced on the chest and centered over the middle of the foot. This opens up the hip joint and low back to 75°.

Front Squat Diagnostic

Math Pt 2 (6)

Math Pt 2 (7)

Comparative Analysis (Varying Weights Across Techniques)

With this analysis we can see some striking differences compared to the last investigation that evaluated each squat at the same weight.

  • The low-bar back squat technique placed dramatically more torque on the low back (lumbar/pelvis joint) and hip joint compared to the other techniques. In this parallel “freeze frame” analysis 717.7 Nm of force was applied to the low back and hip joint compared to the other techniques (522.4 Nm high-bar back squat and 403.5 Nm front squat). Comparatively, the low-bar squat placed 53% more torque on the hip and low back than the high-bar squat and 78% more than the front squat.
  • The low-bar back squat however placed the least amount of torque on the knee joint compared to the other techniques!
  • The high-bar back squat placed relatively the same amount of torque on the knee joint as the front squat. Despite having a longer moment arm in the front squat and a more closed angle, the heavier weight of the back squat increased knee torque to the same level.

Final Thoughts

As you can see with this analysis, changing the weight on the bar can significantly change the amount of torque that was generated on the different joint complexes. The smallest change in variables (weight on the bar, technique used, etc) can greatly change the forces placed on your body.

This allows us as coaches to make exercise recommendations for our athletes based on individual needs. For example, an athlete returning from a knee injury that can’t yet tolerate a more forward knee position during a barbell squat would benefit from using a low-bar back squat compared to a high-bar variation. This is in part because more torque is placed on the knee joint during the high-bar back squat.

Also, an athlete dealing with back pain may benefit from using a front squat during training instead of the conventional back squat. This is because the front squat places less torque on the low back compared to both back squat variations when more realistic weights are used. This recommendation is only practical if the injured athlete is able to perform the front squat with acceptable technique. An athlete with poor core control or restricted thoracic mobility may find it difficult to assume the form.

Exercise recommendations for healthy athletes exercise should not be based solely on the forces sustained at one joint. Research shows that healthy athletes can easily tolerate the forces for any of the 3 squat techniques (1). You shouldn’t worry about injuring the knee using high-bar or low-bar back squat. The ACL and other ligaments inside the knee joint should be completely safe. As long as good technique is used, joint stress values will never come close to exceeding harmful levels (1.).

Athletes should use a training program that employs multiple squat techniques to ensure a more balanced approach and to decrease risk of overuse injuries.

Until next time,

SquatBottom
Dr. Aaron Horschig, PT, DPT, CSCS, USAW

With

Kevin Photo
Dr. Kevin Sonthana, PT, DPT, CSCS

Resources

1) Schoenfeld BJ. Squatting kinematics and kinetics and their appication to exercise performance. JSCR. 2010; 24(12):3497-3506.

2) Rippetoe, M. (2011). Starting Strength. Basic barell Training. 3rd The Aasgaard Company. Wichita Falls, Texas.

3) Diggin D, O’Regan C, Whelan N, Daly S et al. A biomechanical analysis of front versus back squat: injury implications. Protuguese Journal of Sport Sciences. 11(Suppl. 2), 2011; 643-646

4) Wretenberg P, Feng Y, Arborelius UP. High – and low-bar squatting techniques during weight-training. Medicine & Science in Sports & Exercise. February 1996; 28(2):218-24

 

 

Published by

Dr. Aaron Horschig

Doctor of Physical Therapy, CSCS, USAW coach and athlete.

2 thoughts on “The Real Science of the Squat Pt 2

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