Summation of Forces About The Knee Equals Zero | Tom Bailey

# Thread: Summation of Forces About The Knee Equals Zero | Tom Bailey

1. ## Summation of Forces About The Knee Equals Zero | Tom Bailey

Before I found the Starting Strength program, I was a runner and basketball player, with the resulting sore knees consistently limiting what I intended to do. I expected the same knee soreness or worse when I began to low-bar squat in middle age as part of my Novice Linear Progression. I was surprised that my knees felt great as my squats progressed through my NLP...

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Rather, the magnitude and direction of the forces cancel each other out such that the net effect is negligible
I don't think that's correct. Sure, sum forces so that SUM(F) = ma = 0, but that's only translational motion. You forgot the rotational component, SUM(M) = (I)(alpha), and that is definitely not equal to 0 in a lowbar squat, even when the knees are held in position after the initial descent.

And even if you are only looking at the translational component, a net 0 force does not mean the net effect is negligible. It just means the knees aren't accelerating anywhere. The internal forces (and torques) are significant and even desirable. If there were, as you say, negligible, the squat would not make your knees stronger, but it does.

It's been awhile, but I used to work in a lab as a undergrad mechanical engineering student where I wrote software to compute these internal forces and torques from motion capture data.

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Originally Posted by asm44
I don't think that's correct. Sure, sum forces so that SUM(F) = ma = 0, but that's only translational motion. You forgot the rotational component, SUM(M) = (I)(alpha), and that is definitely not equal to 0 in a lowbar squat, even when the knees are held in position after the initial descent.

And even if you are only looking at the translational component, a net 0 force does not mean the net effect is negligible. It just means the knees aren't accelerating anywhere. The internal forces (and torques) are significant and even desirable. If there were, as you say, negligible, the squat would not make your knees stronger, but it does.

It's been awhile, but I used to work in a lab as a undergrad mechanical engineering student where I wrote software to compute these internal forces and torques from motion capture data.
It’s been a while for me as well. The focus of the article is the bottom position which is the concern usually cited by those who claim squats are bad for the knees. Clearly there is acceleration across the range of motion since there is a change in velocity (as well as displacement). But at the bottom position velocity and acceleration are momentarily zero, which is what this article addresses.

“In my day” I had to calculate forces and moments with a scientific calculator, but yes a software model could show internal forces at work. But those are so minor that they approach being negligible in the real world. As far as rotational movement of the knee, bad things happen in any activity in which the knee joint is rotated and not bent in line with the femurs. The article is specific about the knees in line with the femurs at the bottom position and the muscle groups surrounding the knee which prevent the knees from rotating.

Great comment, I appreciate the input. There are great engineers on this forum. Your motion capture data sounds interesting. If there are any lessons to be learned from that lab please share!

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Good article. Thanks

Instead of using the knees as either the brakes
This hits home with me.

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Originally Posted by stef
Before I found the Starting Strength program, I was a runner and basketball player, with the resulting sore knees consistently limiting what I intended to do. I expected the same knee soreness or worse when I began to low-bar squat in middle age as part of my Novice Linear Progression. I was surprised that my knees felt great as my squats progressed through my NLP...

Ultimately, each of us has to answer a question posed by Rip in Strong Enough?: “Are you willing to let medical professionals make excuses for your lack of willingness to do the hardest, most productive exercise in the weight room, an exercise that has been proven safe by decades of use by millions of people? I don’t think you are. Prove me right.”

Impulse buy and bought the audio book. Rip has a chapter called "Silly Bullshit" that is alone worth the price of admission. Wobble board contests!?! Bwahahaha

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When I first started squatting properly, the chronic pain in my knees improved so quickly I felt like I was cheating. It became so clear to me that it was the absolute best way to do the move I can’t think why every other person who has advised me about training told me me to keep my back straight up when I squat. It’s almost like they see a vertical back as a virtue.

7. Originally Posted by asm44
You forgot the rotational component, SUM(M) = (I)(alpha), and that is definitely not equal to 0 in a lowbar squat, even when the knees are held in position after the initial descent.
If the sum of the moment in the squat about the knees were zero, the limb segments wouldn't move. You'd just stay standing at the top. We deliberately introduce moment arms on the joints in order to move through the range of motion. Necessary moment is a good thing. The fact that sum of the moment is not zero is not the point. The sum of the forces is zero which means that the components of the knee are not translating relative to one another. We do not want the distal end of the femur to move away from the proximal end of the tibia and fibula very much. That would be a problem that the ligaments would have to deal with, and we don't want that.

Originally Posted by asm44
And even if you are only looking at the translational component, a net 0 force does not mean the net effect is negligible. It just means the knees aren't accelerating anywhere. The internal forces (and torques) are significant and even desirable. If there were, as you say, negligible, the squat would not make your knees stronger, but it does.
This is actually the basis upon which we do the squat. We WANT the forces to not be zero. We WANT the hamstrings and the quads to produce a lot of force, but we also want the net force on the knee to be zero to prevent translation. This doesn't happen - really what happens is that the ligaments take up the slack which bad because they don't adapt well to this stress.

The reason partial squats are so bad is because the hamstrings get neglected, and you end up with anterior component producing more force than the posterior component which DOES produce a non-zero force summation.
Last edited by AndrewLewis; 01-13-2022 at 06:35 PM.

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Are the anterior and posterior forces generated by the quads and hamstrings assumed to be 0 because the tibia doesn't move move? If so wouldn't this be confounded somewhat by the wait of the body and barbell above this pushing the femur down into the tibia. The more weight there is the more weight it takes to push it laterally. I can push my car unequally from the back but it doesn't move because the force I generate doesn't overcome the weight of the car.

Or is that just splitting hairs? does 5% more anterior force doesn't matter when it's 500 pounds on the bar because it's relatively so small.

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