Squat and Shearing Forces

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KenDowns
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Squat and Shearing Forces

Post by KenDowns » Fri May 20, 2011 10:01 am

I'm trying to work out if I understand the concept of the unhealthy shearing forces that come into play on squat if your knees travel past your toes.

I tend to think of the knee as a hinge. The only forces you need on a hinge either open it or close it. So as I understand it, hamstrings "close" the hinge in a concentric motion (or curl) and quads "open" the hinge in an eccentric motion, or an extension.

Going further, if the lifter "sits back" into a squat, the load up on his back is translated to the knees as a force attempting to close the hinge, or bend the knee. If I understand things, this is good because the knee is exposed only to natural forces. These forces are exaggerated by the weight and the knee actually gets stronger.

So if I've got that right so far, what then is the shearing force you get if the knees go forward past the knee. A shear would be a force pushing two objects across each other. So the weight of the bar is NOT putting a "closing" force on the hinge, but is pushing to break the hinge by bending the two parts (thigh and shin) apart. It seems the only way a force could be acting here would be to push the thighs forward, shearing the knee with a force trying to shove the femur out past the knee.

Is that what is going on?

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Re: Squat and Shearing Forces

Post by Paperclip » Fri May 20, 2011 11:05 am

I think when people say that they mean shear stress working on/between the patella and the knee joint.

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stuward
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Re: Squat and Shearing Forces

Post by stuward » Fri May 20, 2011 5:35 pm

This might help.
http://www.pponline.co.uk/encyc/squats- ... ercise-319" onclick="window.open(this.href);return false;
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mark74
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Re: Squat and Shearing Forces

Post by mark74 » Sat May 21, 2011 10:38 am

stuward wrote:This might help.
http://www.pponline.co.uk/encyc/squats- ... ercise-319" onclick="window.open(this.href);return false;
Some random points that came to me while reading:

Too bad it never actually go further than saying that the ACL is "lightly loaded". They gave numbers for everything else why not squat?

I wonder what exercise/lift would be appropriate for people with injured PCL, as it seems squat may not be the brightest idea. And also, whether those who are engaging in regular parallel squat training could benefit from occasionally doing ATG squats with light loads.

Regarding one-legged squats, it says: "Place the toes of your right foot on a block or step which is six to eight inches high (this is crucial; if you fail to put your trailing foot in an elevated position, you will often unconsciously support some body weight with the rear foot, reducing the strengthening effect for the support leg)"

I have tried this several times and I'm always supporting some weight with the back foot no matter the height. I say why not used assisted pistol squat instead.
I don't know where the blind could lead the sightless
but I'd still like to witness

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