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it's known and written everywhere that knee valgus in squat is bad. But I still haven't found a mechanical explanation of this, and even the best knee setup is not clearly described by most guides (some people say you may point them outwards without limits, others say you should have a slight external rotation etc).

For instance, Stronglift says:

Squatting with your knees caved in is bad for your knees. It twists your knee joints. Keep your knees out when you Squat. Push them to the side. Push them out both when you Squat down and when you Squat back up. External hip rotation is the goal: rotate your right thigh clockwise and your left thigh counter-clockwise. Your toes should be 30° out so your feet and thighs are parallel.

Here, I do not understand:

  1. Why does knees pointing in twist the knee joints whilst knees pointing out don't.

  2. Why should I choice an angle of 30°, rather than a neutral position.

  3. Why is external rotation the goal? I may suppose it is useful to pre-activate the glutes. But I can't see why not externally rotating is a mechanical issue.

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    It's also bad for your hips. I strained my hip/glute not externally rotating and now I have it band issues, and spine issues. So it definitely affects waaaay more than just knee pain – Ace Cabbie Jul 12 at 14:36
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Short answer: you don't actually need to flare your knees out, but what you do need to do is generate torque in your hips which can be accomplished by attempting to rotate forward-facing feet outward ("screwing your feet into the ground") and thinking of this as pointing your knees out is a good physical cue.

Details: The reason you externally rotate your hips is to generate tension in your hips, butt, and lower back. This tension serves to stabilize your lower back (spine) to help you avoid spinal injury (especially when the load is heavy). In addition to the external rotation in your hips, you also want to generate torque/tension in your shoulders through rotation--you may have heard this in a form like "try to bend the bar around your shoulders/neck." The external rotation in your shoulders provides stability for the upper back/spine.

If you fail to stabilize your upper and lower back (with enough torque generated by external rotation) then you risk faults (e.g. arching your back) and consequently damaging your spine. By properly bracing/stabilizing your back you not only minimize risk of injury but you also create a more efficient transfer of power from your legs to the bar. The best way to think about rigid back being better for transfer of power is to imagine pushing a spaghetti noodle with some weight on one end. If the noodle is soft (not rigid) then when you push the noodle just bends. If it is rigid then you get a (near) perfect transfer of power from your push to the weight.

There's a really good book which covers movement like this called: "Becoming A Supple Leopard--the ultimate guide to resolving pain, preventing injury, and optimizing athletic performance" by Kelly Starrett which I highly recommend. My answer is a short summary of chapter 3 in that book.

Final note/experiment: You can do the following experiment to see the difference in how you feel. Stand tall, feet forward, shoulder width apart--hold an empty bar if you want, but don't hold any real weight. Generate some torque via external rotation by trying to twist your feet outward. You should feel your quads, glutes, and lower back tighten up (muscles activating). This position should feel strong to you. Then, instead of trying to twist your feet outward, try to twist them inward. This will feel a bit awkward and you'll feel your muscles just get loose. This is what happens when your knees collapse inward. The real danger is not so much to your knees as it is to your back which you've just left in a really compromised position--especially if you're under any load.

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    As someone who doesn't practice either the 'bend the bar' or 'screw your feet into the ground' cues, I assume you that you can definitely don't need to generate rotational tension in either your hips or shoulders in order to squat. (Also, 'bend the bar' is a shoulder adduction, or maybe internal rotation cue, not external rotation. External shoulder rotation, from a squat stance, would be the act of trying to raise your hands back and off from the bar.) – David Scarlett Jul 21 at 1:53
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    You're absolutely correct on the "bend the bar" not being external rotation (will edit). If you don't generate torque it'll just be harder to keep you back braced and the heavier the weight gets the more you'll risk injury. There may be other ways to help prevent injury (e.g. a belt might help with this too). – TravisJ Jul 21 at 2:55
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    If you have an alternative strategy that helps to brace your back, I'd love to know it (perhaps I misunderstood and you do generate torque in the way I describe, just think about it in other terms than I used). – TravisJ Jul 21 at 2:58
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    I just Valsalva and isometrically clench my core and back musculature. I'd consider that to be the standard form of bracing, and independent of any hip or shoulder actions. – David Scarlett Jul 21 at 3:23

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