Why should the knee point outwards in squat?

Question

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.

Answer 1

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.

Answer 2

I think TravisJ gave a good answer on how to squat but didn't cleared the actual concept of why to externally rotate your knees. now let me give me a disclaimer I am not opposing your knowledge about squatting you are right but I think you don't know the actual cause of externally rotation. This answer is from my personal experience so read it full.

Now to start I will tell about my story of starting workout as I was overweight too I mentioned here. After I joined a local gym I started free hand exercises to build a good foundation so when i used to squat I used to keep my knee straight and used to fall behind while squatting as I was heavy so for 3 weeks I didn't squatted I grabbed 5kgs and started doing lunges for quads, hamstrings and glutes. Till then I have surprisingly lost 4 kgs of weight by extensively doing abs exercises now i started squatting again with no barbell just free hand but still I was unable to do it properly like others used to do in front of me in the gym. One day I noticed my instructor thoroughly during his squats and noticed while he was taking his stance, rotating his knees outwards a little bit, then I started noticing everyone and I too then inculcated that squatting style.

Now after my long story the fact why to externally rotate your knees it's because to create more space in your femur to hip bone joint to squat more deeply to create more fatigue in your quads and more work done by glutes by balancing the weights.

You can even squat deep by keeping your knees straight, feet joined but you can't squat lifting weight as it will not create balance and make your back bend more forward creating unusual stretch in your lower back spine. You can squat normally keeping knees straight, feet shoulder apart but you can't squat deep even if it creates balance in some of them (it depends on individual's body mechanics, my one friend can squat knees straight with heavy weights but can't go deep in squat).

So to squat deep and lift heavy with balance will need you to create more space in femur to hip bone joint (if you don't understand how rotating knees can create more space in joint then my friend it's biomechanics you should study more).

I am linking a video here of famous physical instructor JEFF CAVALIERE running a youtube channel ATHLEAN-X explaining the same fact, if you are a fitness freak I recommend you subscribing his channel.