there is lots of information on the web about feet and knees out in Squat. Firstly, I'd like to summarise it here to make it clear.
As explained here by Athlean X, in the Squat:
- a bit of external rotation of the knees is very useful to make room in the hip for the femurs.
- a bit of external rotation of the feet helps externally rotate the knees.
Catalyst Ahtletics explains more in detail why externally rotating the knees is useful:
There are two reasons to actively push the knees out in a squat. The first is to either prevent knee valgus in an athlete for whom this is a known problem, or to correct valgus as it’s occurring—pushing the knees out maintains or re-establishes the optimal leg orientation. The second reason is to correct forward leaning. If the knees move in, the hips move back and the chest moves forward in response—in other words, it causes you to lean over more than wanted.
However, Catalyst Athletics says also that:
The second reason is to correct forward leaning. If the knees move in, the hips move back and the chest moves forward in response—in other words, it causes you to lean over more than wanted.
My question is: why not pushing the knees very very out (out of the feet) until we get a good spinal position?* I'd say that an excessive knees rotation may rotate too much the feet and reduce the base width in the sagittal plane (hence causing balance issues) and maybe also reduce the athlete strength for the knees and hips extension as some power is wastened to actively push the knees so much out.
There are lots of other coaches saying we must not rotate too much. These videos say the knees should be aligned with the feet, and both must have not more than 5-10° of external rotation. They say a higher external rotation will reduce force and stability (although they do not explain the reason). Here some references: 1, 2, 3, 4, 5, 6, 7.
**In weightlifting Squat, the back is slightly extended. Precisely, the lower back is very slightly extended and the upper back is extended as much as possible. That's explained there enter link description hereand there.