it is known that keeping a neutral lower back under load is crucial to avoid injuries and/or low back pain. This because both an overextended back or a flat/flexed back squeeze the spinal discs, as explained for instance in each stronglift guide.
Also a rounded upper back is known to be bad under load. I mean something like that.
But, I don't understand why keeping an arched upper back (instead of a neutral one) isn't a problem in many exercises. I mean all the exercises where, for any reasons, retracting the shoulder-blades is required.
1) SQUAT: The shoulder-blades are retracted (and elevated) to keep the barbell stable throughout the exercise. Hence, the upper back is arched. It is explicitly written also here:
Arch your upper-back to create support for the bar. Squeeze your shoulder-blades and raise your chest.
2) BENCH PRESS: as explicitly written here (and everywhere), the middle-upper back must be arched in order to be stable on the bench and prevent front delt from activating too much.
Why doesn't an arched upper back squeeze the spinal discs?