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.
3) PLENTY OF OTHER EXERCISES: dumbbell lateral raises, shrugs, the top phase of barbell row etc etc
Why doesn't an arched upper back squeeze the spinal discs?