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NOTE - I've received questions regarding if the butt wink is acceptable. The answer is it depends. Here, I am showing a true ATG full squat, in which case some butt wink is bound to occur, and in my opinion is not an issue for experienced lifters with adept mechanics. https://www.youtube.com/watch?v=b1f6mk1aVuA. However, it MUST be controlled and limited (pause video in bottom position, see I'm not hunched over rather have a flatter back from just minor some loss in natural curve).
Now on the other hand, if you do have a history of back issues, or are simply not particularly concerned with performance or general strength training rather you lift purely for general health, then I would recommend you cut the squat a bit higher to entirely avoid the butt wink.
So in conclusion, it depends on your goals and the significance of the butt wink.
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