Wherever you start the shoulders end up in the same place anyways so the difference is minute. Either way in order for the bar to come up everyone who lifts with their back in extension and the bar against the shins will end up with the shoulder over the bar as it breaks the ground. Biomechanically its just not going to happen otherwise. You can pull the bar off the ground with your shoulders behind the bar when you're in flexion but you're going to fuck yourself up eventually.
Rippetoe knows what he's talking about and what he describes is exactly that your shoulders will end up there anyways so why not start there? Personally I don't start there because I use a dynamic start but I end up there before the bar breaks the ground and I stay in that position until it breaks the knees so - either way its all the same.
NOTICE: Pierre Auge's opinions are subject to change at any time and without prior notice.
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