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I’m a fan of Stuart McGill’s work. I believe he would say the safest would be neutral spine all the time. However, it is evident that some people do some amazing things outside the neutral spine position. a great recent example was in a MWOD episode with Matt Vincent where he snatches with a rounded upper back. he maintains the shape of the rounded upper back which protects it (movement during loaded situations is bad for the spine, but static seems ok in certain conditions) but as he throws the bar up, he unloads his spine and then reshapes it to be neutral/extended for the overhead where it is braced again. so he escapes injury because he changes shape when his spine is unloaded but not when it is.