I'm no guru when it comes to specifics physiology... But how could lifting the feet completely off the ground reduce tension in the upper back? My thoughts are that everything else is going to be fighting that much harder to stop you rolling off the bench!
I have lower back issues, but would rather have my feet on plates to draw my lower back into the bench, and at the same time maintain stability.
In my opinion the legs in the air option is a more challenging option for the stabilisers, similar to if you tried to do an entire bicep track with your feet close together... I don't see how it could be a safer option.
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