Sorry if this has come up before, but why is the upper back rounded during the plank progressions?
To quote someone with much more experience than me, "long spines are strong spines." I always assumed that thoracic extension was still applicable during planks, pushups ect and so that's how I've been training them, but the question has been bugging me for a while.
Is it simply to make the exercise easier, or are there scapula muscles being engaged that I'm not aware of?