Which is more difficult, plank on forearms or plank on hands?
I just like to throw in one comment about the question of shoulder stabilization which some fellow trainers have already alluded to.
I often observe that – in the interest of strengthening ‘the core’ – the issue of shoulder girdle integrity is completely ignored. Even in a position on all fours, I often see shoulders that look truly ‘unhinged’ with the head hanging down and the shoulder blades sticking up. I would suggest that this person does not need to do a plank either way until the shoulder have greater strength to deal with such a load.