This. (At least that's how I interpreted it!)switch the arms and legs repeatedly during the 90 seconds?
I am on level 6 planks in the PBF book. This is where you are one forearm and the opposite with the other arm and leg stretched out. The book says to hold this for 90 seconds. My question is do I do this on each side for 90 second or for 45 seconds on each side for a total of 90 or do switch the arms and legs repeatedly during the 90 seconds?
Is there a video link for it in the ebook?