Revisiting Day 2's Tall Plank today with a very effective add on. Today's plank is about quality hold, for a Max time.
Extend one arm out in front and place it on the ground. Leave at least the distance of one hand print between where your hand was when it was on the ground directly below your shoulder and where you place it. If you want you can use a two step walk out where you place the heel of one hand inline with the finger tips of the other and repeat with both sides.
This plank will hit the core in more challenging way to the tall plank. Everyone's degree of shoulder mobility is different so pay attention to any discomfort there. In a few cases the shoulders may be the weaker link and fail before the core. If this is the case for you, don't force it. Return to tall planks and do alternating hand lifts every 3-5 sec without tilting.