Day 18 puts four exercises in one position and never lets you out of it: 12 plank arm raises, 12 plank leg raises, 12 plank rotations and 6 climber taps, run straight through before the rest. The arm raise is the new one - from a full plank on your hands, extending one arm forward at a time while the rest of you stays exactly where it was.
Taking a hand off the floor is a bigger deal than taking a foot off it. Your base narrows at the end that carries most of your weight, and the body's instinct is to roll toward the supporting side - so the abs, and especially the obliques, have to fight rotation the whole time the arm is out. Plank leg raises then do the same thing at the other end, plank rotations turn the fight into a movement by opening the body to the side and back, and climber taps finish by making the base itself move. Arms, legs, rotation, travel: four ways of destabilising the same plank, in the order that builds on itself.
Set your feet a little wider than usual and it becomes possible to do all this without tipping. Reach the arm forward rather than lifting it upward, and keep the hips absolutely level - if they rotate, shorten the reach. On plank leg raises, keep the toes pointing down and lift from the glute. For climber taps, hips stay low, and the tapping hand goes down to meet the knee rather than the hips rising to meet the hand.
Twelve reps at a time, and the only thing in the room that has to stay still is you.
Make it Harder: Bring your feet together for the arm raises and leg raises - a narrower base turns every lift into a balance problem.
Make it Easier: Take the climber taps from your knees and keep the first three exercises exactly as drawn.
