Day 4 goes to the floor and stays there. Five rounds of 20 climbers and a 10-count stretch hold make one set; 2 minutes rest between sets; three, five or seven sets by Level. Climbers are mountain climbers, counted every time a knee comes forward - 20 is 10 per leg - and the stretch hold is a face-down position that the program returns to later in a different shape.
Climbers start in a plank on straight arms and run in place with the hands planted: one knee drives toward the chest, then the feet switch, and the switching is where the cardio lives. Your shoulders hold the plank while your legs sprint, which is why 20 of them raise the pulse faster than 20 of almost anything else. The stretch hold is the counterweight: lie face down, reach both arms straight out ahead, lift the arms and chest off the floor and keep the legs long. Ten counts of that lengthens the front of the body the climbers just folded shut and asks the lower back and rear shoulders to hold you up.
Keep the hips low in the climbers - level with the shoulders, not bouncing up toward the ceiling - and land each foot quietly. In the stretch hold, lift by squeezing the upper back and glutes rather than cranking the neck; the gaze stays on the floor just ahead of your hands, and the arms stay straight and off the ground for the full count. Breathe in the hold, because it is very easy not to.
Sprint on your hands, then stretch yourself long - Day 4 never leaves the floor.
Make it Harder: Float the legs a fraction above the floor during the stretch hold as well, so the position is held at both ends.
Make it Easier: Walk the climbers - step each foot in and back rather than switching in the air.
