Day 20 adds a core movement to the running-in-place cards. Six 20-second intervals, no rest between them: high knees, butt kicks, high knees, knee-to-elbow, high knees, knee-to-elbow. One round, 2 minutes rest, 3, 4 or 5 rounds for your level.
Knee-to-elbow is drawn standing with both hands behind the head and the elbows wide: lift one knee up and out to the side while the torso bends toward it, bringing the same-side elbow and knee together, then switch. It is a standing side crunch with a balance demand, and it sits in the fourth and sixth slots where march steps used to be - so the recovery intervals have become oblique intervals. High knees still take first, third and fifth, and butt kicks the second. The round is now three sprints, one hamstring interval, and 40 seconds of standing core work with no rest built in anywhere.
In knee-to-elbow, keep both elbows pulled back the entire time so the bend comes from the side of the trunk rather than the shoulders reaching forward. Bring the knee up to the elbow rather than the elbow down to the knee. Move at a controlled pace - it is not a sprint, and a clean rep every second beats a sloppy one every half second. High knees and butt kicks stay as tall and light as they have been.
Knee-to-elbow turns a cardio round into a core round for 40 seconds.
Make it Harder: Keep the elbows pinned wide throughout the knee-to-elbow intervals and hover the lifted foot between reps.
Make it Easier: March the knee-to-elbow rather than hopping between reps.
