Day 13 is abs, one step up from Day 6: 14 knee-to-elbow. That is the set. Rest 2 minutes, then repeat for 3, 4 or 5 sets by level.
Hands behind the head, elbows wide, one knee lifting up and out to the side to meet the same-side elbow, then the other side, alternating - 14 means seven per side. Two more reps than last week is the whole change, and it is enough: at seven per side the last couple of reps on each side begin to ask something of the waist, which is exactly what an abs day is for. The standing leg keeps balancing, the hips keep lifting, the trunk keeps folding.
Lift the knee sideways rather than forward, and fold from the waist so elbow and knee meet in the middle. Stand fully tall between reps. Keep the pace steady - a controlled rep works the trunk, a hurried one just moves the leg.
Fourteen standing reps and the second abs day is done - two more than last time, and you will feel exactly which two.
Make it Harder: Do 20 instead of 14.
Make it Easier: Do 10 instead of 14.
