Day 15 opens the third week with 30 minutes of cardio spread through the day and an abs circuit: 6 sit-ups, 6 reverse crunches and 12 knee in and twists, 3 to 5 sets by level, up to 2 minutes rest between them. Reverse crunches are new: lying on your back with the knees bent over the hips, curl the hips up off the floor and lower them back down. Knee in and twist is done seated, leaning back on the hands with the legs extended: pull both knees in toward the chest and turn them to one side, extend again, and pull in to the other side - 12 in total, alternating.
Three exercises, three directions for the trunk. Sit-ups flex the upper body toward the legs; reverse crunches flex the lower body toward the chest, which is the half of the abs most crunch variations skip; knee in and twists add rotation on top of the flexion, bringing the obliques into every rep. The counts stay low and the reps stay complete - a reverse crunch where the hips actually leave the floor is a different exercise from one where the knees just swing.
Sit-ups keep their Day 3 standard: knees bent, hands reaching past the knees, controlled descent. In the reverse crunch, curl rather than kick - the knees stay over the hips and the movement comes from the lower abs lifting the pelvis. In the knee in and twist, keep the chest lifted, the feet off the floor throughout and the twist coming from the trunk, not from swinging the legs.
Six, six and twelve - small numbers, complete reps, and a trunk that gets worked from every direction it can move in.
Make it Harder: Lower the legs from each reverse crunch at a slow 3-count before the next lift.
Make it Easier: Rest the feet on the floor between knee in and twists, and do 4 reverse crunches per set.
