Starting position: Begin on your hands and knees, arms straight; your hands slightly wider than shoulder width. Extend and raise your right leg behind you, keeping it straight, toe about a foot from the floor. The key is to keep arms straight and back flat. This is the beginning of a circuit, where you add on a new move with each progression.
Target muscles: Core, butt, pectoralis major (chest), and triceps (underside of upper arms).
Assume starting position. Gently tap the toe of the extended right leg on the floor, then return leg to the raised position. Don't swing the leg up and down; this is a small, controlled movement. Do 20 taps; then, keeping the leg extended and raised, do six push-ups. You don't need to bring your chin to the floor—halfway down will do. Return right knee to floor. Repeat on left leg (that's one set). Do four sets; between each, rest for 30 seconds with both knees on the floor. This is Move One.
Perform Move One on right leg and, without resting, continue to Move Two: From starting position, cross the extended right leg over the bent left leg, tap it on the floor, and return to the starting position. Do 20 taps; then do six push-ups (again, with leg extended and raised). Rest for 25 seconds with knees on the floor. Change legs and repeat Moves One and Two. (That's one set.) Do three sets, resting between them for 25 seconds with both knees on the floor.
Perform Moves One and Two on right leg, and, without resting, add Move Three: Bring the right knee in toward the chest no further than your elbows. Hold for one second—your abs will feel it. Then fully extend the leg back out to the starting position. Do this 20 times, followed by six push-ups with leg raised, as before. Rest for 20 seconds. Switch legs and repeat Moves One, Two, and Three. (That's one set.) Do the whole circuit again for a second set. Rest for 20 seconds with both knees on the floor.
Perform Moves One, Two, and Three on the right leg; then, without resting, add Move Four: Bring the right knee in toward the chest and then rotate it outward, like a dog lifting its leg. Bring the knee down, and move the leg back to starting position. Repeat 20 times. (The movement comes from your hip, and the knee shouldn't be lifted higher than your waist.) Next do six push-ups with leg raised as before. Rest for 15 seconds before repeating Moves One through Four with the other leg.