Lie on your back with the soles of your feet pressed together and your knees dropped open to the sides, arms extended on the floor overhead. In one smooth motion, sit all the way up and reach forward to touch your feet, then roll back down with control, one vertebra at a time. The butterfly leg position takes your hip flexors out of the lift, which is why it's harder than a regular sit-up - your abs do all of it.

Works: abs, obliques, hip adductors

Breathing: Exhale as you sit up and reach, inhale as you roll back down.

Common Mistakes: Throwing the arms to yank yourself up with momentum, dropping back down flat instead of rolling down with control, and letting the knees float up toward each other mid-rep.

Make Easier: Cross your arms over your chest to shorten the lever, or work up to the full movement with Sit-Ups or Crunches.

Make Harder: Slow the roll-down to a three-count on every rep, or move on to Sit-Up Punches or Elbow Strike Sit-Ups.