"Fit and healthy abs should be able to resist external forces, flex, extend, and rotate, and this moves does all of these. You’ll also burn more calories because it uses more muscles than just the abdominals," says Marta Montenegro, an exercise physiologist and adjunct professor at Florida International University.
To do it: Start in a plank position with one small towel placed under each ball of your foot, legs together. Bring your left knee in towards the right side of your chest, squeezing your abs. Then, straighten your right leg back out to full plank and bring your right knee in towards the left side of the your chest and back out to full plank. Next, draw both knees into your chest at the same time and then slide your legs back out to full plank. That’s one rep. Build up to 3 sets of 12-15 reps (resting in between).