The possibilities are endless with planks! This interesting variation comes from fitness expert Suzanne Bowen, creator of BarreAmped. "I especially love this exercise because it flexes the spine (which contracts the abs and stretches the lower-back muscles) and then extends the spine (which stretches the abs and contracts the lower-back muscles). This back and forth works the abs at a very intense level and also helps to create balance between the back and front," she says.
Get into full plank position. Lift your left leg off the floor. Contract your abs, round your back, and pull your left knee into your nose. Keeping the core, arms, and legs extremely strong, straighten your left leg behind you as you extend your spine and lower your hips toward the floor (without letting hips or legs touch the ground). Slowly pull your left knee back in. Repeat about 4 times, rest, and then switch sides.