Inching Elbow Plank

You'll feel every inch of this plank in your abs (along with your arms and shoulders too).
How to do it: Begin in an elbow plank position, hands clasped, with your feet slightly wider than hip-width apart. Step your left foot towards your hands as you begin to lift your hips up slightly, quickly taking another step in with your right foot, lifting hips a little more. Step the left foot in again, lifting your hips higher and then again with the right, so that your hips end lifted high in a pike position. Slowly inch back out, reversing the way you walked in, until you return to the starting plank pose. That's one rep. Do up to 3 sets of 5 reps.
Make it harder: Take larger steps in to raise your hips even higher in the pike position, and increase the number of reps per set from 5 to 10.
Mind your muscle tip: Your abs should be drawn in during this entire exercise. As you step your foot in, think of bracing your abs in deeper with each step.source by www.shape.com