The simple motion bringing one knee to the floor at a time works the lower part of the abs, and you will be surprised how it can make your abs burn. It's a great core stabilization exercise for runners, too.
- In an elbow plank, keeping your back flat, slowly tap your left knee to the ground without moving your hips. Lift your left knee back up, returning to the starting position. Repeat the same movement with the right leg.
- Continue alternating knees taps for 30 seconds.
Image Source: POPSUGAR Photography / Kyle Hartman