Think you've tried every kind of plank there is? Guess again! This creative core exercise from Kristin Russo, an ACE certified personal trainer in New York City, works both your transverse abdominis (the deepest core muscles) and obliques (the largest and outermost muscles).
Start in the top of a pushup position with your core braced. Your body should form a straight line from your head to your ankles. Lift your right foot off the floor and slowly raise your knee as close to your right elbow as you can. Keeping your abs braced, bring your right knee down toward your right wrist (use your abs to prevent your knee and foot from touching the floor). Next, use your abdominals to pull your knee up toward your right armpit. Contract the whole core to elevate the knee to this position, slightly rounding your upper back (rather than just moving the knee up and down). Take the knee back down to your wrist and then back up toward the armpit quickly (like an elevator). Do 8-10 reps and then repeat with the left leg.