Start in a plank position with your hands directly under your shoulders and your feet in line with your hips. Lift your left leg off the ground so your heel is even with your pelvis.
Keeping your torso steady, reach your right arm forward. Really brace through your abs by pulling your navel to your spine. Hold this position for about three seconds. It's harder than you think!
Return to plank, then switch sides, lifting your right leg off the ground and reaching your left arm forward. Hold for three seconds and return to plank. This is one rep.