Start: Start on all fours, with your knees and toes on the floor and your palms facedown in front of you.
Finish: Pull in your abs, then straighten one arm and the opposite leg, extending both limbs parallel to the floor while keeping your torso and hips in a straight line. Hold for 1 to 5 seconds, then lower your working limbs and repeat with the opposite arm and leg. That's 1 rep.