Here's how to do it: Lie facedown with your hands under your shoulders, elbows bent, right leg at 90 degrees with your inner thigh on the floor, and your left leg extended straight back on the floor. Push up until your arms are straight, bend your left leg until kneeling (keep your foot off the floor) and extend your right leg straight back. Lower down, pulling your right knee back into a 90-degree angle. Return to start. Do 30 to 40 reps, then repeat on the other side.
Trainer tip: Keep your gaze down so that your neck stays in a neutral position.
Try this move: Army Crawl Leg Extension