Lie on your back, hands under your lower back, palms down. Bend one leg and place that foot flat on the floor or bed; keep your other leg extended. Gently tighten your core muscles. Keeping your elbows on the mattress, slowly elevate your head and shoulders a few inches. Don’t tuck your chin—your head, neck and upper back should stay aligned. Hold for a beat, then lower yourself back down. Perform 15 repetitions, then switch legs and do another set of 15.