Start on your hands and knees, with your wrists under your shoulders and your knees under your hips. Reach your right arm forward and your left leg back, so that you form a long, straight line from your fingers to your toes. Keep your eyes on the floor and your hips level, as though a glass of water were sitting on your lower back. Hold for a moment, return, and switch sides. Do eight to ten reps per side. If it’s too tough, try lifting only one arm or one leg.