Get onto all fours. Make sure your knees are under the hips and wrists are under the shoulders. Tighten your abdominal muscles, then, without changing your lower-back position, raise your right arm and left leg until you have a straight line from fingertips to toes. Keep the neck in line with the rest of the spine. Hold for a few seconds, then return to the starting position. You can repeat on the same side or alternate. Try not to use momentum, but squeeze your glut muscles.