Kneel on all fours, knees under your hips, wrists directly under your shoulders, fingers facing forward. Engage your abdominal muscles to avoid arching your back. Simultaneously raise your left leg and right arm, extending both until they are exactly, or nearly, parallel to the floor. Avoid tilting your hips or shoulders, and keep your head aligned with your torso. Gently return to the starting position, and repeat with the opposite limbs. Do 5 reps on each side, building up to 10.