PLANKS. Planks engage the abdominal muscles and back muscles, stabilizing the spinal column and mimicking proper lumbar alignment. To do them right and protect your back: Start on your forearms to avoid shoulder strain. Keep your abs tight the entire time. Do not let your back sag. Square your hips with the floor. Have someone take a picture or set up a timed photo/video so you can check whether your back is straight. This is a better way to confirm your posture than using a mirror, because the mirror requires you to twist your neck in the middle of the pose.