Back Extension.

(If you have lower-back pain, consult your doctor before doing this exercise.) Lie on your belly, keeping your hips and pelvis flat. With your hands under your chin, contract your lower back muscles, and lift your chest about 30 degrees, or 3 to 5 inches off the floor. Hold, then slowly lower. (For an easier version, rest your arms by your sides, palms facing up. For a more challenging version, extend your arms straight overhead.)