Work your abs from every angle. For optimum back stability, you not only need a strong rectus abdominis (the ab muscle that runs down your front) but also strong obliques (the side muscles) and transverse abs (the deep lateral abdominal muscles that work with the obliques). To strengthen all these muscles, try diagonal crunches with an isometric contraction. Lie back with your knees bent and your feet flat. Clasp your hands behind your head. Raise your head and shoulders together, rotating to one side as you lift off the floor. Hold for 5 to 10 seconds. Lower, and repeat to the other side. Work up to 10 repetitions on each side.

Exercise the extensors. Strong abs do little good if your back is weak. Work your lumbar (lower back) muscles, as well as the erector muscles that run along your spine, with a back extension exercise. Carefully bend forward over an exercise ball (found at most major sporting goods stores), and place your hands on the floor with your stomach and hips on top of the ball. Your feet should stay on the floor. Lift your left arm, chest, and right leg off the floor until the arm and leg are in line with your back. Hold, return to start, and repeat with the opposite limbs. For an easier exercise, do it without the ball first. Do each side 10 times.

Keep your curves. Have a history of disk problems? Skip the traditional advice to tilt your pelvis and flatten your back during ab exercises. “This actually puts more pressure on disks and increases the chance for injury,” says Liemohn. Instead, place your hands, palms down, beneath the curve of your back. Perform crunches from that position.   Support your spine. You need muscular endurance so your back lasts longer during activity without fatiguing. This side-bridge exercise boosts endurance in spine-supporting muscles. Lie on your side with your knees bent. Bend your lower arm, and lift your torso, hips, and thighs off the floor, so only your forearm and lower legs touch the floor. Keep your spine straight. Hold for 5 to 15 seconds. Lower, and repeat on the other side. Work up to holding each side for 30 to 60 seconds.   Stretch your hips. Hip flexibility may be even more important than back flexibility for preventing back injury. Try this stretch after you exercise. Take a giant step forward with your left leg until that knee is directly above that ankle. Drop your right knee toward the floor, and lower your hips toward the floor. Hold for 10 to 15 seconds. Repeat with the opposite leg.   Go easy in the a.m. During the night, your disks fill with fluid, making your spine tight and stiff in the early morning hours and increasing your risk for injury. If you must exercise in the morning, do a longer warm-up—about 15 minutes—to allow some of that fluid to disperse. Or exercise later in the day when your back is loosened up.   More from Prevention: A Flatter Belly At Any Age

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