About 80% of Americans suffer from back pain at some point in their lives. Some have obvious medical problems (such as a herniated disc), but most get a clean bill of health from their doctors -- yet their backs still hurt.

Back pain isn't always caused by a specific injury or activity. It can be due to the ways we use our bodies over time. People who sit for long periods, for example, often have pain because sitting puts more pressure on the back than standing.

Regular exercise helps prevent pain -- but generalized workouts aren't sufficient. Back patients do better with functional exercise -- workouts that mimic the ways we move in real life. To prevent back pain, perform the following exercises and stretches three times a week. They take about 15 minutes to do.

- Trunk Flexion Stretch
- Hook Lying
- Upright Rotation
- Quadruped

Also important: Get at least 20 minutes of aerobic exercise most days of the week to improve circulation to weak and/or sore muscles and to trigger the release of pain-relieving endorphins. At health clubs, the elliptical machine -- which is a cross between a stair climber and a cross-country ski machine -- is ideal for aerobic exercise because it doesn't stress the lower back. Also good are bicycling, walking and tai chi.

Trunk Flexion Stretch

Most people who have back pain need to elongate and strengthen muscles in the lower back and hips. This stretch hits all of the lower back muscles. It also relieves pressure on the sacroiliac joint, which connects the triangular bone at the base of the spine (the sacrum) with the pelvis (iliac crest).

How to do it: Sit on the floor with your knees bent and your heels on the floor. Grip the tops of your shoes with both hands. Keeping your knees pointing toward the ceiling, pull your upper body forward toward your feet. Bend your elbows as you pull forward, keeping your knees outside your shoulders.

Tuck your head down so that the top of your head is aimed at the floor.

Flex your spine as far forward as you comfortably can. Hold for a count of three. Repeat five times.

Hook Lying

People who have weak backs and pelvic floor muscles tend to walk with the pelvis thrust slightly forward. This produces a slight sway in the lower back that increases muscular stress and the risk of injury. This exercise targets muscles in the abdomen, back and pelvic floor and improves posture.

How to do it: Lie on your back with your knees bent and your feet flat on the floor. Reach your arms straight out behind you, flat on the floor, palms up. Tighten your stomach muscles so that there's as little space as possible between your spine and the floor. This move is called a pelvic tilt.

Keeping your arms straight, lift your head, neck and shoulders off the floor, reaching forward with your hands until they touch your knees. While in this position, aim to complete 10 "pulses" (tightening and releasing the stomach muscles), maintaining the pelvic tilt with the small of the back remaining on the floor. Then return to the resting position. Increase the number of repetitions as you get stronger.

Upright Rotation

Strengthening the core muscles in the abdomen and lower back is the best way to reduce or prevent back pain. Trainers often recommend lying crunches for strengthening the core.

Better: A standing rotational exercise that involves more than one plane of movement and mimics the ways we move in real life.

How to do it: Stand with your legs hip-width apart and your arms extended about shoulder-width apart. Hold a rope, towel or resistance band tautly between your hands. Tighten your stomach muscles and rotate your body to the right as far around as you comfortably can. As you move, lift the heel of your left foot and piŽvot on the toes. Keep your right foot stable.

Repeat the movement in the same direction 10 times. Then repeat the movement going in the opposite direction.

Quadruped

This therapeutic movement is especially good for people with chronic back pain. It stretches while strengthening muscles that stabilize the spine.

How to do it: Get on your hands and knees, with your arms directly under your shoulders and your feet hip-width apart. Lift and straighten your right arm and left leg simultaneously, keeping shoulders down. Look straight down at the ground while doing this, to prevent neck strain.

Stretch the arm and leg as far as you comfortably can. Hold for five seconds. Repeat 10 times.
Reverse the movement by lifting and straightening your left arm and your right leg.