"A lot of people think 'core muscles' refers to their abs," says John Jay Wooldridge, a Reebok Master Fitness Trainer, "but it includes all the muscles of the trunk that help with stabilizing and moving the spine and pelvis."

Having strong core muscles, also called trunk muscles or the "inner unit," that work together properly is important for all movement, from sports to basic functions of daily life.

"A strong inner unit is a prerequisite for prevention of injury, enhanced sports performance, improved balance, and proper muscular development," says Clare Dunphy, a Pilates instructor and personal trainer with Progressive Bodyworks.

The Core Muscles

Core muscles include "anything that has to do with one of the five major postures: standing, sitting, lying on the back, lying on the stomach, or quadriped (on hands and knees)," explains Wooldridge. This means that in addition to abdominal muscles, some consider the core muscles include all the muscles of the lower back and shoulders, the internal and external obliques, pelvic muscles, gluteal muscles, and hamstrings. Deep core muscles, such as the multifidi and pelvic floor muscles, are endurance type muscles that work constantly to stabilize posture. Superficial core muscles, such as rectus abdominus (abdominal muscle), are more powerful and are typically involved in producing forceful motion.

Therefore, to maximize core strength, working one or two isolated groups of muscles isn't enough. "You need to focus on integrated training that works several muscle groups together," advises Wooldridge.

Avoiding Injury

While no one is ever completely safe from injury, strong core muscles are thought to go a long way toward injury prevention in both sports and routine activities like carrying groceries or picking up a child. Weakness of core muscles also may be related to the development of chronic back pain, one of the most common medical complaints.

Strengthening—Back injuries often result from weak trunk muscles, "but by strengthening the core, your spine will be more stable and you'll have less risk of herniation or a bulging disc," says Dale Huff, RD, CSCS (certified strength and conditioning specialist). Evidence supports that core strengthening can benefit in decreasing back pain.

Balancing—Sports that involve repetitive motions that stress the two sides of your body differently—like golf or tennis—can lead to imbalances. Since core training works all of your major muscle groups, such imbalances can be minimized.

Coordinating—Strong core muscles help your extremities to work in conjunction with the rest of your body. So when you're teeing off or serving, muscles throughout your body contribute to the effort and cushion the strain on your joints, decreasing your risk of tennis elbow or a rotator cuff injury.

Sport-specific Training

Core strength plays a role in many sports:

Baseball and softball—The best throw starts with your legs. According to Huff, strong core muscles help transfer energy from your legs through your trunk and arms and into the throw.

Basketball—A strong core provides stability and balance to help endure the repetitive stop and go motions of basketball and improve the ability to do quick direction changes.

Golf—Strong core muscles help correct the imbalance caused by using only one side of your body. The golf swing also relies on the ability to rotate the spine and pelvis in an integrated fashion.

Hockey—On the ice (or on the field), hockey players spend most of their time bent at the waist and leaning over their sticks, which can strain the back. "Strong trunk muscles help stabilize the lower back," Wooldridge says.

Racquet sports—In all racquet sports—tennis, squash, and racquetball—there's "a lot of flexion, extension, and rotation" of the trunk, explains Wooldridge. These "multi-planar" activities rely heavily on core muscle strength.

Running—A strong core helps runners maintain good posture and balance, and avoid injury from the slight but repetitive rotation of the spine that occurs with every stride.

Volleyball—Whether arching for a serve or lunging for a dig, volleyball players are constantly extending and flexing their spines. Strong core muscles provide a "fuller range of motion" for sports like this, Huff says.

All activities—The more efficiently your body works the more energy you have to spend on other activities. If your core is strong you will spend less energy working on things like balance and stability, meaning you have more energy to poor into your sport.

Get Strong

There are several ways to strengthen your core muscles. Talk to a certified personal trainer or instructor to see which is best for you, and to make sure you're doing it right. Work on strengthening core muscles three times per week for at least 15 minutes per session (it may take longer) if you want to notice a real difference, suggests Angelica Tinio, an ACE-certified fitness instructor. "Focus on the quality of the exercises rather than the quantity of repetitions," she adds.

Physio balls—"When you do a crunch or oblique twist on the ball rather than the floor, you're using many more muscles to stabilize your body," notes Wooldridge. In fact, Huff says, you get two times the abdominal activity and four times the oblique activity that you would on the floor.

Push-ups—This basic exercise works more than your arms; holding the push-up position forces your shoulders, abdominals, and lower back muscles to work together.

The "swimmer"—Lie on your stomach and lift one arm and the opposite leg. Hold for 10 seconds; do 8-10 repetitions. Repeat with the other arm and leg.

Pilates—A series of exercises designed to strengthen the body from the inside out, Pilates is done on machines and mats and should be taught by a well-trained instructor. "Pilates really teaches your muscles to work together," Dunphy says.