Yes, a toned core is great for bikini season. But, when you learn to access the deepest muscles, you will get benefits that are more than skin deep.

Katrine Naleid

It's summer—and chances are good that people all over America want to tone up their tummies, and they're adding crunches to their routines to do it. But sit-ups alone won't make a potbelly disappear. In fact, they just might have the reverse effect!

To achieve a toned and healthy midsection, you need to work with a number of muscles that are commonly called the "core." Your core includes more than just abdominal muscles. Most exercises that target your abdominals can actually tighten the muscles around your tummy in such a way that they prevent abdominal tone and, if done without proper awareness, can push your belly out and even hurt your back.

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It's often the psoas that will cause you to lose the battle of the bulge. A key muscle at the very center of your core, the psoas is all too often overworked in ab exercises. You'll benefit most from your core work if you learn the actions that not only tone your tummy, but also tame a tight psoas.

The Deep-Down Muscle

The psoas is the deepest and one of the largest muscles in the body. On each side of your lumbar spine, it attaches to the vertebrae and stretches over the hip joint—like the strings of a violin stretching from the neck over the bridge—to attach at your femur (inner thigh). You use the psoas when you walk: It initiates every step you take by exerting a powerful pull on your leg at your inner thigh. It also plays a critical role in forward bends, working in tandem with your abdominals to flex your spine.

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Especially important, the psoas provides structural support for the curves of your spine. In fact, it runs so deep that when you're lying down, your abdominal organs literally sit on top of it, which is why the psoas can have a profound effect on the appearance of your abdomen. Thanks to the way the psoas contracts to flex your legs toward your spine, it's almost impossible to avoid tightening the psoas in any abdominal exercise. And this can be a problem unless you actively incorporate poses and techniques that release and lengthen your psoas.

See also YOGA SHRED: Go With the Flow + Power Up Your Psoas

Rethink Your Core

Surrounding the psoas you'll find what is most commonly referred to as the core—three layers of muscle that provide much-needed control and support for the movements of your spine.

First, the outermost layer consists of the abdominals, which, in addition to moving the torso into forward bends, are also involved in twists. The rectus abdominis is the most visible member. It gives you that six-pack look and is emphasized in many popular abdominal routines. The rectus abdominis does make your belly look trimmer by providing support for your abdominal organs, and its active function is to bend the spine forward. It's worked strongly in poses such as Ardha Navasana (Half Boat Pose) and arm balances such as Bakasana (Crane Pose).

The other members of the outermost layer are the internal and external obliques. These start at the side and front body at the ribs and sternum and wrap around the front torso to your pelvis. Their primary function is to twist your torso as well as to bend it sideways. They join with the rectus abdominis to add power to your forward bending. The obliques have a protective function in twisting: They ensure that the spine twists evenly, so that the vertebrae do not turn too strongly in any one place and injure an intervertebral disk. You will find them at work in poses such as Marichyasana III and Ardha Matsyendrasana (Half Lord of the Fishes Pose). The obliques also get a workout and a stretch in lateral sidebending poses such as Utthita Trikonasana (Extended Triangle Pose).

The second, or middle, layer plays quite a different role. It supports your spine by bracing it, especially when you're picking up something heavy. This layer is really a system of muscles whose prime member is the transverse abdominis. These muscles wrap around your torso—from back near your lumbar spine around to the front—covering your entire abdomen from sternum to pubic bone. They're often described as a muscular corset.

The transverse abdominis works in combination with the diaphragm and pelvic-floor muscles to pressurize your torso, protecting your spine from stressful loads. To feel this system at work, take a small breath in and then hold it; tense the abdominals as though you were going to lift something heavy, and firm your pelvic floor (as though you were trying to "hold it" on the way to the bathroom).

These actions firm the entire torso, supporting your lumbar spine in particular. They're at work whether you're a weightlifter who grunts during a heavy lift, or a yogi who uses the Ujjayi Breath and the bandhas to steady your core for a challenging pose. Your transverse abdominis works strongly in poses such as Plank Pose and Chaturanga Dandasana (Four-Limbed Staff Pose).

Finally, though you might not think of them as core muscles, the tiny muscles that fine-tune the movements of your vertebrae make up the deepest layer. The powerful river of the psoas flows right alongside these muscles.

The Power of the Psoas

If the psoas is like a river flowing through the core, the transverse abdominis forms the sturdy riverbanks. The support of the transverse abdominis strengthens the action of your psoas. When the torso is held steady by this corset of muscle, the pull of the psoas acts powerfully on the leg and hip. But if the transverse abdominis is weak (if the torso is not pressurized and held steady by the transverse abdominis), the psoas will pull your lumbar spine out of alignment and into an exaggerated concave curve—when you're walking, doing your abdominal exercises, or even just standing or sitting—as it drags the front of your vertebrae toward your hip.

Therein lies the danger of many core workouts: If your transverse abdominis is weak, your psoas will pull too strongly on your spine. A prime example of the danger comes from doing exercises such as leg lifts. The transverse abdominis should do the job of holding your spine steady while your psoas and thigh muscles lift and lower your legs. Your effort to maintain a neutral curve in your spine throughout the exercise is what gives your abdominals a workout. Your transverse abdominis tightens to prevent your psoas from pulling your back into an exaggerated arch as you lift and lower your legs.

But the exercise essentially pits your core muscles and psoas against each other. The problem is that your core is most often no match for the combined power of your psoas and gravity. The end result is that tremendous pressure is placed on your lumbar spine, which causes your low back to overarch and can lead to low-back pain or even to injury.

The physiotherapist Leon Chaitow, an osteopath and senior lecturer at the University of Westminster in London, points out that when you practice a sit-up (lifting your entire torso all the way up and off the floor), the pull of the psoas compresses the disk between the vertebrae known as L5 and S1 (the place where your lumbar spine meets your sacrum) with the force of about 100 kilograms—or 220 pounds! That's quite a squeeze to put on your low back for the sake of a flat belly. The pressure can wreak havoc on the health of your low back, bringing stiffness, muscle spasms, and even the risk of damage to the lumbar disks.

The effect of overworking your psoas is also postural. A tight psoas will tilt the bowl of your pelvis forward, spilling the contents of your belly over your waistline. The result? A potbelly!

The good news is that it is possible to work your core without stressing your psoas. And it doesn't necessarily take more abdominal work, but rather smarter work. You can learn to tame a tight psoas and support a posture that is supple and tension free while developing core strength. It starts with learning two key techniques that you can use in your daily yoga practice or in any abdominal routine.

See also Yoga For Beginners: Build a Strong Core with Plank Pose

The Drawstring and the Zip-Up

By locating the action of the transverse abdominis in the following exercise, you can experience the support it gives to your spine and the release it provides your psoas, and apply that understanding to any core work you do.

Lie on your back with your legs straight, about hip-width apart, your knees and toes pointing toward the ceiling. Place your fingertips on your hip points, the bony protrusions at the front of your hipbones closest to the surface.

With your legs straight and firm, activate both legs at once as though you're trying to lift them off the floor. But don't actually lift your feet off the floor, since that risks hurting your back.

The first thing you're likely to feel, apart from the effort in your thighs, is a firming of your abdomen in the space between your hip points. That firming is the result of your transverse abdominis engaging to provide support as your psoas works to lift your legs. Engaging the transverse abdominis in this way is like tightening the drawstring on a pair of sweatpants: It narrows your waist, pulling your hip points slightly toward each other. You'll also notice that your sitting bones move back and apart, the arch in your lower back increases slightly, and your thighs effortlessly spiral inward.

To give your transverse abdominis the support it needs, you'll now learn to engage the rectus abdominis and control the tilt of your pelvis. The rectus abdominis regulates the tilt of the pelvis through its attachment at the pubic bone. To engage it, simply draw your belly below your navel slightly back toward your spine and up toward your heart, as though you were zipping up a tight pair of pants. You'll feel your tailbone lengthen away from your waistline at the back.

See also A Safe, Core-Supported Backbending Sequence

Conscious Core

These two basic actions—the drawstring and the zip-up—allow you to work crosswise (via the transverse abdominis) and lengthwise (via the rectus abdominis). Their combined power brings full integration to the layers of the abdominals while allowing the power of the psoas to be focused on moving your legs instead of pulling on your lumbar spine.

True core strength is developed through a conscious awareness of these two actions during your exercise or yoga routine. Once you find this balance in your yoga practice, you won't even think about adding sit-ups to your practice to get your belly ready for the beach!

A Core-Toning Sequence for Summer

1. Supta Padangusthasana (Reclining Hand-to-Big-Toe Pose) Supta Padangusthasana will tone your abdominal muscles and lengthen your psoas. Lie on your back with your left leg extended. Use a strap to hold your right foot and then lift your right leg. Before moving your right leg closer to your torso, tilt your pelvis forward slightly to establish the natural inward curve to your low back, using the drawstring action to help: Work your left leg as though you were trying to lift it, firming your transverse abdominis and narrowing the space between your hip points. Feel your left leg spiral inward and the arch in your low back increase slightly, signaling a release in your left psoas. Now use the zipper effect by drawing the pit of your abdomen in and up to engage your rectus abdominis, and press out through the mound of your big toe. If your left leg turns outward and your low back touches the floor here, your psoas is gripping and shortening. Maintain these lengthening actions as you take your right leg closer to your torso. Hold for 10 breaths or longer, remaining firm but not tense in the pose. Release your right leg and repeat on the second side. See also YOGA SHRED: A Sequence to Cool Down and Release Your Psoas 1 / 8

Doug Keller, who studied at the Siddha Yoga ashram in Ganeshpuri, India, for nearly a decade, teaches workshops and trainings worldwide.