According to bodybuilder Karen Sessions, the deep muscles in the abdominal wall, such as the transversus abdominus and lumbar multifidus, form a belt around the vertebrae that supports posture and helps to control breathing. These are the deep abdominal muscles that are difficult to reach and often neglected during workouts. Common exercises like crunches affect the superficial muscles, but only certain exercises will touch the deeply embedded abdominal muscles.
One method to target the deep abdominal muscles is the stomach vacuum, which is an isometric contraction that tenses the muscles without dramatically moving them. Stand upright and place your hands on your hips. Expand your chest, and pull your stomach inward so that it will retreat as far as it can go. This can last for as long as you wish--20, 40 or possibly 60 seconds--and you should spread it out across a few sets. It can be done while standing, kneeling, seated, lying and even during other abdominal workouts. The exercise should get more intense and last longer with time.
Toe tap Pilates is another great deep abdominal exercise. Lay on the floor with your knees bent, feet flat and pelvis in neutral. Raise your right leg until it is at a 90 degree angle at the hip and knee, and then raise the left leg to match it. Lower one leg down and tap the toe on the floor; bring it back up. Do the same motion with the other leg. Inhale and exhale as you alternate legs. It helps to place your fingers under your lower back so that your back maintains the same amount of pressure on your fingers throughout the exercise.
The plank pose is another isometric exercise. Lie face down on the floor. Let your elbows and the tip of your toes support you; both should be spaced about hip length. Straighten your back, pull your stomach in and lift your body up. Hold this position for 30 to 60 seconds, and spread it out across three to five sets. There are also a few variations on the plank pose. While in the air, you can lift one leg up, and for an added challenge you can also twist your trunk and bring your knee toward your hips.