Swimming Anatomy

The rectus abdominis, popularly known as the six pack, attaches superiorly to the sternum and the surrounding cartilage of ribs 5 through 7. The fibers then run vertically to attach to the middle of the pelvis at the pubic symphysis and pubic crest. The six-pack appearance results because the muscle is divided by and encased in a sheath of tissue called a fascia. The visible line running along the midline of the body dividing the muscle in two halves is known as the linea alba. Contraction of the upper fibers of the rectus abdominis curls the upper trunk downward, whereas contraction of the lower fibers pulls the pelvis upward toward the chest. Combined contraction of both the upper and lower fibers rolls the trunk into a ball.

The muscles of the two lateral groups are arranged into three layers. The external oblique forms the most superficial layer. From its attachment on the external surface of ribs 5 through 12, the fibers run obliquely (diagonally) to attach at the midline of the body along the linea alba and pelvis. If you were to think of your fingers as the fibers of this muscle, the fibers would run in the same direction as your fingers do when you put your hand into the front pocket of a pair of pants. Unilateral (single-sided) contraction of the muscle results in trunk rotation to the opposite side, meaning that contraction of the right external oblique rotates the trunk to the left. Bilateral contraction results in trunk flexion.

The next layer is formed by the internal oblique. The orientation of its fibers is perpendicular to those of the external oblique. This muscle originates from the upper part of the pelvis and from a structure known as the thoracolumbar fascia, which is a broad band of dense connective tissue that attaches to the spine in the upper- and lower-back region. From its posterior attachment, the internal oblique wraps around to the front of the abdomen, inserting at the linea alba and pubis. Unilateral contraction rotates the trunk to the same side, and bilateral contraction leads to trunk flexion. The deepest of the three layers is formed by the transversus abdominis, so named because the muscle fibers run transversely (horizontally) across the abdomen. The transversus abdominis arises from the internal surface of the cartilage of ribs 5 through 12, the upper part of pelvis, and the thoracolumbar fascia. The muscle joins with the internal oblique to attach along the midline of the body at the linea alba and pubis. Contraction of the transversus abdominis does not result in significant trunk motion, but it does join the other muscles of the lateral group to function as a core stabilizer. An analogy that often helps people grasp the core-stabilizing function of the muscles of the lateral group is to think of them as a corset that, when tightened, holds the core in a stabilized position.

Note that other muscles, including the serratus anterior and hip flexors, can be recruited along with the abdominal muscles when many of the exercises in this chapter are performed. The serratus anterior commonly functions as a stabilizer of the scapula, as described in chapter 3, but it is also activated during many of the exercises that target the external and internal obliques. The two primary hip flexors are the rectus femoris and the iliopsoas. As described in chapter 7, these muscles can either flex the hip or flex the lower trunk, depending on whether the lower extremity or trunk is stabilized.

The role of the core abdominal muscles in swimming can be easily broken down according to their roles as trunk flexors, trunk rotators, and trunk stabilizers. Through their ability to flex the trunk, the rectus abdominis, external oblique, and internal oblique all play important roles in the movements that take place during swimming. For example, flexion of the trunk during flip turns is initiated by the upper fibers of the rectus abdominis, sustained by the lower fibers of the rectus abdominis, and helped to completion by both obliques. The trunk flexors are also important contributors to the wavelike, or undulating, body roll that takes place during butterfly, breaststroke, and underwater dolphin kicking. Besides contributing to flexion of the trunk, the obliques are responsible for trunk rotational movements. Strong obliques are vital to enhancing the speed of open turns performed during butterfly and breaststroke. The obliques are active during the body-rolling movements that take place during freestyle and backstroke, functioning to link the movements of the arms with the movements of the hips and legs. As previously mentioned, through their ability to function like a corset, the abdominal muscles are central to stabilizing the trunk. Trunk stability is one of the keys to efficient movement through the water, because it ensures a firm base of support upon which the arms and legs can generate their propulsive forces.