The transversus abdominus — or, more accurately, the transversus abdominis — is a large flat muscle lying deep within the muscle layers of the abdomen and extending from its attachments at the pelvis up into the lower rib area. It is a paired muscle, meaning that it has a mirror-image duplicate on the opposite site of the body. It is one of four sets of major core, or trunk, muscles in the chest and abdomen that provide fundamental structural support for the body and protection for vital internal organs. Although many people spell the name of this muscle "transversus abdominus," this is incorrect; the correct spelling of the second term is "abdominis."
The transversus abdominus is also called the transversalis, or transverse abdominal muscle. It crosses the abdominal wall horizontally beneath the oblique muscles and wraps around to the lower back. It lies so deeply beneath the other abdominal muscles that it is unable to be touched or felt. This muscle helps stabilize the center of the body during lateral flexion movements.
In addition to providing trunk stability, the transversus abdominus is vitally important to the breathing process. It is not connected to the ribs, but it provides the main support for the diaphragm, allowing it to lift the ribs in order for them to freely expand and contract with breathing movements. Other core muscles — the external obliques, the internal obliques, and the rectus abdominis — also work together to sustain the breathing process.
Trunk muscles are also called torso muscles, which include the pectorals in the chest and the laterals within the abdomen. However, these muscles are not part of the core muscles, since their main function is to move the upper limbs, rather than the trunk. They also do not maintain essential central body support along the abdominal wall, or aid in breathing.
The transversus abdominus is similar to the cardiac and smooth types of muscles in that it can move involuntarily, without conscious control. This muscle moves involuntarily during forceful bodily reactions, such as coughing or sneezing. It is under voluntary, or conscious, control as well, however. In addition, the transversus abdominus is basically a type of skeletal muscle, which is the most predominant muscle type in the body. It is defined as a skeletal muscle because it has attachments to bone, is largely voluntary, and has striations — bands of light and dark cells found in skeletal muscles.
The core abdominal muscles have become closely identified in recent years with weight loss and physical fitness programs. These programs commonly target the abdominals, or “abs,” as being a key element in improving body conditioning and overall fitness. Toward this end, a wide variety of methods have been utilized for targeting specific core muscles in abdominal strengthening programs, including machine-assisted exercises, floor crunches, and ball crunches. Exercises that focus specifically on the transversus abdominus include leg-lifting, bellybutton tensing, and traditional situps.