Transversus abdominis muscle

# Overview
The transversus abdominis muscle, also known as the transversalis muscle and transverse abdominal muscle, is a muscle layer of the anterior and lateral abdominal wall which is just deep to (layered below) the internal oblique muscle. It is a major muscle of the functional core of the human body.

# Structure
The transversus abdominis, so called for the direction of its fibers, is the innermost of the flat muscles of the abdomen, being placed immediately beneath the internal oblique muscle.

## Origin
It arises, as fleshy fibers, from the lateral third of the inguinal ligament, from the anterior three-fourths of the inner lip of the iliac crest, from the inner surfaces of the cartilages of the lower six ribs, interdigitating with the diaphragm, and from the lumbodorsal fascia.

## Insertion
The muscle ends in front in a broad aponeurosis, the lower fibers of which curve downward and medialward, and are inserted, together with those of the internal oblique muscle, into the crest of the pubis and pectineal line, forming the inguinal aponeurotic falx. In layperson's terminology, the muscle ends in the middle line of a person's abdomen.
Throughout the rest of its extent the aponeurosis passes horizontally to the middle line, and is inserted into the linea alba; its upper three-fourths lie behind the rectus muscle and blend with the posterior lamella of the aponeurosis of the internal oblique; its lower fourth is in front of the rectus abdominis.

## Layperson's description
- The transvers abs run from our sides (lateral) to the front (medial), its fibers running horizontally (transverse).
- The lateral beginnings of the muscle (origin) run from the front of the inside part of the hip bone (anterior iliac crest and inguinal ligament) to the last rib of the rib cage.  It also is connected to the diaphragm which helps with inhalation.
- The muscle runs transverse and is the deepest of the major abdominal muscles (the others being the rectus abdominis, and the internal and external obliques).
- It ends (the muscle insertion) by joining with the large vertical abdominal muscle in the middle (the linea alba), where the fibers begin to curve downward and upward depending on what direction it has to go to meet the linea alba, and below the sternum it combines with next most superficial muscle (the internal oblique).  This insertion runs down by the belly button where it passes over the thick abdomen muscle (the "6/8-pack") and all the ab muscle fibers join together.

## Innervation
The transversus abdominis is innervated by the lower intercostal nerves, as well as the iliohypogastric nerve and the ilioinguinal nerve.

## Actions
The transversus abdominis (TVA) helps to compress the ribs and viscera, providing thoracic and pelvic stability. This is explained further here.

# Variations
It may be more or less fused with the Obliquus internus or absent. The spermatic cord may pierce its lower border. Slender muscle slips from the ileopectineal line to transversalis fascia, the aponeurosis of the Transversus abdominis or the outer end of the linea semicircularis and other slender slips are occasionally found.

# The muscle in movement and training
The most well known and effective method of strengthening it is the vacuum exercise, as well as engaging it during lifts. The transversus is the body's natural weight-lifting belt, stabilizing the spine and pelvis during lifting movements. Failure to engage the muscle at higher intensity lifts is dangerous and encourages injury. It acts as a girdle or corset in creating hoop tension around the midsection, tensing before contraction of the extremities.
Without a stable spine, the nervous system fails to recruit the muscles in extremities efficiently, and functional movements cannot be properly performed. Stabilization must then occur at the segmental level. The transversus abdominis and the segmental stabilizers of the spine are designed to work in tandem. This kind of lifting eventually overloads segmental stabilizers, and can result in massive lower back pain, early degeneration and many orthopedic problems.

# Additional images
- Diagram of a transverse section of the posterior abdominal wall, to show the disposition of the lumbodorsal fascia.
- Posterior surface of sternum and costal cartilages, showing Transversus thoracis.
- The interfoveolar ligament, seen from in front.
- Diagram of sheath of Rectus.
- Diagram of a transverse section through the anterior abdomina wall, below the linea semicircularis.
- The abdominal inguinal ring.
- The abdominal aorta and its branches.