Several sets of muscles support and propel the torso. The abdominal wall muscles help transfer force between the upper and lower body, and they also protect the delicate internal organs. Their most important function is to support the back. The muscles of the torso extend in several directions. They help maintain posture and aid the spinal muscles when bending, twisting, and other movements.
The transversus abdominis is a broad, flat muscle of the abdomen that is named after the direction of its fibers. The muscle lies just below the internal oblique and spans the area from the pelvis to the six lower ribs and the lumbar region of the spine, where it connects to the lumbar fascia. This muscle assists in breathing.
The internal oblique is a small, thin, deep muscle of the abdomen. It runs diagonally opposite underneath the external obliques. It has a quadrilateral form which extends from the hip bone (crest of the ilium) to the cartilage of the lower ribs (the seventh, eighth, and ninth ribs). This muscle protects a weak point in the abdominal wall and works with the external oblique to help twist the torso.
The external oblique is a large, thin sheet of overlapping muscle that runs along the side of the torso and partly on the front from the lower ribs to the rectus, the pubis bone, and the iliac crest of the hip. The muscle is divided into two portions; and upper thoracic portion and a lower flank portion. The thoracic portion is located along the rib cage. The individual ribs can be seen beneath this muscle when it is relaxed. The lower flank portion is located along the side of the abdomen between the rib cage and the pelvis. Most of this muscle is concealed by fat. The two portions meet at the waist. This muscle assists the rectus abdominis muscle in flexing (bending) the spine when the trunk twists or turns. It also supports the abdominal organ tissue.
The rectus abdominis is a long, flat, vertical muscle located along the front of the abdomen. It is wide and thin at its point of origin along the crest of the pubic bone (pelvis) and tapers below to insert in the cartilage of the fifth, sixth, and seventh ribs. This muscle is used when you bend forward. It also tenses the abdominal wall and aids in compressing the contents of the abdomen. The muscle is enclosed in a sheath of fascia that holds it in position, but does not restrict its movement during its contractions.
The pyramidalis is a small, triangular muscle located towards the lower
part of the abdomen. It is located just in front of the rectus abdominis
muscle.