The abdominal muscles consist of six muscle groups that extend from the ribs to the pelvis. These muscles include: the transverse abdominals; a pair of internal obliques; a pair of external obliques; and, the
rectus abdominis. These muscles provide for upper body movement and support the trunk. Strong abdominal muscles also help provide good posture. In the exercise world, these muscles are often called "the core" because of their importance in providing overall body support.
The transverse abdominals are the deepest abdominal muscles. They provide support for the trunk of the body. They also work in conjunction with the deep back muscles, the multifidus, to provide spine stability and protect against back injury or strain during movement or normal posture.
The internal and external obliques are responsible for twisting and rotation of the trunk. The internal oblique and opposite external oblique work in conjuction with each other to cause body rotation. The left internal oblique and right external oblique contract to rotate the trunk to the left. The right internal oblique and left external oblique contract to rotate the trunk to the right.
The rectus abdominis muscles are located on either side of the abdominal midline. The are often called the "sixpack abs" because of their external appearance on the stomach or abdomen.