Oblique Muscles

In a recent post, I discussed the structure and function of the Rectus Abdominus muscle which is one of the four groups of muscles that makeup the abdominal group. These muscles are arranged in four layers from superficial to deep, with the Rectus Abdominus group being the superficial of the group.
The next two layers of the abdominal group are called the Obliques secondary to the diagonal orientation of their muscle fibers. They consist of the following:  

External Oblique (Second Layer)
•    The muscle fibers of the external oblique are oriented to run diagonally on the side from posterior (back) on the rib cage to anterior (front) on the pelvis.
Internal Oblique (Third Layer) 
•    The muscle fibers of the internal oblique are oriented to run diagonally on the side from anterior on the rib cage to posterior on the pelvis.

Both muscles function to provide spinal stabilization and trunk movement. Due to the unique orientation of their fibers and their overlapping locations to one another (think of the design of the belts of a radial tire) they provide the following trunk movements.
o    When all contract simultaneously on both sides they provide stabilization to the trunk by neutralization of trunk rotation and allow the rectus abdominus to function in   moving the pelvis toward the bottom of the rib cage.
o    If one external on one side contracts with the internal on the opposite side then rotation to the side of the internal oblique occurs
o    If both the external and internal oblique on the same side contract, then trunk side bending to that side occurs.

As a valued segment of the core musculature their main function is to stabilize the pelvis and thus prevent excessive torque on the lumbar spine.