The different muscles that comprise the abdominal muscles are activated (recruited) for various – but specific activities that are unique to each group. No one particular activity has been conclusively isolated as having an effect on ALL of the abdominal muscles as a group – in any particular sequence – throughout the performance of that activity. It therefore appears that understanding the various activities that recruit the different abdominal muscle groups is more beneficial than the sequence of recruitment of the abdominal muscles.
The external oblique (EO) muscles are the outermost fibers of the trunk, and are located on each side of the rectus abdominis (RA). Beneath the EO muscles, running at approximately right angles to them, are the internal oblique (IO) muscles which form an inverted “V” shape. Both the external and internal oblique muscles are recruited for activities such as spinal flexion, side bending and trunk rotation. Due to the muscle fiber arrangement, the EOs are referred to as “opposite side” rotators i.e. rotation to the left is facilitated by the right EO fibers. On the other hand, the IO muscles are “same side” rotators. Thus, rotation to the left is facilitated by the fibers of the left IO muscles. The lower fibers of the IO muscles run horizontally and are thus also strong trunk rotators. Their fan-like fibers extend all the way around the sides to the lower back in much the same way a corset fits a person.
The rectus abdominis flexes the spinal column and assists in lateral trunk flexion (side bending). The RA is also actively involved in stabilizing the trunk when the head is raised while lying in supine.
The transversus abdominis (TA) is the deepest layer of abdominal muscles and is not involved in movements of the trunk. Instead, this respiratory muscle plays an important function in forceful expiration of air from the lungs as well as compression of the internal organs (as seen in abdominal hollowing).
Electromyographic (EMG) analysis of the abdominal muscles carried out by Sheffield & Major in 1962 demonstrated that the abdominal muscles are quite inactive during walking on a flat surface. Other similar studies show that the RA and EO muscles exhibit little electrical activity in an erect posture unless the trunk is inclined backward or laterally and/or resistance is applied to the torso. The IOs on the other hand, are in a state of tension while sitting, standing and walking, to help stabilize the pelvis.
For illustrations of the various abdominal muscle groups, their origins and insertions, their actions and the results of various studies carried out on them, kindly visit Reference 1 below.
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