The oblique muscles consist of external oblique muscle and internal oblique muscle. They are a group of muscles of the abdomen (belly) acting together forming a firm wall. This wall supports muscles of the spine and internal organs help maintain an erect posture and protects the organs from getting injured by keeping them in their anatomical (expected) position. Invertebrates, the abdomen is a large body cavity enclosed by layers of the muscular wall at the front and sides. The vertebral column at the back houses and protects major elements of the gut, such as part of the esophagus, stomach, small intestine, large intestine, liver, pancreas, gall bladder, and spleen, as well as parts of the urinary system, such as kidneys and ureters. The contraction of the abdominal muscles helps to breathe out and increases the intra-abdominal pressure, such as during sneezing, coughing, passing stools, urinating, and childbirth.
Basic anatomy of the oblique muscles
Abdominal muscles work together to produce movements of the spine as well as to compress the abdominal viscera. There are two types of oblique muscles: external oblique muscle and internal oblique muscle, with distinctive origin and functions. The oblique muscles make up two of the three muscle layers that surround the abdominal wall with the third layer of transversus abdominis muscle.
External oblique muscle
It is the outermost muscle located on both sides of the trunk. The muscle fibers lie beneath the thoracic (chest) and abdominal skin originating from the lower ribs to the pelvis (hip bone). The muscular part makes up the lateral part of the abdominal wall. The muscle contributes to the variety of trunk movements and is mainly responsible for performing functions, such as twisting the sides of the trunk, pulling the chest downwards, rotating the spine. Any form of strain, injury, or trauma to the muscle can be debilitating.
Functions of the external oblique muscle:
The external abdominal oblique muscle has a variety of functions depending on whether it contracts unilaterally (one side) or bilaterally (both sides). When acting unilaterally and in synergy with the internal oblique muscle, it rotates the trunk to the opposite side. When working together with back muscles, it contributes to bending of the trunk sideways. When contracting on both sides, it leads to the forward flexion (forward bending) of the trunk. This activity also increases the tone of the abdominal wall and positive intra-abdominal pressure, which is part of various physiological processes, such as forced exhalation, defecation (passing stools), micturition (urinating), and labor (childbirth).
Internal oblique muscle
It is a broad, thin muscular sheet found on the sides of the abdomen arranged perpendicular (at right angles) and deep to the external oblique muscle. It has multiple sites of origin and is divided into anterior (front) fibers, posterior (hind) fibers, and lateral (sideways) fibers according to its origin sites. This muscle is an opposing force to the diaphragm reducing upper chest cavity volume during exhalation. The contraction of this muscle also rotates and bends the trunk sideways. The muscle is also called the “same side rotator.”
Functions of the internal oblique muscle:
Along with other muscles of the abdominal wall, internal abdominal oblique muscle is important for maintaining normal abdominal wall tension. The contraction of these muscles has a protective and supportive role. The bilateral (both sides) contraction of this muscle also increases the intra-abdominal pressure and aids in functions, such as forced expiration (exhalation), micturition (urination), defecation (passing stools). The weakness of these muscles increases the risk of abdominal hernias.
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Science Direct. Abdominal External Oblique Muscle. https://www.sciencedirect.com/topics/neuroscience/abdominal-external-oblique-muscle