Internal oblique

The OBLIQUE INTERNAL MUSCLE OF THE ABDOMEN constitutes the deep layer of the anterolateral wall of the abdomen; it lies, in fact, below the external oblique muscle.

It originates from the intermediate line of the anterior superior iliac spine and from the deep leaf of the thoracolumbar fascia (posteriorly). Some fibers can also originate from the lateral third of the inguinal ligament (anteriorly).

The muscle fibers follow an ascending (fan) course and are inserted in three distinct zones dividing the muscle into three parts: cranial, medial and caudal. The cranial part is inserted at the lower edge of the last three costal cartilages; the medial part continues in a fibrous band forming the aponeurosis of the internal oblique muscle which after being divided into two sheets (internal and external) joins the alba line contributing to the formation of the sheath of the rectus abdominal muscles.

The lower part of the aponeurosis forms, together with that of the transversus abdominal muscle, the joint tendon, which is inserted at the upper margin of the pubis, at the pubic tubercle and at the medial margin of the pectine crest. It has actions similar to those of the external oblique, lowers the ribs (expiratory muscle), flexes and inclines the chest laterally but, unlike the external oblique muscle, rotates it on the same side; its contraction also determines an increase in abdominal pressure, necessary to withstand the forces acting on the spine during certain exercises.

ORIGIN Inguinal ligament, anterosuperior iliac spine, intermediate line of the iliac crest, posterior face lumbodorsal fascia (or thoracolumbar)
INSERTION Lower margin of the costal cartilage of the last three ribs (10a-12a); aponeurosis in the upper part of the linea alba; continue in the caudal part with the cremaster muscle
ACTION Intervenes in forced expiration; flexes and tilts the chest sideways and rotates it on its side; increases abdominal pressure
INNERVATION Intercostal nerves (T10-T12), ileoipogastric and ileoinguinal nerves of the lumbar plexus (L1)

Upper limb

Lower limb

Trunk

Abdomen

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