The external oblique causes ipsilateral nutation by pulling the ilium posteriorly, while rotating the rib cage down and towards the contralateral pelvis, increasing the ipsilateral lordosis. It also assists the contralateral internal oblique in inducing contralateral counternutation.

Origin: Anterior 2/3 of the outer lip of the iliac crest.

Insertion: Outside of the lower 8 ribs, linea alba and contralateral rectus sheaths. [1]^p521-527 [2]^p329. Posteriorly, it interdigitates with the serratus anterior and latissimus dorsi.

Functions:

• Flexes the trunk by posteriorly tilting the pelvis and bringing the ribs closer to the pelvis [1]^p521-527 [2]^p329.
• Works with contralateral internal oblique to rotate the rib cage anteriorly and inferiorly, toward the side of the internal oblique (contralateral rotation of trunk) [1]^p521-527 [2]^p329.
•  Compresses the abdominal contents, increasing Internal Abdominal Pressure (IAP) [1]^p521-527
• Helps stabilize the pelvis during leg movement [3]

 

References:

1. Warwick, R. and P.L. Williams, eds. Gray’s Anatomy. 35 ed., ed. R. Warwick and P.L. Williams. 1973, W.B. Saunders Company: New York.
2. Neumann, D., Kinesiology of the Musculoskeletal System. Foundations for Physical Medicine. 2002: Mosby.
3. Floyd, W.F. and P.H. Silver, Electromyographic study of patterns of activity of the anterior abdominal wall muscles in man. J Anat, 1950. 84(2): p. 132-45.