Oblique crunches take their name from the major muscle group they work: the external and internal oblique muscles of your abdomen. These muscle layers wrap around your torso and form a natural internal corset at your core. Another major muscle group, the rectus abdominis, forms the bumpy, segmented washboard abs you see on bodybuilders. Crunches also incorporate the fan-shape serratus muscles of the ribs.
Rectus Abdominis
Although you use this large segmented sheet of muscle more during straight crunches, the rectus abdominis muscles also get a workout during oblique crunches. This layer of muscle lies just below your skin and runs along the front of your abdomen. The rectus abdominis supplies the initial lift to your upper torso as you begin an oblique crunch. Because this muscle group lies so close to the surface of your skin, it's the most prominent feature of a lean, toned abdomen.
External Obliques
The external obliques comprise the outermost layer of muscle along the sides of your abdomen. The muscle bundles attach to your ribs at one end and to the crests of your hip bones at the other. When you perform an oblique crunch that takes your upper body to the right, you're working the external obliques on your left side. If you place your hand against your side as you stretch, you'll feel your external obliques tighten as you rotate your abdomen in an oblique crunch.
Internal Obliques
Internal oblique muscles lie beneath the external obliques and run perpendicular to them. They attach along your iliac crest -- the flare of your hip bone -- at the base and extend to the lowermost ribs. During oblique crunches, these deep muscles rotate your upper torso as they contract. As external obliques provide the power behind an opposite-side stretch, internal obliques pull your torso into a same-side stretch. These core muscles wrap around your waist and facilitate rotational movement.
Serratus Anterior
Serratus muscle groups attach to the upper ribs and run beneath your arms. They assist in moving the shoulder blades and arms, but they also come into play during oblique crunches. The serratus anterior muscle group activates when you push your arms and shoulders into an oblique crunch. As this fan-like series of muscles contracts, it lifts the shoulder opposite the direction of rotation.
Transversus Abdominis
The deepest set of abdominal muscles, the transversus abdominis, runs from the base of the ribs to the crest of the hip. Its fibers run almost horizontally. Supine oblique crunches activate these muscles. While this muscle group facilitates deep breathing, it also acts as a support system for the lumbar region of the back. A 2006 University of Queensland study published in the journal "Spine" concluded that a strengthened transversus abdominis muscle "most likely improves the stabilization of the lumbopelvic region."
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