The muscles on the sides of your abdomen are called the obliques. You have two layers on each side: the external obliques and the internal obliques. Together, they form the bulk of what people mean when they refer to “side abs,” and they’re responsible for twisting, bending sideways, and bracing your core during nearly every movement you make.
The Two Oblique Muscles
Your external obliques are the larger, more superficial pair. They sit on either side of the rectus abdominis (your “six-pack” muscle) and run diagonally from your lower ribs toward the center of your body. Because they’re closer to the surface, they’re the ones you actually see when someone has visible side-ab definition.
Underneath them sit the internal obliques, a thinner, smaller pair layered just inside your hip bones. Their fibers run in the opposite diagonal direction, creating a crisscross pattern with the external obliques. This two-layer design is what makes your torso so effective at rotating and resisting rotation at the same time.
Deeper still is a third flat muscle called the transversus abdominis. It wraps around your midsection like a corset and plays a major role in spinal stability, but it isn’t typically what people picture when they think of side abs because it sits too deep to be visible.
What the Obliques Actually Do
The obliques handle three main jobs. First, they rotate your trunk. Any time you twist to throw a ball, swing a golf club, or look over your shoulder while turning your torso, your obliques are driving that movement. Second, they bend your spine sideways, which biomechanists call lateral flexion. Third, when both sides contract together along with the rectus abdominis, they flex your trunk forward, the same motion as a crunch or sit-up. This makes them far more than cosmetic muscles. They’re essential for balance, posture, and transferring force between your upper and lower body.
Other Muscles That Create Side Definition
Two other structures contribute to the chiseled “side ab” look, even though they aren’t technically part of the abdominal wall.
The serratus anterior is a fan-shaped muscle along the side of your ribcage, just below the armpit. Its finger-like segments attach to the first through eighth or ninth ribs, giving it a saw-toothed appearance that can look like extra abs running up the side of your torso. In leaner, more muscular individuals, these serrations are clearly visible and are sometimes mistaken for oblique definition. In about 10% of people, the lowest attachment of the serratus extends to the 10th rib and even blends directly into the external oblique.
Lower down, the so-called “Adonis belt” or “V-cut” refers to the diagonal lines running from the hip bones toward the pelvis. These are formed by the inguinal ligament and the lower edge of the transversus abdominis, not the obliques themselves, but they’re part of the overall side-ab aesthetic many people train for.
When Obliques Become Visible
Oblique definition depends heavily on body fat percentage. For men, some external oblique definition typically starts appearing around 10 to 14 percent body fat, though the lower abs may still look soft at that range. For women, oblique lines tend to remain distinct at roughly 15 to 19 percent body fat, even as lower abdominal definition begins to fade. Below those thresholds, the diagonal muscle fibers and the separation between the obliques and rectus abdominis become progressively sharper.
Genetics also play a role. Muscle belly length, tendon placement, and fat distribution patterns vary from person to person, so two people at the same body fat percentage can look quite different.
Exercises That Target the Obliques
Not all core exercises hit the obliques equally. A study published in the journal Physical Therapy measured electrical activity in the oblique muscles during a range of common exercises. The results showed clear winners.
For the external obliques, the highest activation came from pike movements performed on an ab wheel (96% of maximum contraction), followed by knee-ups on an ab wheel (80%) and hanging knee raises with straps (79%). For the internal obliques, incline reverse crunches at 30 degrees produced the strongest signal (86%), with hanging knee raises (85%) and ab wheel pikes (83%) close behind.
What stands out is that exercises involving full-body tension and instability consistently outperformed traditional moves. Simple side crunches and basic twisting exercises ranked much lower. If visible or stronger obliques are the goal, movements that force your entire core to stabilize against gravity or an unstable implement will recruit far more muscle fiber than isolated floor work alone. Hanging leg raises, pallof presses, and loaded carries like suitcase walks are all practical gym-friendly options that follow the same principle.