The sides of your torso are built from several layers of muscle, stacked on top of each other like sheets of fabric running in different directions. The most prominent are the external and internal obliques, but they’re far from alone. From just under your skin down to your spine, at least seven distinct muscles wrap around or attach to the lateral part of your body.
External Obliques
The external obliques are the outermost layer of muscle on your sides and the ones you can most easily see and feel. They run diagonally from your lower ribs down toward your pelvis, with fibers angling in the same direction as if you were sliding your hands into your front pockets. When both sides contract together, they help you crunch forward. When only one side fires, it bends your trunk sideways or rotates it to the opposite direction.
These muscles are active in almost every trunk movement, from twisting to throw a ball to bracing your core when you lift something heavy. They also play a role in forced breathing, like when you cough or blow out candles. Because they sit right on the surface, they’re the muscles that create the visible diagonal lines on a lean torso.
Internal Obliques
Sitting directly beneath the external obliques, the internal obliques run at a roughly perpendicular angle to them. If the external obliques angle downward and inward, the internal obliques angle upward and inward. This crisscross design gives your torso rotational strength in both directions.
When both internal obliques contract at the same time, they flex the trunk forward and compress the abdominal contents, increasing pressure inside your abdomen. That pressure is what helps you exhale forcefully, bear down during heavy lifting, or stabilize your spine during quick movements. When only one side contracts, it bends and rotates your trunk toward that same side, the opposite pattern of the external obliques. Together, the two oblique layers work as a team: one internal oblique and the opposite external oblique fire together to produce any given rotation.
Transversus Abdominis
The deepest of the six abdominal muscles, the transversus abdominis wraps around your trunk from front to back like a built-in corset. Its fibers run horizontally, extending between the ribs and the pelvis. You can’t see it from the outside, but it’s one of the most important core muscles you have.
Rather than producing large movements, the transversus abdominis specializes in stabilization. It keeps your abdominal organs in place, supports your lower back, and maintains tension across your midsection during virtually every movement you make. When this muscle is weak or fails to activate properly, it contributes to low back pain and instability in the lumbar spine. It’s the muscle physical therapists often target first in core rehabilitation programs.
Serratus Anterior
The serratus anterior is the saw-toothed muscle visible on the upper sides of your ribcage, just below and in front of your armpit. It fans out from the inner edge of your shoulder blade and attaches to the top eight or nine ribs. On a lean person, it looks like finger-like projections along the side of the chest.
This muscle is the prime mover for pulling your shoulder blade forward and rotating it upward. It’s sometimes called the “boxer’s muscle” because it drives the forward punch of the shoulder blade when you throw a jab. But it’s essential for any overhead movement too, working alongside the trapezius to rotate the shoulder blade so you can reach above your head. It also keeps the shoulder blade pressed flat against your ribcage. When it’s weak, the inner border of the shoulder blade lifts off your back, a visible condition called winging. As a bonus, the serratus anterior assists breathing by helping lift the ribcage when you’re winded.
Intercostal Muscles
Between each pair of ribs sit three thin layers of intercostal muscle: the external, internal, and innermost intercostals. You have 11 sets of them on each side of your body, and their primary job is breathing.
The external intercostals contract when you inhale, pulling the ribs upward and outward to expand the chest cavity and draw air into your lungs. The internal intercostals do the reverse, helping shrink the rib cage when you exhale. The innermost intercostals support the internal layer during forceful exhalation, like sighing, coughing, or blowing up a balloon. You rarely think about these muscles, but they’re working with every breath you take.
Quadratus Lumborum
Tucked deep in your lower back on each side of the spine, the quadratus lumborum connects the top of your pelvis to your lowest rib and the lumbar vertebrae. It’s technically a posterior muscle, but it sits far enough to the side that it plays a major role in lateral movement and is a common source of side-body pain.
When one side contracts, it bends your trunk in that direction. Both sides working together help stabilize the spine and pelvis during walking, running, and carrying loads. The quadratus lumborum is also important for anchoring the 12th rib, which provides a stable base for the diaphragm to pull against during breathing. This muscle is notorious for tightness and spasm, particularly in people who sit for long periods or have pelvic imbalances that force it to overwork as a stabilizer.
Latissimus Dorsi
The latissimus dorsi is a massive, flat muscle that covers the lower two-thirds of your back and wraps around to form part of your side body. Its lower border traces all the way to the top of your pelvis at the iliac crest, and you can feel it along your entire lateral torso when you pull your arm down against resistance. It forms the rear wall of your armpit and gives width to the lower torso.
Primarily a shoulder muscle, the latissimus dorsi pulls your arm down, back, and inward. It’s the main muscle working during pull-ups, rows, and swimming strokes. But because of how broadly it covers the side and back of the trunk, it also stabilizes the shoulder blade against the ribcage and assists with deep breathing when you’re exerting yourself.
When Side Muscles Get Strained
The obliques are the most commonly strained muscles on the side of the torso, particularly during sports that involve sudden twisting or powerful rotation. A strain typically causes sharp pain along the side that gets worse when you cough, sneeze, laugh, or try to sit up from a lying position. You might also notice swelling, bruising, muscle spasms, or stiffness that makes it hard to twist or bend.
Milder strains usually resolve on their own with rest and gradual return to activity. More severe pulls, where the pain is intense and movement is significantly limited, sometimes require imaging to rule out rib fractures or spinal injury. Persistent side-body pain that doesn’t follow a clear injury pattern can also point to the quadratus lumborum, which tends to develop trigger points and chronic tightness rather than acute tears.