Your core muscles form a cylinder of tissue that wraps entirely around your midsection, from your ribcage down to your pelvis. Most people picture the core as just the “abs” on the front of the stomach, but the core actually includes muscles on all four sides of your trunk plus a roof and a floor. Think of it as a pressurized canister: the diaphragm sits on top, the pelvic floor muscles form the bottom, abdominal muscles wrap around the front and sides, and deep spinal muscles run along the back.
The Core as a Cylinder
The easiest way to understand where your core muscles are is to picture a balloon-shaped cavity inside your torso. The front and sides are formed by your abdominal muscles. The back is lined by muscles running along your spine. The ceiling is your diaphragm, the dome-shaped breathing muscle tucked up under your lower ribs. The floor is a hammock of muscle stretching across the base of your pelvis, connecting from your pubic bone in front to your tailbone in back.
All of these muscles work together to regulate pressure inside the abdominal cavity. When you breathe in, your diaphragm pushes downward and your pelvic floor relaxes to make room. When you cough, laugh, or brace to lift something heavy, the diaphragm, pelvic floor, and abdominal muscles all contract simultaneously, increasing internal pressure to stiffen and stabilize your spine. This pressure system is the reason the core is described as a unit rather than a collection of separate muscles.
Front and Side Muscles
The abdominal wall has four layers stacked on top of each other, each running in a different direction. On the very outside is the external oblique, the largest and most superficial layer. Its fibers originate from your fifth through twelfth ribs and angle downward and inward toward the midline. If you put your hands in your coat pockets, your fingers roughly follow the direction of these fibers.
Directly underneath is the internal oblique, whose fibers run in the opposite direction, angling upward and inward. Together, the two oblique layers form a crisscrossing mesh along your sides and front, from the lower edge of your ribcage down to your hip bones. They’re responsible for rotation and side-bending of the trunk.
The rectus abdominis is the long, flat muscle running vertically from your sternum and lower ribs down to your pubic bone. This is the “six-pack” muscle, and it sits in a sheath formed by the other abdominal layers. It flexes your trunk forward, like during a sit-up, but it’s actually one of the more superficial core muscles and contributes less to deep spinal stability than the layers beneath it.
The deepest abdominal layer is the transversus abdominis. It wraps horizontally around your torso like a corset, originating from the inner surfaces of your lower six ribs, the thick connective tissue along your lower back, and the top of your hip bones. Its fibers run straight across, and it attaches to a tendinous line down the center of your abdomen. Because it wraps all the way around, it’s the primary muscle that compresses the abdominal cavity and creates the stiffening pressure that stabilizes your spine.
Back Muscles
The posterior wall of the core runs along both sides of your lumbar spine. The deepest muscle here is the multifidus, a series of small, thick segments that connect one vertebra to the next. These muscles sit right against the spine, filling the grooves between the bony projections you can feel when you run your fingers down your lower back. They act as local stabilizers, controlling the fine, segment-by-segment motion of each vertebra.
Layered over the multifidus are the erector spinae, a group of longer muscles that run vertically from the pelvis up to the ribcage and beyond. These have both deep and superficial portions. The deep lateral fibers help with side-bending and work alongside the multifidus for stability, while the more superficial medial fibers generate the force for back extension, like when you lift your chest during a back raise.
Sitting off to the side, the quadratus lumborum connects the top of your pelvis to your lowest rib and the sides of your lumbar vertebrae. It’s located deep in the flank, behind the kidneys. This muscle is especially active during side-bending and continues working even when the larger back muscles relax during full forward bending, serving as a stabilizer when the spine is in vulnerable positions.
The Connective Tissue That Links Them
A large sheet of connective tissue called the thoracolumbar fascia plays a critical role in tying the core together. Located across the lower and mid-back, it consists of multiple layers that wrap around the spinal muscles like a retaining sleeve. Along the sides of this sleeve, a thickened ridge of tissue runs from the top of the hip bone up to the twelfth rib. This ridge is where the horizontal fibers of the transversus abdominis merge with the sheath surrounding the spinal muscles, essentially connecting the front of the core to the back through a continuous band of tension.
At the base of the lower back, all the layers of this fascia fuse into a thick composite that anchors firmly to the back of the pelvis and a ligament connecting the pelvis to the tailbone. This anchoring point helps maintain the integrity of both the lower lumbar spine and the sacroiliac joint, the junction where your spine meets your pelvis.
Top and Bottom of the Core
The diaphragm forms the roof. It’s a thin, dome-shaped muscle that attaches to the inside of your lower ribs, the bottom of your breastbone, and the front of your lumbar spine. Most people only think of it as a breathing muscle, but it also plays a direct stabilizing role. When you brace your trunk before a heavy lift, your diaphragm contracts downward at the same time your abdominal muscles tighten, compressing the contents of the abdominal cavity to create a rigid column of support around your spine.
The pelvic floor forms the base. It’s a group of muscles that span the opening at the bottom of your pelvis, running from the pubic bone in front to the coccyx (tailbone) in back and out to the sit bones on each side. These muscles don’t work in isolation. They contract and relax in coordination with the diaphragm and the abdominal wall, rising and falling with each breath and bracing automatically during actions like coughing, speaking, or lifting. Their constant adjustments help control internal pressure while also supporting the organs of the pelvis from below.
Hip Muscles That Extend the Core
Several muscles crossing the hip joint are functionally part of the core even though they’re technically “extremity” muscles. The most important is the iliopsoas, a deep muscle that runs from the front of your lumbar vertebrae and the inside of your pelvis down to the top of your thigh bone. It flexes the hip, but it also directly stabilizes the lumbar spine because of where it attaches. The gluteus maximus, the large muscle of your buttock, connects the back of the pelvis to the thigh and plays a similar stabilizing role from the opposite side.
This broader grouping is sometimes called the lumbopelvic-hip complex. It includes all the musculature connecting the abdomen, pelvis, trunk, spine, and upper thigh, and it functions as the central link between your upper and lower body. Every force you generate with your arms or legs passes through this region, which is why weakness or poor coordination anywhere in the cylinder affects movement quality throughout the entire body.
How to Feel Your Deep Core Muscles
You can locate your transversus abdominis by placing your fingertips about two inches inward from the bony points at the front of your hip bones. Gently cough or clear your throat. The deep tension you feel pushing back against your fingers, before the superficial muscles clench, is the transversus abdominis tightening. It should feel like a subtle tautening of a sheet rather than a bulging crunch.
To sense the multifidus, place your thumbs alongside the bony bumps of your lower spine and gently arch your back. The small, firm mounds of tissue that swell under your thumbs are the multifidus segments. For the pelvic floor, the simplest cue is the sensation of gently stopping the flow of urine. That lifting, closing feeling comes from the pelvic floor muscles contracting at the base of the cylinder.