Your butt is primarily muscle, yes. The bulk of what you feel when you sit down comes from the gluteus maximus, which is actually the biggest and strongest muscle in the entire human body. But your buttocks aren’t purely muscle. A layer of fat and connective tissue sits on top, and that combination of muscle underneath and padding on top is what gives the area its distinctive shape.
What’s Actually in There
Three separate muscles make up the gluteal group. The gluteus maximus is the large, powerful outer muscle that creates most of the shape and size you see. Beneath it sit two smaller muscles: the gluteus medius and the gluteus minimus. Together, these three muscles form the muscular core of your buttocks.
On top of those muscles lies a layer of subcutaneous fat, the soft tissue that sits between your skin and the muscle beneath. This fat layer is thicker in the buttocks than almost anywhere else on the body, often exceeding 3 centimeters. People with higher estrogen levels tend to store even more fat in the buttocks, hips, and thighs, which is why body shape varies so much from person to person. That fat layer acts as cushioning, absorbs impact, and allows your skin to glide smoothly over the muscles underneath rather than rubbing directly against them.
So the short version: your butt is a muscle group wrapped in a generous layer of fat. The ratio between the two varies by genetics, hormones, activity level, and overall body composition.
What Your Glutes Actually Do
The gluteus maximus powers hip extension, which is the motion of driving your leg backward. Every time you stand up from a chair, climb stairs, sprint, or jump, the gluteus maximus is doing the heavy lifting. It also plays a significant role in core stability and keeping your pelvis level, a job most people don’t associate with their butt at all.
The gluteus medius handles a different set of tasks. It rotates your hip inward and outward, moves your leg out to the side, and stabilizes your pelvis during any movement where you’re standing on one leg. That includes every single step you take while walking or running. Without it firing properly, your pelvis would drop to one side with each stride, and your knees would collapse inward.
The gluteus minimus works alongside the medius as a stabilizer, helping keep the pelvis steady and assisting with hip rotation.
Why Weak Glutes Cause Problems Elsewhere
Because the glutes are so central to movement, weakness in these muscles doesn’t just stay in your butt. When the gluteus maximus and medius aren’t pulling their weight, other muscles have to compensate. Your lower back, hamstrings, and hip flexors pick up the slack during everyday movements like walking, standing, and lifting. Over time, this creates chronic tension, fatigue, and pain in those areas.
Weak glutes can also tilt your pelvis forward, placing extra strain on your lumbar spine. Strong glutes absorb impact forces during running, jumping, and bending. Without that absorption, your lower back becomes the body’s default shock absorber, a role it was never designed for. This chain reaction is one of the more common and overlooked contributors to recurring back pain.
Dead Butt Syndrome Is a Real Thing
Sitting for long hours at a desk or in a car gradually weakens the gluteus medius. When that muscle loses enough strength, it can cause numbness or tingling throughout your buttocks. This condition, sometimes called “dead butt syndrome” or lower cross syndrome, can eventually lead to broader pain in the hips and lower back as the muscle imbalance worsens.
It’s not just an office worker problem. Runners who skip cross-training and stretching can develop the same issue from chronic muscle tightness. The fix in both cases is targeted strengthening of the glutes, particularly the medius, to restore the balance between muscle groups.
Can You Change Your Butt’s Size and Shape?
Because the buttocks are largely muscle, resistance training can meaningfully change their size. A nine-week study published in Frontiers in Physiology measured gluteal muscle growth using MRI scans and found that both squats and hip thrusts produced similar increases in gluteal muscle cross-sectional area. The gluteus maximus responded well to both exercises, though the smaller stabilizer muscles (the medius and minimus) showed little to no measurable growth from either movement.
This means you can build a larger, more defined gluteus maximus through consistent resistance training, but the deeper stabilizing muscles may need more targeted exercises like lateral band walks or single-leg balance work. The fat layer on top of the muscle is influenced more by overall body composition than by any specific exercise, so the visible shape of your butt reflects both the muscle you’ve built and the fat distribution determined largely by your genetics and hormones.