A flat butt comes down to some combination of muscle size, fat distribution, and skeletal structure, all of which vary enormously from person to person. The gluteus maximus is the largest and most powerful muscle in the human body, but it is also surprisingly prone to weakening and shrinking when it is not regularly challenged. Layer genetics, hormonal changes, hours of daily sitting, and simple aging on top of that, and you have a long list of reasons why some people end up with less volume behind them than others. The real answer is rarely just one of these factors acting alone.
What Actually Creates Buttock Shape
The roundness or flatness of your backside is determined by three tissue layers stacked on top of each other: the gluteal muscles, the subcutaneous fat that sits over them, and the skin and connective tissue holding it all together. The gluteus maximus does the heavy lifting in terms of shape. It is a thick, fan-shaped muscle that spans from the pelvis and sacrum down to the upper thigh bone, and its primary job is extending and externally rotating the hip. Despite being the body’s biggest muscle, it actually does relatively little during ordinary walking. Research suggests that its enlargement during human evolution was driven more by running than by walking, which helps explain why people who only walk and never sprint or climb tend to have less developed glutes.
Underneath and beside the gluteus maximus sit the gluteus medius and gluteus minimus, which handle side-to-side stability. These muscles contribute to the upper and lateral fullness of the buttock. On top of all three muscles lies a layer of fat whose thickness and distribution are heavily influenced by your sex, hormones, age, and genes. Someone with well-developed gluteal muscles but very little subcutaneous fat will have a firm, angular shape. Someone with moderate fat but small muscles might have a softer but flatter appearance. The interplay between these layers is why two people of the same weight can have dramatically different buttock contours.
The Sitting Problem
Modern life involves an extraordinary amount of sitting, and the gluteal muscles pay a price. Hours in a chair mean hours during which the glutes are stretched and inactive, and over time this can lead to what trainers and physical therapists informally call “gluteal amnesia,” where the muscles become inhibited and fire less efficiently even when you try to use them. A clinical commentary in the International Journal of Sports Physical Therapy notes that the gluteus maximus is prone to inhibition and weakness, which contributes to chronic pain, injury, and reduced athletic performance.1PubMed Central. Assessing and Treating Gluteus Maximus Weakness – A Clinical Commentary The muscle does not vanish overnight, but when it stops being recruited properly, it gradually loses size.
There is also a direct mechanical effect. When you sit, your body weight compresses the soft tissue between your sit bones and the chair surface. MRI-based research has shown that the gluteus maximus muscle can be compressed by roughly 70% of its resting thickness directly beneath the ischial tuberosity during sitting, dropping from about 22.5 mm to just 7.2 mm in one case study.2Frontiers in Physics. A novel approach for quantitative imaging of the seated anatomy of the buttocks using ultrasound and MRI That kind of sustained compression reduces blood flow to the tissue and may contribute to long-term changes in muscle quality, though the research on chronic effects in otherwise healthy people is still limited.
Genetics and Where Your Body Stores Fat
Genes play a major role in determining not just how much body fat you carry, but where you carry it. A large genome-wide study identified 13 genetic loci associated with waist-to-hip ratio independent of overall body fat, and seven of those loci showed a much stronger effect in women than in men.3PubMed Central. Meta-analysis identifies 13 new loci associated with waist-hip ratio and reveals sexual dimorphism in the genetic basis of fat distribution In plain terms, your DNA influences whether extra calories end up padding your hips and buttocks or settling around your midsection, and that influence differs between the sexes.
More recent MRI-based genetic research has gone further, pinpointing genetic variants tied to fat accumulation at specific anatomical sites, including thigh subcutaneous fat and pelvic fat depots.4PubMed Central. MRI-Based Genetic Studies Reveal Specific Genetic Variants and Disease Risks Associated With Fat Distribution Across Anatomical Sites Researchers have also found that gene expression patterns differ between abdominal fat and gluteal fat, suggesting these depots are not just the same tissue in different locations but are regulated differently at a molecular level.5PLOS Genetics. Coexpression Network Analysis in Abdominal and Gluteal Adipose Tissue Reveals Regulatory Genetic Loci for Metabolic Syndrome and Related Phenotypes This is why some families reliably produce members with fuller backsides regardless of fitness level, while others tend toward flatter profiles. You cannot outrun your fat-distribution genetics entirely, though you can change the muscle layer underneath.
Hormones, Menopause, and the Shift Toward Central Fat
Sex hormones are among the strongest regulators of where fat accumulates. Estrogen promotes fat storage in the hips, thighs, and buttocks, which is why premenopausal women generally carry more lower-body fat than men of the same age. When estrogen levels drop during menopause, fat tends to redistribute away from the lower body and toward the abdomen. Studies of women going through menopause have found that total and visceral fat area, body fat percentage, and waist-to-hip ratio all increase, reflecting a clear shift toward central adiposity.6PubMed Central. The Impact of the Menopausal Transition on Body Composition and Abdominal Fat Redistribution The hormonal changes at menopause are associated with an increase in total body fat and a more android (apple-shaped) pattern of distribution.7Maturitas. Differential expression of factors involved in fat metabolism with age and the menopause transition
The practical result is that many women notice their buttocks becoming flatter in their 40s and 50s even if their overall weight stays the same or increases. The fat that once gave the area its shape migrates toward the waist. Men experience a more gradual version of this as testosterone declines with age, though the shift is generally less dramatic because men tend to carry less gluteal fat to begin with. Age, sex hormones, and genetics all interact to shape fat distribution, and much about the fine details of that regulation remains unknown.8PubMed Central. Determinants of body fat distribution in humans may provide insight about obesity-related health risks
Aging and Muscle Atrophy
Even setting hormones aside, aging itself shrinks muscles throughout the lower body. A study of elderly women found that muscle thickness in nearly all lower-limb muscles was significantly smaller compared to younger women, and that those who had stopped walking showed more severe atrophy, particularly in the thigh muscles.9PubMed. Atrophy of the lower limbs in elderly women: is it related to walking ability? Longitudinal follow-up work confirmed that muscles like the erector spinae and quadriceps lost the most volume, though the glutes were not spared.10PubMed Central. Association between walking ability and trunk and lower-limb muscle atrophy in institutionalized elderly women: a longitudinal pilot study
Research in Masters athletes offers a useful contrast. Older athletes who remained highly active still experienced some decline in muscle volume and increases in intramuscular fat, but far less than sedentary peers. The findings suggest that intrinsic muscle weakness (not just muscle shrinkage) is an important contributor to age-related declines in power.11PubMed Central. Leg and hip muscles show muscle-specific effects of ageing and sport on muscle volume and fat fraction in male Masters athletes In other words, staying active does not freeze the clock, but it slows it considerably. The flat-butt phenomenon in older adults is often a combination of genuine muscle loss, fat redistribution, and reduced muscle quality, with inactivity accelerating all three.
Why Flat Glutes Are a Health Concern, Not Just a Cosmetic One
Weak or underdeveloped glutes affect more than how your jeans fit. The gluteus medius, which sits on the outer side of the hip, plays a critical role in stabilizing the pelvis during walking. When it is weak, the pelvis drops on the unsupported side with each step, a pattern known as Trendelenburg gait. This creates uneven pressure on the spinal discs and contributes to low back pain.12PubMed Central. Gluteus medius muscle function in people with and without low back pain: a systematic review Gluteus medius weakness is recognized as the primary contributor to this gait pattern.13PubMed. The use of electromyogram biofeedback to reduce Trendelenburg gait
Adding gluteal strengthening to a rehab program produces measurable benefits. One study of people with chronic low back pain found that combining lumbar stabilization exercises with gluteal strengthening led to greater reductions in disability, greater improvements in lumbar muscle strength, and better balance compared to lumbar exercises alone.14Journal of Physical Therapy Science. The effects of gluteus muscle strengthening exercise and lumbar stabilization exercise on lumbar muscle strength and balance in chronic low back pain patients The connection between flat or weak glutes and back problems, knee problems, and hip problems is well documented in rehabilitation literature. People who sit most of the day and then develop low back pain may assume the problem is their spine when it is really an underperforming set of hip muscles.
Building a Bigger Gluteus Maximus
The good news is that the gluteus maximus responds well to resistance training. A meta-analysis pooling data from 12 studies found that resistance training produces a moderate hypertrophy effect on the glutes, regardless of whether people used a single exercise or combined multiple movements.15PubMed Central. The impact of resistance training on gluteus maximus hypertrophy: a systematic review and meta-analysis That same analysis found that when the goal is specifically glute growth rather than general lower-body development, the barbell hip thrust should be prioritized, while back squats done to parallel or deeper also produce significant gluteal hypertrophy.
A systematic review of muscle activation data ranked exercises by how strongly they fire the glutes. Step-ups and their variations produced the highest activation levels, followed by deadlifts, hip thrusts, lunges, and squats.16PubMed Central. Gluteus Maximus Activation during Common Strength and Hypertrophy Exercises: A Systematic Review Comparing the hip thrust directly against the back squat and split squat, the hip thrust produced significantly higher peak glute activation.17PubMed. Activation of the Gluteus Maximus During Performance of the Back Squat, Split Squat, and Barbell Hip Thrust and the Relationship With Maximal Sprinting A randomized trial comparing hip thrusts and back squats over a training period found that both produced similar gluteal hypertrophy and similar transfer to deadlift strength, though the hip thrust generated higher muscle activity in the upper and middle portions of the glute during each set.18PubMed Central. Hip thrust and back squat training elicit similar gluteus muscle hypertrophy and transfer similarly to the deadlift
The practical takeaway is that you do not need a complicated program. Consistently training some combination of hip thrusts, squats, deadlifts, lunges, or step-ups two to three times per week with progressive resistance will build the muscle. People starting from a very flat baseline often see visible changes within a few months because the gluteus maximus, being so large, has substantial growth potential. That said, genetics still set the ceiling for how much subcutaneous fat sits on top of the muscle and where, so two people following the same program will end up with different shapes.
Medical Conditions That Cause Unusual Fat Loss
In rare cases, a flat or hollow-looking buttock is not the result of lifestyle or aging but of an underlying medical condition. Lipodystrophy syndromes are a group of disorders characterized by varying degrees of body fat loss, and they come in both genetic and acquired forms. The genetic forms include congenital generalized lipodystrophy and familial partial lipodystrophy, the latter of which can selectively deplete fat from the limbs and buttocks while sparing or even increasing fat elsewhere. Acquired lipodystrophy can also occur as a side effect of certain antiretroviral medications used to treat HIV.19PubMed Central. Lipodystrophy Syndromes These conditions affect a small number of people, but they are worth knowing about because the fat loss pattern can look strikingly different from normal variation and often comes with metabolic complications like insulin resistance.
Other medical causes include prolonged corticosteroid use, which redistributes fat centrally and depletes it from the extremities, and certain neuromuscular conditions that cause muscle wasting in the hip and thigh. If someone experiences rapid or unexplained changes in buttock shape without changes in diet or activity, it is worth discussing with a doctor rather than assuming it is purely cosmetic.
Cosmetic Approaches to Gluteal Augmentation
For people whose concerns are primarily aesthetic, surgical options exist. The most popular is the Brazilian butt lift, which transfers a person’s own fat from other areas (usually the abdomen or flanks) into the buttocks. Anatomical research has identified seven distinct deep gluteal fat compartments that sit above the muscle layer, each with clear boundaries that keep injected fat contained. Surgeons can selectively expand these compartments to sculpt specific areas of the buttock.20PubMed. Deep Subcutaneous Gluteal Fat Compartments: Anatomy and Clinical Implications Safety has been a serious concern with this procedure: injecting fat below the muscle fascia risks hitting large blood vessels. Anatomical studies have confirmed that when injection is kept in the subcutaneous layer using proper technique, the fat does not penetrate the gluteal fascia or muscle.21Aesthetic Surgery Journal. Accurate Plane Fat Grafting in Gluteal Augmentation: An Anatomic Study
Implants are another option, typically made of solid silicone placed within or on top of the gluteal muscle. They produce a more predictable volume increase than fat transfer but come with their own risks, including capsular contracture and implant displacement. Neither procedure changes the underlying muscle, so they do not address the functional weakness that often accompanies flat glutes.
Pelvic Tilt and the Illusion of Flatness
Sometimes a butt that looks flat is not actually lacking in muscle or fat but is being presented that way by posture. Anterior pelvic tilt, where the front of the pelvis tips forward, tends to push the buttocks backward and make them appear more prominent. Posterior pelvic tilt does the opposite, tucking the pelvis under and flattening the visual profile. Research into postural assessment has found that clinicians’ perception of spinal curvature is heavily influenced by gluteal prominence, suggesting that the contour of the buttocks shapes how posture is perceived rather than the other way around.22PubMed. Investigation of the validity of postural evaluation skills in assessing lumbar lordosis using photographs of clothed subjects In everyday life, people with a posterior tilt can look significantly flatter than someone with the same muscle and fat mass who stands with a slight anterior tilt. Correcting habitual posture through awareness and hip-flexor stretching can sometimes improve the appearance of the buttocks without any change in tissue composition.
Body Image in the Age of the Slim-Thick Ideal
It is worth acknowledging that what counts as a “flat” butt is partly a cultural judgment that has shifted over time. The current Western beauty standard heavily favors a large, round backside paired with a small waist, a body type sometimes called the slim-thick ideal. Research on young women exposed to slim-thick imagery on Instagram found that those who compared themselves to these images experienced more weight and appearance dissatisfaction, and lower body satisfaction, than women who compared themselves to the older thin-ideal standard.23Body Image. The effect of physical appearance perfectionism and social comparison to thin-, slim-thick-, and fit-ideal Instagram imagery on young women’s body image The rise of the slim-thick ideal means more people perceive their natural body shape as deficient, even when their gluteal development is perfectly normal and healthy. Two decades ago, the same buttock proportions might have attracted no attention at all.
That does not mean concerns about flat glutes are purely cosmetic vanity. As the evidence on back pain, gait stability, and age-related muscle loss shows, gluteal strength genuinely matters for long-term health. But separating the health question from the appearance question helps clarify which changes are worth pursuing and why. Building stronger glutes because they protect your spine and improve your movement is a different motivation from chasing an Instagram silhouette, and the two goals sometimes point to different approaches.