Flat feet develop when the arches along the inside of your feet are low or completely absent, allowing the entire sole to touch the ground when you stand. The causes range from normal childhood development that never fully progresses, to tendon damage, weight gain, genetics, and chronic disease. In adults, the prevalence sits between 5 and 14 percent depending on the population studied, and the condition can be something you’re born with or something that develops later in life.
Every Child Starts With Flat Feet
All typically developing children are born with flexible flat feet. The medial longitudinal arch, the curved structure running along the inner edge of your foot, forms gradually over the first decade of life. In most kids, the arch becomes visible by age five or six and continues to strengthen into the preteen years. Some children simply never develop a pronounced arch, and their feet remain flat into adulthood without ever causing problems.
Because the arch is still forming during childhood, prevalence estimates of flat feet in children swing wildly, from under 1 percent to nearly 78 percent depending on the age group measured and the method used. The consistent finding across studies is that the number drops steadily as children get older, which reflects normal arch development rather than any treatment effect.
Flexible vs. Rigid Flat Feet
The most important distinction is whether a flat foot is flexible or rigid, because the causes and implications are very different.
A flexible flat foot has a visible arch when you sit or stand on your toes, but the arch disappears under your full body weight. This is by far the more common type and is often painless. Many people with flexible flat feet inherited the trait and live their entire lives without symptoms.
A rigid flat foot has no arch regardless of whether you’re bearing weight or not. This type is rare and usually traces back to a structural abnormality. Tarsal coalition, where two or more bones in the back of the foot are fused together by bone, cartilage, or fibrous tissue, is one of the most common causes. The two joints most frequently affected are the connection between the heel bone and the talus, and the connection between the heel bone and the navicular. Other structural causes include a congenital vertical talus (where a bone in the ankle is positioned abnormally from birth) or an accessory navicular bone, an extra piece of bone on the inner side of the foot.
Rigid flat feet often become symptomatic after a sprain or minor injury to the affected foot, which triggers pain and stiffness that reveals the underlying coalition.
Posterior Tibial Tendon Dysfunction
In adults, the single most common cause of a newly flattening foot is damage to the posterior tibial tendon. This tendon runs from the calf muscle, hooks around the bony bump on the inside of your ankle, and attaches to bones in the middle of your foot. Its job is to hold up the arch and help your foot push off the ground with each step.
The tendon is vulnerable because of its anatomy. It makes a sharp turn around the inside of the ankle, which puts significant mechanical stress on the fibers in that area. Over time, the tendon can stretch, partially tear, or rupture entirely. When it fails, the arch gradually collapses, the foot rolls inward, and the surrounding ligaments begin to stretch as well. Left unchecked, this progression eventually affects the bones and joints themselves, leading to a more rigid deformity.
Risk factors for posterior tibial tendon dysfunction include age (it’s most common after 40), obesity, high blood pressure, diabetes, and activities that place repetitive stress on the tendon like running or high-impact sports.
Obesity and Pregnancy
Excess body weight places continuous extra load on the arch, and research has shown that obesity can directly cause the arch to collapse. The mechanism is straightforward: the soft tissues supporting the arch, including tendons, ligaments, and the thick band of tissue along the sole, are not designed to bear weight beyond a certain threshold for prolonged periods. Over months and years, they stretch and weaken.
Pregnancy creates a double effect. The additional weight of pregnancy loads the arch in the same way obesity does, but hormonal changes compound the problem. Rising levels of relaxin, progesterone, and estradiol during pregnancy loosen ligaments throughout the body to prepare for delivery. That same loosening affects the ligaments in the feet, reducing their ability to hold the arch in place. Many women notice their shoe size increases permanently during pregnancy, which reflects a lasting change in foot structure rather than simple swelling.
Connective Tissue and Genetic Conditions
Some people develop flat feet because the building materials of their connective tissue are inherently weaker. Conditions like Ehlers-Danlos syndrome involve gene variations that produce abnormal collagen, the protein that gives strength and structure to tendons, ligaments, skin, and bone. When collagen is weaker or stretchier than normal, the ligaments supporting the arch can’t maintain their tension, and flat feet are a recognized feature of the condition.
Marfan syndrome, Down syndrome, and other genetic conditions that affect connective tissue carry a similar elevated risk. In these cases, flat feet tend to appear early in life and are flexible rather than rigid.
Even without a named connective tissue disorder, flat feet run in families. If one or both of your parents have flat arches, you’re more likely to have them too. The hereditary component involves the shape of your foot bones, the elasticity of your ligaments, and the overall structure of your foot, all of which are influenced by multiple genes.
Inflammatory and Neurological Conditions
Rheumatoid arthritis can cause flat feet through a specific chain of events. The chronic inflammation attacks the lining of joints in the foot and ankle, causing swelling and tissue damage. In the hindfoot, this inflammatory process leads to joint erosion, instability, and swelling around the posterior tibial tendon. As the tendon becomes dysfunctional and the surrounding joints lose their structural integrity, the arch collapses inward. People with rheumatoid arthritis may notice their feet gradually flattening and turning outward over years.
Diabetes contributes through a different pathway. Diabetic neuropathy damages the nerves in the feet, which can reduce your ability to sense the position and stress on your foot joints. In severe cases, this leads to Charcot foot, a condition where repeated unnoticed injuries cause the bones to fracture and the foot to collapse. Diabetes also impairs blood flow and tissue healing, which can weaken the tendons and ligaments over time.
Injury and Overuse
A single traumatic event, like a fracture of the heel bone, a severe ankle sprain, or a dislocation of the midfoot joints, can disrupt the architecture of the foot enough to flatten the arch permanently. Falls from a height, car accidents, and sports injuries are typical scenarios.
More commonly, flat feet develop from cumulative overuse. Occupations that require long hours of standing or walking on hard surfaces stress the posterior tibial tendon and plantar ligaments day after day. Runners and dancers accumulate similar repetitive forces. The tendon and ligament damage is gradual, and people often don’t notice the arch loss until pain or fatigue brings it to their attention.
How Flat Feet Are Identified
A simple visual check is the starting point. If you stand on a hard floor and can slide a finger under the inside of your foot, you have some arch. If the entire sole contacts the ground, you likely have flat feet. The “wet footprint test,” where you step onto a dry surface with a wet foot and look at the print, gives a quick visual of your arch height.
When a more precise assessment is needed, X-rays taken while you’re standing provide measurable angles. One of the most reliable is the calcaneal pitch, which measures the angle of your heel bone relative to the ground. A value below about 12 degrees indicates a flat foot. Doctors may also measure the angle formed between the talus bone in your ankle and the first metatarsal bone in your forefoot. In a normal foot, these two bones line up in a relatively straight line. In a flat foot, they form a noticeable angle as the arch sags.
When Flat Feet Cause Problems
Most people with flexible flat feet never experience symptoms. The condition only becomes a medical concern when it causes pain, limits activity, or progressively worsens. Pain typically shows up along the inner ankle (where the posterior tibial tendon runs), in the arch itself, or on the outer side of the ankle as the foot rolls inward and compresses the structures there.
Because flat feet alter the alignment of the entire lower limb, they can contribute to problems further up the chain. Shin pain, knee pain, and even hip or lower back discomfort can trace back to flat arches changing the way forces travel through your body during walking and running. Flat feet also increase the mechanical load on the plantar fascia, the thick band of tissue along the sole, which is why plantar fasciitis is more common in people with low arches.
The key factor that separates a harmless flat foot from a problematic one is whether the condition is stable or progressive. A foot that has been flat since childhood and causes no pain rarely needs intervention. A foot that is newly flattening in adulthood, especially on one side, signals active tendon or joint damage that benefits from early attention.