Fallen arches develop when the structures holding up the foot’s natural curve weaken, stretch, or tear. The single most common cause is gradual breakdown of the posterior tibial tendon, a thick band of tissue that runs from the calf muscle to the bones along the inner arch. When this tendon can no longer do its job, the arch sags, and the foot begins to flatten and roll inward. Estimated prevalence of this condition is over 3% in adults.
How the Arch Holds Itself Up
Your foot’s arch isn’t just bone. It’s an active structure maintained by a combination of tendons, ligaments, and muscles working together under load. The posterior tibial tendon is the primary support. It attaches one of the calf muscles to the bones on the inside of the arch, pulling the arch upward every time you take a step. It also helps you stand on your toes and turn your foot inward.
Beneath the arch, a group of ligaments called the spring ligament complex acts like a hammock connecting the heel bone to the navicular bone (the keystone of the arch). This complex has two main bundles that prevent the bones from sliding apart. Together, the tendon and these ligaments share the load of keeping the arch’s shape intact under your body weight.
The Most Common Cause: Tendon Breakdown
In most adults who develop fallen arches, the problem starts with chronic inflammation of the posterior tibial tendon. Years of repetitive stress cause small amounts of damage that the tendon can’t fully repair. The tendon thickens, weakens, and eventually stretches out like an old rubber band. Once it elongates, it can no longer hold the arch at its normal height.
This sets off a vicious cycle. As the tendon weakens, the arch drops, shifting extra stress onto the spring ligament and other supporting structures. Those structures then stretch and weaken too, forcing the already-damaged tendon to work even harder. Eventually, the tendon and ligaments may tear completely, and the arch collapses further. Research on cadaver feet has shown that cutting the spring ligament alone shifts weight toward the outside of the heel and causes the hindfoot to angle outward, a hallmark of flatfoot deformity. The combination of losing both the tendon and the spring ligament is particularly devastating to the arch’s stability.
Risk Factors That Speed Up the Process
Several factors make the posterior tibial tendon more vulnerable to this kind of breakdown.
- Excess body weight. Carrying extra weight increases the mechanical load on the arch with every step. Over time, this can physically change the structure of the foot. Obesity also raises the risk of diabetes, which compounds the problem (more on that below).
- Age. Tendons lose elasticity and blood supply as you get older, making them slower to heal from daily wear. Adult-acquired flatfoot is most common in middle-aged and older adults.
- Diabetes. Nerve damage from diabetes can reduce sensation in the feet, leading to a condition called Charcot foot. The bones in the foot gradually break down and the supporting ligaments fail, causing the arch to collapse. This type of arch collapse can be severe.
- Arthritis. Both degenerative arthritis (from wear and tear) and inflammatory arthritis (like rheumatoid arthritis) cause swelling and deformity in the foot joints. This disrupts the precise alignment of bones that gives the arch its shape.
- Genetics. Some people inherit a foot structure that makes them more prone to flat feet. One example is an accessory navicular bone, an extra piece of bone on the inner side of the foot present from birth. This extra bone changes where the posterior tibial tendon attaches, weakening its mechanical advantage. The tendon effectively becomes longer and less efficient at pulling the arch upward, leading to imbalance and inward rolling of the foot.
Sudden Injury vs. Gradual Collapse
Not all fallen arches develop slowly. A fracture in the foot or a torn ligament from a sports injury or fall can undermine the arch overnight. In these cases, the structural damage is immediate rather than cumulative. Even the recovery period itself can contribute: going too long without walking while healing allows the muscles that support the arch to weaken, making it harder for the arch to hold its shape once you’re back on your feet.
The gradual version is far more common. It typically unfolds over months or years, starting as mild soreness along the inner ankle and progressing to visible flattening of the foot. Many people don’t realize the arch is collapsing until the foot has already changed shape significantly.
How Fallen Arches Progress
Clinicians describe the progression in four stages, and understanding them helps explain why early attention matters.
In the earliest stage, the tendon is inflamed but not yet stretched out. The arch looks normal, and you can still rise onto your toes on one foot without difficulty. Pain is focused on the inner ankle. At this point, the damage is mostly reversible with rest and support.
In stage two, the tendon has actually elongated. The arch visibly drops, and the heel tilts outward. A telltale sign: when someone looks at your feet from behind, more toes are visible on the outer side of the affected foot than on the healthy one (clinicians call this the “too many toes” sign, and it indicates the forefoot has rotated outward). At this stage, you likely can’t do a single-leg heel rise, meaning you can’t stand on one foot and push up onto your toes, because the tendon is too weak to lift your body weight.
By stage three, the deformity has become rigid. The heel is locked in its tilted position and can’t be manually corrected. Pain spreads to the outer ankle because the collapsed arch forces bones on that side to jam together. Stage four adds arthritis in the ankle joint itself, not just the foot.
What Keeps the Collapse Going
One reason fallen arches rarely reverse on their own is that the foot’s alignment changes create new mechanical problems. As the arch drops, the heel tilts outward and the forefoot splays. This shifts your body weight to the inner edge of the foot, placing even more strain on the already-damaged tendon and ligaments. Walking and standing reinforce the collapsed position with every step.
The muscles in the lower leg also adapt. The calf muscle, which powers the posterior tibial tendon, loses its mechanical leverage as the tendon stretches. It contracts less effectively, and over time the muscle itself can weaken from disuse. Meanwhile, the tendons on the outer side of the ankle tighten in response to the new foot position, pulling the heel further out of alignment. The result is a self-reinforcing deformity that tends to worsen without intervention.
Early Signs to Watch For
The first symptom is usually aching or swelling along the inner ankle, right where the posterior tibial tendon runs behind the ankle bone. This may feel worse after long walks or standing for extended periods. Over time, you might notice your shoes wearing unevenly, with more breakdown on the inner heel, or that one foot seems to roll inward more than the other.
A simple self-check: stand on one leg and try to rise onto your toes. If you can’t do it, or if it causes pain along the inner ankle, the posterior tibial tendon may be compromised. Another clue is looking at your feet from behind in a mirror. If you can see three or four toes peeking out on the outer side of one foot but not the other, the arch on that side has likely dropped and the forefoot has started to rotate outward.