Flatfoot, sometimes called pes planus, is a condition in which the medial longitudinal arch of the foot sits lower than usual or contacts the ground entirely during standing. It is extremely common, especially in children, and in most cases causes no pain or functional limitation at all. The story gets more complicated in adults who develop a flat arch later in life, where it can signal progressive tendon damage that benefits from early treatment. Whether your flat feet are something to worry about depends almost entirely on the type you have, when it appeared, and whether it hurts.
What Holds the Arch Up in the First Place
Your foot’s arch is not a single structure but a collaboration between bones, ligaments, and a key tendon. The spring ligament, which connects the calcaneus (heel bone) to the navicular bone farther forward, acts as the primary static stabilizer of the medial arch, essentially functioning as a sling that holds the arch’s keystone in place even when your muscles are relaxed.1PubMed. Spring ligament complex: Illustrated normal anatomy and spectrum of pathologies on 3T MR imaging The posterior tibial tendon (PTT) adds dynamic support on top of that. Running along the inside of your ankle and attaching beneath the foot, the PTT pulls upward on the arch with every step, particularly during the push-off phase of walking. Biomechanical studies confirm that loading the PTT significantly changes the positions of the foot’s bones in ways that restore arch height.2PubMed. Effect of the posterior tibial tendon on the arch of the foot during simulated weightbearing: biomechanical analysis When the PTT is deficient, the arch tends to collapse, and the bones lose their normal alignment.3PubMed. Tibialis posterior: a review of anatomy and biomechanics in relation to support of the medial longitudinal arch
The PTT also locks the bones of the arch and hindfoot into a stable configuration at heel-off, which is the moment your heel lifts from the ground during a stride. Flatfoot deformity compromises this locking ability, which helps explain why people with flat arches sometimes feel unstable when pushing off.4PubMed. The effect of posterior tibialis tendon dysfunction on the plantar pressure characteristics and the kinematics of the arch and the hindfoot Interestingly, a study using shear-wave elastography found that flat-foot shape is associated with increased stiffness of the spring ligament in certain foot positions, likely because that ligament is being asked to do more work when the tendon above it is underperforming.5PubMed Central. Biomechanical evaluation of the spring ligament and the posterior tibial tendon by shear-waves elastography: validation of a reliable and reproducible measurement protocol
Children and Flat Feet
If your toddler has flat feet, you are in very large company. A study of preschool-aged children found that about 54% of three-year-olds had flat feet, and by age six that number had dropped to around 24%.6Pediatrics. Prevalence of Flat Foot in Preschool-Aged Children The arch simply has not finished developing in most young children. A thick fat pad on the sole obscures whatever arch is present, and the ligaments and muscles that will eventually hold the arch taut are still maturing. For most kids, the arch rises on its own with normal growth and activity, and no treatment is needed.
The exception is rigid flatfoot. In a flexible flat foot, the arch disappears when the child stands but reappears when they stand on tiptoe or when the foot is unloaded. In a rigid flat foot, the arch stays flat regardless of position. Rigid flatfoot in children and adolescents is most commonly caused by tarsal coalition, a condition where two bones in the back of the foot are connected by abnormal bone, cartilage, or fibrous tissue.7Foot and Ankle Clinics. THE RIGID FLATFOOT: Tarsal Coalitions Talocalcaneal coalition, one particular type, often leads to flatfoot deformity in children and can cause pain during activity.8PubMed Central. Talocalcaneal coalition combined with flatfoot in children: diagnosis and treatment: a review This is the kind of flatfoot that genuinely needs medical attention in a child, because it will not resolve on its own.
Connective tissue disorders can also produce a more severe form of childhood flatfoot. Hypermobile Ehlers-Danlos Syndrome, which affects the body’s connective tissues and makes joints excessively lax, can lead to an extreme and progressive form of pes planus. In these cases, the soft tissues of the foot lack the intrinsic stability to maintain the arch, and the deformity can compound problems in the knee, hip, and beyond.9The Journal of Foot and Ankle Surgery. Surgical management of pes planus in children with hypermobile Ehlers-Danlos: A case report and review of literature
When Adults Develop Flatfoot Later in Life
Adult-acquired flatfoot is a different animal from the flexible flat feet most people carry from childhood. Its most common cause is posterior tibial tendon dysfunction (PTTD), a progressive deterioration of the tendon that serves as the arch’s main dynamic support.10PubMed Central. Posterior tibial tendon dysfunction: an overlooked cause of foot deformity The condition is aptly named “overlooked” in the medical literature: early on, it presents as vague pain and swelling along the inside of the ankle, and many people chalk it up to a strain that will pass. But PTTD is generally progressive; without intervention, the tendon weakens further, the arch collapses, and degenerative changes can spread to surrounding joints.11PubMed Central. Posterior Tibial Tendon Dysfunction: An Overview
Although PTTD is the headline cause, adult-acquired flatfoot deformity (AAFD) is really a spectrum of ligament and tendon failure that has been linked to multiple demographic factors and medical comorbidities.12PubMed Central. Adult-Acquired Flatfoot Deformity Risk factors include obesity, diabetes, hypertension, and prior injury to the foot or ankle. The disorder does not only involve the tendon; it includes a spectrum of pathologic changes in associated ligaments and joint structures of the ankle, hindfoot, and midfoot.13PubMed. Posterior tibial tendon dysfunction That is why staging matters: clinicians classify AAFD from Stage I (tendon inflammation with minimal deformity) through Stage IV (rigid deformity with ankle joint involvement), and treatment differs substantially at each level.
Does Having Flat Feet Actually Cause Problems
This is the question that probably brings most people here, and the honest answer is: less often than you might think. A prospective study of Royal Australian Air Force recruits provided some of the strongest evidence on this point. After tracking recruits through a training period, the researchers found that foot shape was not related to injury. Recruits with flat feet did report poorer subjective physical health than those with normal arches, but they did not sustain more injuries or experience more pain.14Military Medicine. Foot Shape and Its Effect on Functioning in Royal Australian Air Force Recruits. Part 1: Prospective Cohort Study This is meaningful because military training puts feet under heavy, repetitive load, exactly the situation where you would expect flat feet to show up as a liability if they were going to.
That said, flat feet are not universally benign. When the arch collapses significantly, the foot tends to roll inward (pronate) more than it should. That excessive pronation does not stop at the ankle. It drives the tibia and femur to rotate inward, which increases forces across the kneecap and may raise the risk of patellofemoral pain, sometimes called runner’s knee. This is particularly relevant for recreational athletes whose sport involves running and jumping.15PubMed Central. The Association of Foot Arch Variations With Patellofemoral Pain Syndrome in Recreational Athletes In children with flexible flatfoot, researchers have found reduced ankle strength, lower hindfoot inversion velocity at push-off, and decreased ankle energy generation during running compared to children with normal arches.16PubMed. Foot kinematics and running dynamics in children and adolescents with flexible flatfoot So while flatfoot does not automatically mean trouble, it can alter how your entire lower limb moves, and those alterations sometimes show up as pain somewhere upstream.
Flexible Versus Rigid and Why the Distinction Matters
The single most important clinical distinction in flatfoot is whether the deformity is flexible or rigid. In a flexible flatfoot, the arch can still be recreated: standing on tiptoe brings it back, and the foot can be manually repositioned. In a rigid flatfoot, the arch stays flat no matter what. This distinction drives almost every decision about imaging, treatment, and prognosis.17PubMed. Multimodality imaging of the paediatric flatfoot
For flexible flatfoot, imaging is often unnecessary unless there is pain. Weight-bearing X-rays are the standard starting point when evaluation is warranted, with Meary’s angle and talonavicular coverage among the most commonly assessed radiographic parameters.18Journal of Pediatric Orthopaedics B. Diagnosis and treatment of flexible flatfoot: results of 2019 flexible flatfoot survey from the European Paediatric Orthopedic Society CT and MRI are reserved for symptomatic cases or when a structural abnormality like tarsal coalition is suspected. Rigid flatfoot almost always warrants advanced imaging because it usually has a treatable structural cause hiding beneath it.
Conservative Treatment Options
For the majority of people with symptomatic flatfoot, treatment starts without surgery and often stays there. The main conservative approaches are orthotic insoles, targeted exercises, and in some cases footwear modifications.
Custom insoles are the most commonly prescribed intervention. A comparative study found that custom 3D-printed insoles outperformed prefabricated (off-the-shelf) insoles for symptomatic flatfoot, reducing metatarsal pressure by distributing load to the mid-foot area and offering better comfort.19PubMed Central. Comparative Study of the Effects of Customized 3D printed insole and Prefabricated Insole on Plantar Pressure and Comfort in Patients with Symptomatic Flatfoot That does not mean over-the-counter insoles are useless; for mild cases, a decent prefabricated arch support can reduce discomfort considerably. But if you have tried store-bought insoles and they have not helped, the evidence suggests custom versions are worth the extra cost.
Exercise is the other major pillar. A comparative study found that both core stability exercises and intrinsic foot muscle training improved arch height, foot function, and pain in people with flat feet. The intrinsic foot exercises, which directly target the small muscles in the sole of the foot, showed greater improvement on the Foot Function Index, a measure of how well people can perform daily activities.20PubMed Central. Efficacy of Core Stability Exercises and Intrinsic Foot Training on Patients With Flat Foot: A Comparative Study These exercises include things like toe curls, short-foot exercises (where you try to shorten the foot by pulling the ball of the foot toward the heel without curling the toes), and single-leg balance work. They are free, have no side effects, and can be done at home.
There is also growing interest in whether footwear choice itself can strengthen the arch. Research on minimally shod runners found that wearing minimal shoes increased the size of certain intrinsic foot muscles and boosted longitudinal arch stiffness by about 60%.21Journal of Sport and Health Science. The effect of minimal shoes on arch structure and intrinsic foot muscle strength A separate randomized controlled trial looked at a four-week minimalist shoe walking program in young adults and found a significant group difference in a measure of foot posture, though the interaction over time was not significant, suggesting any structural changes may take longer than a month to show up clearly.22PLoS ONE. A four-week minimalist shoe walking intervention influences foot posture and balance in young adults–a randomized controlled trial Transitioning to minimal shoes should be done gradually, because the feet and calves need time to adapt to the increased workload.
When Surgery Becomes the Conversation
Surgery for flatfoot is reserved for cases where conservative treatment has failed and the deformity is causing significant pain or disability. The specific procedure depends heavily on the stage and flexibility of the deformity. For flexible flatfoot that has not responded to months of bracing and physical therapy, joint-sparing reconstructive procedures are the typical first choice. These often involve calcaneal osteotomies, where the heel bone is cut and repositioned to shift the weight-bearing axis back under the leg.
A study of stepcut lengthening calcaneal osteotomy for adult-acquired flatfoot reported substantial improvements: the talonavicular coverage angle improved from about 34 degrees to roughly 9 degrees, and patients showed significant gains in pain scores, daily activities, sport activities, and quality of life. All osteotomies healed at an average of about eight weeks with no cases of delayed union or nonunion.23PubMed. Outcomes of a Stepcut Lengthening Calcaneal Osteotomy for Adult-Acquired Flatfoot Deformity A similar study on an extended Z-cut osteotomy found the Foot Function Index improved from about 52 down to roughly 10, with a 100% union rate and deformity correction well maintained in most patients at final follow-up.24PubMed. Outcomes of the Extended Z-Cut Osteotomy for Correction of Adult Acquired Flatfoot Deformity
When the deformity is rigid or the joints have developed arthritis, fusion procedures become necessary. In a study comparing joint-sparing reconstructions to fusions, the vast majority of patients (about 87%) underwent the joint-sparing approach, with fusions reserved for the remaining cases. Fusion patients actually achieved greater radiographic correction, but both groups had meaningful improvement. About a quarter of joint-sparing patients needed a reoperation, compared to roughly a third of fusion patients, though the difference was not statistically significant.25PubMed Central. Fusion Versus Joint-Sparing Reconstruction for Patients With Flexible Flatfoot Fusion trades some foot mobility for stability and pain relief, which is a reasonable exchange for someone whose joints are already arthritic and painful.
What the Kinetic Chain Actually Looks Like
The downstream effects of flatfoot on the rest of the body are often discussed in vague terms, so it is worth being specific about the mechanism. When the arch collapses during the middle phase of a stride, the subtalar joint pronates excessively. That pronation forces the tibia to rotate inward. Normally, the tibia needs to rotate outward as the knee extends to lock into full extension through what is called the screw-home mechanism. When excessive pronation prevents that outward rotation, the femur compensates by rotating inward over the tibia instead. The result is an increased angle at the kneecap and a greater lateral pull from the quadriceps, which overloads the cartilage on the outside of the kneecap joint.26PubMed Central. The Association of Foot Arch Variations With Patellofemoral Pain Syndrome in Recreational Athletes
This chain of events helps explain why some people with flat feet develop knee pain that no amount of knee-focused physical therapy seems to fix. If the root cause is at the foot, treating the knee alone is treating a symptom. Addressing the pronation with orthotic support or foot-strengthening exercises can sometimes resolve knee pain that was otherwise stubborn. It also explains why clinicians evaluating chronic knee problems will often look at how a patient’s feet behave during walking and running, even if the patient has never complained about their feet.
Flatfoot in Athletes and Military Populations
The old idea that flat feet disqualify someone from military service or athletic performance is largely outdated. As the Australian military recruit study showed, flat feet did not predict injury during intense physical training.27Military Medicine. Foot Shape and Its Effect on Functioning in Royal Australian Air Force Recruits. Part 1: Prospective Cohort Study Many elite athletes compete successfully with flat arches. The critical variable is whether the flatfoot is symptomatic. An asymptomatic flat arch that functions well under load is not a performance limiter. A symptomatic one with reduced push-off power and pain, as seen in the pediatric running study where flexible flatfoot was associated with lower ankle strength and reduced energy generation at the ankle, could limit performance in power-dependent sports.28PubMed. Foot kinematics and running dynamics in children and adolescents with flexible flatfoot
For recreational runners and athletes who do have symptomatic flat feet, the practical advice is straightforward: start with intrinsic foot exercises and, if needed, supportive insoles. Monitor for knee or shin pain that might indicate the pronation chain is causing problems upstream. And avoid the two extremes of panic (flat feet do not doom you) and denial (if they hurt, ignoring them allows preventable damage to accumulate). If you ran a marathon with flat feet and nothing hurts, you have flat feet that work. If walking to the grocery store leaves the inside of your ankle aching, that is worth investigating before the tendon deterioration described in adult-acquired flatfoot gets harder to reverse.