Flat Feet and the Tibialis Posterior: The Muscle Holding Your Arch Together
Most people think of flat feet as a shape problem, an arch that never formed properly. In reality, for a large proportion of cases, it's a muscle problem. Specifically, it's a failure of one muscle most patients have never heard of: the tibialis posterior.
The Muscle Doing the Real Work
Your foot's arch isn't held up by bone alone. It's a dynamic structure, actively supported by the tibialis posterior, a deep calf muscle that runs down the inside of the shin, passes behind the inner ankle bone, and attaches into the underside of the midfoot. Every time you push off while walking or running, tib posterior contracts to lift and support the arch, control how much your foot rolls inward (pronation), and stabilise the whole lower limb during single-leg loading.
When this muscle is doing its job well, you don't notice it. When it isn't, the arch has nothing left to hold it up, and that's when flat feet, in adults, tend to develop or worsen.
What Happens When Tib Posterior Fails
Tibialis posterior tendinopathy (TPT) is a common overuse injury, particularly in runners, and one of the most under-recognised causes of progressive adult flat foot. It develops when the tendon is subjected to load beyond what it can handle, leading to inflammation, pain, and a gradual loss of its ability to support the arch. Left unaddressed, the arch flattens further, the extra strain accelerates wear on the tendon, and the cycle reinforces itself.
Common contributing factors include:
Overuse. Increasing training volume or intensity too quickly, particularly a sudden spike in running mileage or speed, can overload the tendon before it's adapted.
Biomechanical issues. Abnormal foot mechanics such as overpronation or flat feet place excessive strain on the tendon. Runners with high arches can also be at risk, since a rigid arch absorbs less shock.
Weak calf muscles. Inadequate calf strength forces the tibialis posterior to compensate for stability it wasn't designed to provide alone.
Tightness in the calf or Achilles. Tight calf muscles or Achilles tendon alter running mechanics and add extra load onto the tibialis posterior.
Sudden changes in activity. Moving to harder running surfaces or adding hill running increases strain on the tendon before it's had time to adjust.
Left untreated, this can progress from mild arch collapse to a more rigid, fixed flat foot deformity, along with pain along the inner ankle and shin, swelling, and a visible change in
foot shape over time.
The Impact on Running
For runners, a weak or dysfunctional tib posterior doesn't just affect the foot. It changes load distribution up the entire kinetic chain. An arch that collapses excessively during stance:
Increases internal rotation through the tibia and knee
Alters hip mechanics and glute activation during stance phase
Reduces the foot's ability to act as a rigid lever at push-off, costing running efficiency
Increases strain on the plantar fascia, Achilles, and shin, all common sites for overuse injury
This is why many runners with recurring shin splints, plantar fasciitis, or medial knee pain never fully resolve their symptoms by treating the painful site alone. If the tib posterior isn't controlling the arch properly, the downstream structures keep absorbing load they weren't designed for.
How Physios Diagnose It
A proper diagnosis starts before any hands-on assessment. Your physiotherapist will first take a detailed history: how the pain started, what it feels like, exactly where it's located, when it worsens, and which activities (running, in particular) tend to trigger it. This step is critical for building an accurate picture and an effective treatment plan.
From there, a physical examination follows, typically including specific muscle tests to assess strength and flexibility, and palpation to pinpoint areas of tenderness. The goal is to establish how well the tendon is functioning, identify any weaknesses or imbalances, and understand how the ankle and foot are moving under load.
Not Every Flat Foot Needs Treatment
It's worth saying clearly: flat feet alone aren't automatically a problem. Plenty of people run and train pain-free with low arches their entire lives. What matters is whether the tib posterior is functioning well enough to control the arch under load. The warning signs worth getting assessed are:
Pain or tenderness along the inner ankle or shin
A visible change in arch height over months or years
Difficulty performing a single-leg heel raise
Recurring lower limb injuries that don't respond to standard treatment
How Urban Physiotherapy Can Help
At Urban Physiotherapy, assessing flat feet and tibialis posterior issues means looking beyond the foot itself. Treatment is tailored to your findings and may include:
Manual strengthening techniques, using specific mobilisation methods to improve tendon function, restore proper alignment, and gradually rebuild strength.
Soft tissue therapy and massage, releasing tension in the calf and Achilles to reduce the load being transferred onto the tibialis posterior tendon and support healing.
Joint mobilisations, improving ankle and foot mobility, reducing stiffness, and restoring the range of motion needed for normal tendon function.
Taping and offloading techniques, using methods like kinesiology taping to reduce strain on the tendon and provide support while it recovers.
Footwear and posture advice, assessing your foot alignment and recommending footwear that reduces strain on the tendon during daily activity and running.
Where there's chronic tendon involvement that hasn't responded to standard rehab, we also offer shockwave therapy to stimulate healing in degenerated tendon tissue and support a faster return to full function.
If you've been dealing with recurring shin, ankle, or arch pain, don't just treat the symptom. Get the underlying mechanics assessed properly.
Get in touch with us at (02) 8526 4000 or info@urbanphysiotherapy.com.au
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