Pes planus

Pes planus (or flat foot) is a lowering or dropping of the foot arch. About 20% of the adult population have flat feet, and in the majority it does not cause symptoms. Flat foot may be a risk factor for chronic pain in the foot and other structures of the lower limb and lower back because of altered biomechanics associated with flat foot.

The foot arch is formed by numerous foot bones (tarsal and metatarsal bones) and is supported by four layers of muscles in the sole of your foot, six muscles arising from your leg, a number of foot ligaments and a very strong thick band of connective tissue that acts like a bowstring, called plantar fascia.

Like an arched bridge the plantar fascia helps distribute your weight through your foot, providing it with maximum stability in weight bearing and minimises energy costs by maximising leverage for your muscles and spring of your ligaments when walking and running.

Failure (weakness) of your foot and/or leg muscles (especially the tibialis posterior and peroneus muscles) due tendinosis is the most common underlying cause of flat foot.  Failure of support from the muscles exposes the passive tissues of the arch, including ligaments, the plantar fascia and bone, to strain and pain.

A person with flat feet will likely stand and walk with their feet pronated, where the rolls inward. This makes the person susceptible to heel pain (plantar fasciitis and heel spurs).