Under the arch towards the heel: plantar fasciitisStrong evidence
The NHS describes plantar fasciitis as pain on the bottom of the foot, around the heel and arch. It's more likely if the pain is much worse when you first start walking after sleeping or resting, gets better as you move around but worse if you stand, walk or run for a long time, and hurts when you stretch the sole, for example raising your toes off the floor or walking upstairs. The NHS page on pain in the bottom of the foot calls it sharp pain between the arch and the heel.
It's more likely if you're 40 to 60, have recently started exercising on hard surfaces, exercise with a tight calf or heel, do a lot of standing, walking or running (or a lot more than usual), wear shoes with poor cushioning or support, or are overweight, the NHS says.
Sources 1, 2
Along the inner arch and ankle: the tibialis posterior tendonStrong evidence
The tibialis posterior tendon runs behind the inner ankle bone to the underside of the arch and helps hold it up. A clinical review calls its failure the commonest cause of a flat foot that develops in adulthood in otherwise healthy people, with women over 40 most at risk. It is usually diagnosed from the story and an examination, but most people are first told they have a sprain or arthritis. From behind, more toes show on the outside of the affected foot, and while someone with a normal foot can do 8 to 10 single heel raises, with this problem it's hard or impossible.
In a survey of women over 40 at one English general practice, followed by examinations, 3.3% of those who replied had early-stage disease, all undiagnosed despite long-standing symptoms. The American Academy of Orthopaedic Surgeons (AAOS) lists pain in the arch or along the inside of the foot and ankle, swelling and pain just behind and below the inner ankle bone, and difficulty rising onto tiptoe on one leg, and says it is more common in women, over 40, and with obesity, diabetes and high blood pressure.
Sources 3, 4, 5
Flat feet: usually not the problemEmerging evidence
The NHS says flat feet usually cause no problems, shouldn't stop you doing any activities, including sport, and are rarely a sign of anything serious; children usually develop arches between 3 and 10. A flexible flat foot's arch comes back when you sit or rise onto tiptoe, and it is usually painless, the AAOS says.
In 3,378 adults in the Framingham foot study, a flat foot went with arch pain in men (odds ratio 1.38) but not in women. Across 21 cohort studies of 6,228 people, a flat (rolled-in) foot was a small risk factor for shin splints and, on very limited evidence, kneecap pain, but not for foot and ankle injuries or bone stress injuries; in 927 novice runners in neutral shoes, rolled-in feet weren't at higher risk of injury. The NHS says see a GP if flat feet are painful, stiff, weak or numb, new, or only on one side.
Sources 6, 7, 8, 9, 10
A stiff flat foot in a teenagerEmerging evidence
A tarsal coalition is a bridge of bone, cartilage or fibrous tissue joining two foot bones. The AAOS estimates it affects about 1 in 100 people, both feet in about half; it usually hardens between 8 and 16, and can cause stiff, painful feet, a rigid flat foot, repeated ankle sprains, and more pain or a limp with more activity. Many coalitions show on X-rays, and a computed tomography (CT) scan shows them best.
Source 11
Nerves, lumps, bones and crampEmerging evidence
Tarsal tunnel syndrome is a squeezed nerve behind the inner ankle. A clinical review says its symptoms in the sole tend to worsen with walking and can look like diabetic nerve damage or a back problem, and that a magnetic resonance imaging (MRI) scan can show things that aren't the cause. A plantar fibroma is a harmless growth in the band under the arch, mostly in middle age, that can form painful lumps but usually doesn't cause the tightening (contracture) the hand form does; treatment ranges from watching it to surgery.
A navicular stress fracture causes pain that is hard to pinpoint, worse when you put weight on the foot and with sprinting or jumping; the AAOS describes the navicular as a bone on the top of the midfoot, so where it's felt in the arch is our inference. Pooled case series favour a cast with no weight on the foot. A sudden pop in the arch is rarely a torn plantar fascia: a case report lists sudden increases in activity, a direct injury, steroid injections and long-standing plantar fasciitis as things that make it more likely. Cramps can affect the feet and last from a few seconds to 10 minutes, the NHS says.
Sources 12, 13, 14, 15, 16, 17
Pregnancy and the archEmerging evidence
In 49 women measured early in pregnancy and again 19 weeks after giving birth, arch height and stiffness had fallen and feet had got longer, and the first pregnancy accounted for the loss of stiffness and the lengthening. The researchers think the loss of arch height is permanent, though they followed women only to 19 weeks, and they call for research into exercises to prevent it.
Source 18
When a scan or test helpsStrong evidence
A tibialis posterior tendon problem is usually diagnosed from the story and an examination, with the single-leg heel raise as the key test. X-rays taken standing show the shape of a collapsing flat foot and any arthritis, and an MRI scan can show inflammation or a tear when the diagnosis is in doubt, the AAOS says. A teenager's stiff flat foot needs X-rays, and a CT scan shows a coalition best; a suspected navicular stress fracture may need a CT or MRI scan.
Burning or numbness that persists needs a GP; in tarsal tunnel syndrome, an MRI can show a squeezed nerve but can also mislead. With diabetes, a suspected Charcot foot gets an X-ray taken standing, and an MRI if the X-ray is normal but the doubt remains, the National Institute for Health and Care Excellence (NICE) says.
Sources 3, 5, 11, 14, 12, 19
How it usually goesEmerging evidence
The AAOS says more than 90% of people with plantar fasciitis improve within 10 months of starting simple treatment, and the NHS says see a GP if it hasn't improved after 2 weeks of looking after it yourself. For a tibialis posterior tendon problem, the AAOS says most people's symptoms are relieved without surgery, though the pain may last longer than 3 months even with early treatment; left alone, the problem can slowly flatten the foot. Flat feet that don't hurt need no treatment. No one can promise you a date.
Sources 20, 1, 5, 3, 6