True plantar fasciitis has to be caused by one thing: too much force going through the plantar fascia.
True plantar fasciitis can only be fixed by one thing: lower the force going through the plantar fascia.
Many risk factors for plantar fasciitis have been documented, with the most common ones being tight calf muscles and body weight. Both of these must increase the load in the plantar fascia. The most likely mechanisms being the weight carried by the plantar fascia (should be obvious in those who are overweight) and the tight calf muscles as many of the fibres from the achilles tendon pass through to the plantar fascia.
No one has yet documented a pronated foot as a risk factor, yet you often see it written that it is the cause! Foot orthotics have been shown to be successful in treating plantar fasciitis, but that does not prove that a pronated foot was the cause.
At the Boot Camp, I talk about the device that we have built that measures the force to dorsiflex the hallux. In one study we used the device in those with unilateral plantar fasciitis. The force was measured to dorsiflex the hallux to 0, 4, 8, 12, 20 and 30 degrees. The forces needed to do this was approximately double on the symptomatic side compared to the asymptomatic side (eg at 8 degrees, it was 19.7N compared to 37.5N).
This could mean one of two things. It could mean that this high force is the cause of plantar fasciitis or it could mean that this high force is the result of plantar fasciitis (in that the changes in tissue properties as a result of the plantar fasciitis).
Whichever it is, that force has to be lowered if the plantar fasciitis is to be resolved. One of the most effective ways to lower it is via low dye strapping. Using the same device, we have shown that this can reduce the force close to zero. Try it yourself: Have the patient stand; use your hand to dorsiflex the hallux; get a feel for how much force is needed; apply the low dye strapping and repeat the test; it’s really easy to now dorsiflex the hallux.
Foot orthotic design parameters also have to lower that force if they are to be successful.
For the latest on plantar fasciitis, see Run Research Junkie and I blogged here on its snake oil! And here is an article I wrote that got translated into Vietnammese.