The plantar fascia is a thick band of gristle that runs from the heel to the ball of the foot, just behind the big toe. Unlike a tendon or a muscle, the fibres that make up a fascia are completely non-stretch.
When your foot is sitting properly, the band stretches across the arch, travelling a particular distance. Looking at the line drawing below, you can see that both triangles have the two upper arms of the same length. The more arched one has a shorter bottom arm.
The same is true in the foot.
If the arch of the foot drops (flatfoot, pronation, dropped arches) the distance that the plantar fascia has to cross increases. Because the band cannot stretch, it will pull on its weakest attachment. For most people this is directly under the heel. It can also be around the rim of the heel or through the arch. The area becomes inflamed and, if present for long enough, the body may lay down extra bone to try to bring the two attachment points closer together. This is called a heel spur. It is important to know that a heel spur does not cause the pain. It is the pulling that causes the heel spur to grow and the pulling that causes the pain.
A heel spur almost never needs to be cut out and surgery is very rare these days. Also, pads commonly for sale to cushion the heel are not useful, as the problem is not downward pressure on the heel (although it feels like it is) but elongation pressure along the length of the foot. Pain is usually bad for the first couple of minutes of standing after rest or sleep. It will then often improve before worsening again with ongoing use. Standing still may be as bad as, or worse, for pain than walking.
When pain occurs in the heel, your body will often recruit a muscle running from the front of the shin bone to the midpoint of the inside edge of the arch (shown in the diagram at X) to reduce pressure on the fascia. This can result in pain in either of these areas.
What can be done for it?
The key to eliminating the pain is to reposition the foot so that the arch is normal again. This can be done in a simple way if the problem is recent or mild, or a more potent way for worse problems.
In assessing you, the podiatrist will discuss these options with you. If one method is clearly better for you, it may be that a decision is made on how to progress at the first meeting. If there is doubt about how much intervention is necessary to get better, the two approaches can be trialled in temporary form over a week to get a very good idea of likely outcomes.
Steroid injection may be used as a last resort as, although effective, it is quite painful and ultimately weakens the tissue making the problem more likely to recur in the future.