A whole body approach to healthy plantar fascia
If you are suffering pain in the heel or sole of the foot, it might be wise to consider not only the symptoms and the structures, but the pre-symptomatic condition of those structures of the foot, the usage and/or lack of usage of the foot and how that can be essentially a "pre-existing condition." In other words, we don't think there's anything wrong with our feet until they hurt. But it could be the case (just like a disc injury) that the damage was cumulative and it was the proverbial straw that broke the camel's back that tipped the scales to the diagnosis.
So what is Plantar Fasciitis and how is it diagnosed? The definition - itis the suffix for inflammation, is an inflamed plantar fascia. This is a structure on the sole of your foot, under and next to the skin that runs from the front edge of the heel bone to the toes at the ball of the foot. In fact, a lot of researchers believe plantar fasciitis is degeneration (known by the suffix osis or plantar fasciosis) not inflammation, so taking NSAIDS (with their associated side effects) might not be as helpful as we wish.
Plantar fasciitis is usually experienced as pain in the medial (inside, or big toe side) heel. Occasionally it can spread through the whole foot and toes. It's usually only in one foot, 30% of sufferers have symptoms in both feet. If you have a diagnostic to see the inside of the foot, a thickened plantar fascia is indicative of fasciitis, and a thickness greater than 4mm is considered a diagnosis (normal being 2-3mm). What (besides inflammation) could render a tissue thicker (and in the words of one study - "gritty")?
So what other structures/tissues does the plantar fascia affect? Well, surprisingly, it attaches to the skin, particularly at the heel and ball of the foot, through septa that run vertically from the plantar fascia to the skin. This provides traction at these parts of the foot, that connect to the ground. Otherwise, the tissues would slide on each other, and your foot would be on "loose skin" when it needed to grip instead. Isn't that fascinating?
Even more fascinating is the quality of the skin of your foot. In 2018 I broke my ankle in three places which required surgery and 13 weeks of immobilisation in a cast. When the cast came off, I was left with baby skin on the sole of the foot, but thick rough skin on the top that didn't move, rendering me unable to spread my toes. So even the skin on your foot can tell you a lot about the condition of your foot as a whole.
The plantar fascia also connects to the muscles beside it, through their associated fascia. These muscles move the big and baby toe respectively. So if you spend a lot of time in shoes with narrow toe boxes that don't allow toe spread, these muscles atrophy and weaken and the big and baby toes reside too close to their neighbours, which could also affect the (lack of) tension on the plantar fascia from the sides (perhaps narrowing and thus thickening it). In the area where the plantar fascia connects to these muscles, it is well innervated (nerves, hence pain) and also where it connects to the muscles deep to it (including muscles that flex the toes).
The plantar fascia also has mechanoreceptors (movement sensors), particularly on the sides again, and for this reason the plantar fascia has a role in proprioception (it perceives the state of contraction and position of various foot muscles). If your feet lack varied positions, experiences, sensations, surfaces, the message rarely differs and is given less "brain space."
The relationship of the plantar fascia with the paratenon (covering) of the achilles tendon is consistent with the idea that the calf musculature is involved in plantar fasciitis pathology. The calf musculature can be shortened passively from constant heel wearing and sitting. The exercise most commonly prescribed for plantar fasciitis is calf stretching! However, it is thought a poor response to treatment might be due to inappropriate and nonspecific stretching techniques. The Restorative Exercise Calf Stretch is performed in an objective manner so that everyone can do it safely (and not over-do it).
The hamstrings cross the back of the knee as well, hence the whole posterior leg needs to be addressed as a solution for plantar fasciitis.
So you might see with just this minimal amount of information about the plantar fascia, it is not a simple "stretch your calves" kind of solution. In fact, it requires a whole body solution. Very few issues in the human body are isolated ones.