Plantar fasciitis or plantar heel pain is a painful disorder of the fasciaplugin-autotooltip__plain plugin-autotooltip_bigThe Fascia

Fascia are whitish bands or sheets of connective tissue - mainly composed of collagen and fat, which stabilise, enclose, and separate muscles and other internal organs.

An exact definition has been much disputed over the years. Aside from 'compartmentalising' the various organs etc, its functions are poorly described and not well understood. of the heel and sole of the foot.

The pain typically starts gradually, especially after exercise, and can affect both feet (in about one-third of cases). It can be so severe as to make normal walking impossible. It's the most common cause of acquired sub-calcaneal heel pain in adults, and recovery can take several weeks

The cause or causes of Plantar fasciitis are not yet fully understood.

Although poorly understood, the development of plantar fasciitis is thought to have a mechanical origin. In particular, pes planus [flat] foot types and lower-limb biomechanics that result in a lowered medial longitudinal arch are thought to create excessive tensile strain within the fascia, producing microscopic tears and chronic inflammation. However, contrary to clinical doctrine, histological evidence does not support this concept, with inflammation rarely observed in chronic plantar fasciitis. Similarly, scientific support for the role of arch mechanics in the development of plantar fasciitis is equivocal, despite an abundance of anecdotal evidence indicating a causal link between arch function and heel pain.

Source : Journal of Sports Medicine; 36 (7)

Note that because it's now generally thought to involve little (or no) inflammation, there are proposals to rename the condition plantar fasciopathy.(ref.)