Heel or foot pain from plantar fasciitis is a very common problem that can occur at any age, usually occuring after trauma to the foot and so typically only affects one foot , but can occur on both feet at the same time.
The plantar fascia is an extension of the Achilles tendon which runs under the heel then spreads out across the sole of the foot and inserts into the base of the toes. Its function is to provide shock absorption on the sole of the foot as well as to lift the arch of the foot when walking.
Plantar fasciitis is inflammation of the plantar fascia which causes pain under the heel or in the instep which is usually worst either first thing in the morning when one gets up or after a period of non-weight bearing rest and one puts weight on the foot. The pain will often initially ease with movement, but after a while overuse will aggravate it too.