Flat feet
Flat feet, also called pes planus or fallen arches, is a condition in which the arch of the foot collapses, with the entire sole of the foot coming into complete or near-complete contact with the ground. In some individuals (an estimated 20-30% of the general population) the arch simply never develops in one foot (unilaterally) or both feet (bilaterally). It should be noted that being flatfooted does not decrease footspeed.

# Flat Feet in Children
The appearance of flat feet is normal and common in infants, partly due to "baby fat" which masks the developing arch and partly because the arch has not yet fully developed. The human arch develops in infancy and early childhood as part of normal muscle, tendon, ligament and bone growth. Training of the feet, especially by foot gymnastics and going barefoot on varying terrain, can facilitate the formation of arches during childhood, with a developed arch occurring for most by the age of four to six years.
Because young children are unlikely to suspect or identify flat feet on their own, it is a good idea for parents or other adult caregivers to check on this themselves. Besides visual inspection, parents should notice whether a child begins to walk oddly, for example on the outer edges of the feet, or to limp, during long walks, and to ask the child whether he or she feels foot pain (which some have described as feeling like a nail going through the foot) during such walks.
Children who complain about calf muscle pains or any other pains around the foot area, are likely to have flat feet. Recent medical research indicates that arch support inserts and certain heel cups, inserted into a growing child's shoes, can facilitate the proper development of the longitudinal arch, by holding the foot in the correct neutral position while it is growing. There is little debate, however, that going barefoot, particularly over terrain such as a beach where muscles are given a good workout, is good for all but the most extremely flatfooted, or those with certain related conditions such as plantar fasciitis. One medical study in India with a large sample size of children who had grown up wearing shoes and others going barefoot, found that the longitudinal arches of the barefooters were generally strongest and highest as a group, and that flat feet were less common in children who had grown up wearing sandals or slippers than among those who had worn closed-toe shoes.

# Flat Feet in Adults
Flat feet can also develop as an adult ("adult acquired flatfoot") due to injury, illness, unusual or prolonged stress to the foot, faulty biomechanics, or as part of the normal aging process. Flat feet can also occur in pregnant women as a result of temporary changes, due to increased elastin (elasticity) during pregnancy. However, if developed by adulthood, flat feet generally remain flat permanently.
If a youth or adult appears flatfooted while standing in a full weight-bearing position, but an arch appears when the person dorsiflexes (stands on tip-toe or pulls the toes back with the rest of the foot flat on the floor), this condition is called flexible flatfoot. Muscular training of the feet, while generally helpful, will usually not result in increased arch height in adults, because the muscles in the human foot are so short that exercise will generally not make much difference, regardless of the variety or amount of exercise. However, as long as the foot is still growing, there is still a possibility that a lasting arch can be created.

# Diagnosis and Treatment
A podiatrist can easily diagnose a flat foot condition during an office visit. An easy and traditional home diagnosis is the "wet foot" test, performed by wetting the feet in water and then standing on a surface such as cement or heavy paper. If the impression that the wet foot leaves does not show a dry area where the arch should be, it is a good indication of flat feet.
Most flexible flat feet are asymptomatic; not painful. In these cases there is no real cause for concern. Rigid flatfoot, a condition where the sole of the foot is rigidly flat even when a person is not standing, can be legitimate cause for concern, however.  Other flatfoot-related conditions, such as various forms of tarsal coalition (two or more bones in the midfoot or hindfoot abnormally joined) or an accessory navicular (extra bone on the side of the foot) should be treated promptly, usually by the very early teen years, before a child's bone structure firms up permanently as a young adult. Both tarsal coalition and an accessory navicular can be confirmed by x-ray.
Treatment of flat feet may also be appropriate if there is associated foot or lower leg pain, or if the condition affects the knees or the back. Treatment may include using arch supports/orthotics, foot gymnastics or other exercises as recommended by a podiatrist or other physician.  Surgery, while a last resort, can provide lasting relief, and even create an arch where none existed before, but is usually very costly.
Several studies of soldiers explored the link between arch height and stress fractures. One study of 287 Israeli Defense Force recruits found that those with high arches suffered almost four times as many stress fractures as those with the lowest arches. One later study of 449 US naval special warfare trainees found no significant difference in the incidence of stress fractures among soldiers with different arch heights and another was inconclusive.