When standing on both feet the mean height of the arch at the tangent to the medial plantar imprint is 5 mm at the age of 4 years. The "critical limit" is approx. 2 mm. The height of the medial longitudinal arch increases each year. The increase is greater at the age of three than at six years, and should be at least 1 mm between the third and fourth years of life. Girls have higher arches than boys of the same age. The medial longitudinal arch is lower in children with superextensible knee joints and hypermobile ankle joints. The talocalcaneonavicular joint becomes more mobile as the height of the medial longitudinal arch increases.