The flat foot gains significant clinical attention because of its harmful effects not only on the ligamentous structures of foot but on the lower leg musculature, knee and hip joint. And especially the flexible flat foot changes with dynamic body weight appliance. So we studied 40 normal person's three dimensional motion analysis of foot and ankle complex during stance phase of walking to assess the dynamic change of medial longitudinal arch and other structures.

We concluded as below;

1) Medial longitudinal arch was mostly widen when 45% of gait cycle. And mean widening of angle was 4.2degree and standard deviation was 3.3degree.

2) If a navicular angle without weight bearing would be greater than 155.4degree, then that foot could be diagnosed as rigid flat foot.

3) If a navicular angle widening during stance phase would be greater than 10.8 degree, then that foot could be diagnosed as flexible flat foot.

In addition to proposing the diagnostic criteria for rigid and flexible flat foot, we present the relationship among the dynamic angles of MP joint, subtalar joint and arch height which needs further biomechanical research.