Box plot of the NH (a), V
(c), and AVI (d) in groups from ages two to six. The box includes observations from the 25th to the 75th percentile; the horizontal line within the box represents the median value, and the square represents the mean value. Lines outside the box represent the 1th and 99th percentiles. NH: navicular height Vn: the volume under foot arch in a sitting position Vf: the volume under foot arch in a standing position AVI: arch volume index.