Automatic calculation of the medial axis of foot outline and its flexion angles

As one of the morphological characteristics of human foot, flexion angles of medial axis of foot outline have been proposed in order to improve shoe comfort. Foot outlines were easily obtained by a scriber, whereas calculation of the medial axis and its flexion angles from the foot outline had inter-operator errors and a lack of reproducibility. This is because foot outlines were inputted by a digitizer, and flexion points of the medial axis were determined by eye inspection. An automatic method of calculation was developed for the following purposes: (1) to decrease inter-operator errors caused by manual operation; (2) to improve the reproducibility of the medial axis and its flexion angles; and (3) to save time and trouble in processing a large amount of data. The flexion angle reproducibility of the present method was within 1 degree. The errors in the flexion angles determined by the present method are half those determined by the conventional method. The flexion angles of over 5000 foot outlines that were obtained in 1987 were calculated when the present method was developed. The mean posterior flexion angle of the adults was 8.4 degrees. Since the majority of Japanese feet are outflared, this characteristic should not be ignored in designing/modifying shoe lasts.