足と靴のバイオメカニズム

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  • BIOMECHANISM OF FOOT AND SHOE

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The foot is composed of approximately thirty bones and several ligaments, and its size and shape are deformed by the force applied while walking. Deformation is not only an important factor of the fitness of the foot to the shoe, but also the cause of many problems concerning both elements. In this study, we attempted to measure the dynamic change of the foot-size and the force applied while walking. The foot-length and the foot-width were measured by a newly developed light displacement sensor using strain gages. The footheight and the whole motion of the foot were measured by the 16mm cine-film analysis system. The force applied to the heel, the medial fore part and the lateral fore part of the foot were measured independently by a special designed three-parted force plate. Each plate is capable of measuring the three components of the floor reaction force and the center of application. Further, in order to know the pressure distribution of the foot in the shoe, small and thin pressure gages were fixed on the heel and on the side part and the sole part of the big toe and the little toe. Using these measuring instruments and a microcomputer, we composed a total measuring system for shoe-walking. The experimental results from nine subjects show that the average length chage of arch chords is about 10 to 15mm during normal level walking, but that the tarsalia part is little deformed at any stage of walking. According to the characteristic of foot deformation, the foot structure can be simplified by three rigid link models such as the big toe link, other toes link and the heel link which is connected at the ankle joint. The pointed ends of these links correspond to the base of transverse and the longitudinal arch of the foot. From the mathematical link model and the measured data, we calculated the muscle and the ligament forces of arch support. And also, comparing barefoot-walking with shoe-walking, we discussed the reasonable design of the shoe. Consequently, the following results were obtained. (1) The maximum deformation of the foot during the normal level walking is almost 16mm at the medial longitudinal arch. (2) The maximum percent is 15% at the foot-height. (3) The big toe is always flexed while walking, like a grasping motion. (4) The long planter ligament exerts about 30% of arch-support force. (5) The suitable heel-height decreases the muscle load while walking. (6) The height of pressure at the medial fore part of the foot effects comfort in shoe-walking. (7) The rolling motion of the foot decreases with shoe-walking. Especially the application point of the floor reaction force to shoes with flexible soles moves straight and smooth like a wheel.

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