This study examined the safety and accessibility of stairs for elderly people. A motion capture system was used to measure the heel trajectory while descending the stairs. In our earlier study, we found that the heel trajectories tended to become more linear the faster the subjects descended the stairs, so we developed a mathematical model to reflect the linear trajectory. Using this model, we could calculate the K-value, which is the shortest distance between the heel and the stair nosing. The

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... value varied with the dimensions of the stairs, the tread and the rise, and other foot positioning parameters for normal descending and non-normal descending. A negative K-value indicated that the heel trajectory might hit the stair nosing if the subject descended hastily. In this study, it was assumed that such stair dimensions could be dangerous. As a result, the stair dimensions recommended by "The act on buildings accessible and usable for the elderly and physically disabled" were judged to be safe provided that elderly peopled descended normally. In addition, we considered that the elderly could not descend normally stairs with dimensions of a rise of less than 10 cm and a tread of more than 30 cm, which are used in public places. The dimensions recommended by the "Guidelines of housing design device to cope with aged society" were judged to be rather dangerous. However it was unsuitable that stair dimensions of our recommendation need more capacity for housing in Japan.