Abstract
A motion capture system was used to measure the heel trajectory during descending stair of twenty young healthy male and female subjects. Two kinds of the trajectory were found. One was the straight line type trajectory, and the other was the curved line type trajectory as if the heel kept away from the nosing of the stair. The former were found often when the descending cadence was high. In this condition, the heel clearance lengths were short. A mathematical model was developed to express this line type trajectory. Using this new model we could calculate the K-value which shows the shortest value of the distance between the heel and the nosing of the stair. These values were corresponded well to the values of the actual heel clearance. The K-value varies with the dimension of the stair, the tread and the riser, and other foot positioning parameters. When the K-value is negative, it means that the heel trajectory might hit the nosing of the stair if he or she walks down hastily. This kind of stair dimension supposed to be dangerous. When the K-value stays positive on the wide range of the parameter set, this dimension is adequate as the safe stair. The dimension of the minimum standard of Guidlines of Housing Design Device to Cope with Aged Society was judged as safe.
