The effect of cyclic loading on the stiffness and damping factors in the midsoles of running shoes

摘要

McMahon and Greene showed, in 1979, that it is possible to design a running track with a compliance that will reduce the shock loading on the legs of the athletes, and yet will enable them to return fast times. Since then, attempts have been made to find a combination of properties in a running shoe that would achieve the same ends. The extent to which the stiffness of the midsole may be reduced is limited by the need for the athlete's ankle to maintain a high degree of stability. On the other hand, it has been predicted from modelling studies that the damping properties of running shoe midsoles maybe controlled to reduce the impact shock that is imparted to each leg of a runner, as the heel strikes the ground at the initiation of the contact phase. A method has been developed for deriving the damping parameters (logarithmic decrement and damping factor) from an analysis of the complete load deformation curve, both for a single cycle and after several thousand load cycles. The stiffness and damping properties of five types of running shoes have been measured under low frequency loading, and also at 1 Hz - close to the frequency of heelstrike. It was found that although the damping factors range from 0.02 to 0.04 at low frequency, they are reduced to a range between 0.013 and 0.03 when the load is cycled at 1 Hz. In a similar fashion, the stiffness values increased under cyclic loading, so that the energy absorbed by the midsole was reduced by about a factor of two. These findings were found to be repeated across all the types of shoes that were tested, and related to the basic deformation mechanisms in the polymers used in the midsoles.