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

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.