The purpose of this study is to proposes viscoelastic model of landing shock load during running at a steady pace. The vertical components of landing shock loads are measured by force platform under various running speeds in the range of 2.54 m/s to 5.68 m/s. A unique set of parameters of the model for each trial is computed by nonlinear programming, minimizing the sum of the relative standard error of landing shock load and the relative error of rebound velocity. The results are summarized as follows: (1) The viscoelastic model with variable mass proposed in this study can estimate accurately not only the landing shock load but also the rebound velocity. (2) Nonlinear unsymmetrical viscosity behaves as an active element because of its minus viscosity in the restore phase.
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