The purpose of this study was to investigate the relationship between plantar flexion angle and underwater dolphin kick performance using a simulation model, SWUM (SWimming hUman Model), in which the entire body was represented as a series of elliptic cylinders. Three unsteady fluid forces (added mass force, normal and tangential resistive force), buoyancy and gravity acting on the elliptic cylinders were computed from the shape and density of the cylinders, and from the joint motion for one cycle. Eight elite competitive swimmers participated in this study. Their body characteristics were measured and input as the simulation data (the shape and density of the elliptic cylinders). The joint motion data for one cycle were obtained by the 2-D DLT method during the underwater dolphin kick. To recreate the dynamics of the underwater dolphin kick in the simulation, three fluid coefficients, used to calculate the three unsteady fluid forces, were identified to minimize the difference between measured and simulated swimming velocity by the least-squares method. This identification was conducted for each of the input data. Simulations of the increased plantar flexion angle in the knee extension phase were executed with the optimal fluid coefficients for each swimmer in order to analyze the change in swimming velocity and thrust generated by the feet. The conclusions can be summarized as follows : (1) Swimming velocity during the underwater dolphin kick increased due to the increment of the thrust generated by the feet after an increase in the plantar flexion angle. (2) It was suggested that flexibility of plantar flexion is important, although it is not a determinant of underwater dolphin kick performance. (3) Swimmers with low flexibility of plantar flexion tended to achieve a faster swimming velocity after increasing the plantar flexion angle.
