The purpose of this study was to gain knowledge into the technical factors that maintain the mechanical effectiveness in the second half of the men's 20-km walking race. Video data in the first and second halves of the race were collected by two high speed cameras. 58 subjects were analysed, and the kinematic and energetic variables were calculated using the three-dimensional DLT method. The differences between the first and second halves of the race about these variables were calculated. Based on amount of change in the walking speed index and the effectiveness index (EI), the subjects were classified into three groups by cluster analysis. Differences in the amount of change between the groups and the trunk movement within the groups were examined. The results showed that among the athletes who maintained the walking speed index, those who lowered their EI had significantly greater trunk forward leaning during the entire support phase in the second half of the race. In addition, the amount of change in the velocity of center of gravity during the deceleration phase in the support phase was increased. Furthermore, the torsion angle of the trunk joints increased significantly, and the mechanical energy transfer decreased. These results suggest that it is important to control changes in trunk forward leaning and trunk joint torsion to maintain mechanical effectiveness under the fatiguing conditions of the latter half of a men's 20-km walking race.
