In running, the muscles exert forces acting internally on the body and externally on the ground to move the body's center of mass forward. The energetics of running can be studied by calculating efficiency: mechanical work done/net energy consumed. Variations in efficiency, however, can be attributed not only to the human machinery but also to methodological differences. Mechanical work has been calculated primarily by two procedures: one is from energy changes taking place on and around the body's center of mass, and the other is from segmental analysis. Using the former and assuming the energy cost to be 1 kcal kg-1 km-1, we have shown that efficiency increases with running speed from 45 to 70%, with elastic energy recoil playing a role (Cavagna and Kaneko, J. Physiol. 268, 467-481, 1977). However, the efficiency value decreases with speed if the oxygen debt is included in the calculation of the energy cost. With the same method we have also shown that the efficiency of distance runners is higher than that of sprinters if the running speed is less than 25 km h-1, and that there is a most economical step rate for maximizing the efficiency at a given constant running speed.