The purpose of this study was to investigate whether or not the neuro-muscular locomotor system is optimized at a unique speed by examining the variability of the ground reaction force (GRF) pattern during walking in relation to different constant speeds. Ten healthy male subjects were required to walk on a treadmill at 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 km/h. Three components (vertical (Fz), anteroposterior (Fy), and mediolateral (Fx)) of the GRF were independently measured for about 35 steps consecutively for each leg. To quantify the GRF pattern, five indexes (Fzp1, Fzp2, Fyp1, Fyp2, and Fxp) were defined. Coefficients of variation (CVs) were calculated for these five indexes to evaluate the GRF variability for each walking speed. It became clear for Fzp1, Fzp2, and Fxp that index variabilities increased in relation to increments in walking speed, whereas there was a speed (5.5 - 5.8 km/h) at which variability was minimum for Fyp1 and Fyp2, which were related to forward propulsion of the body. These results suggest that there is `an optimum speed' for the neuro-muscular locomotor system, but only for the propulsion control mechanism.