The purpose of this study was to investigate whether or not the neuromuscular 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 (F_z), anteroposterior (F_y), and mediolateral (F_x) force] of the GRF were independently measured for ~35 steps consecutively for each leg. To quantify the GRF pattern, five indexes (first and second peaks of F_z, first and second peaks of F_y, and F_x peak) were defined. Coefficients of variation were calculated for these five indexes to evaluate the GRF variability for each walking speed. It became clear for first and second peaks of F_z and F_x peak 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 first and second peaks of F_y, which were related to forward propulsion of the body. These results suggest that there is "an optimum speed" for the neuromuscular locomotor system but only for the propulsion control mechanism.