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.