Human adults walk at a characteristic speed, but the mechanisms responsible for this ubiquitous and reproducible behavior remain unknown. In this study, preferred walking speed (PWS) was 4.7 +/- 0.1 km h -1 in 12 overnight-fasted adults, mean age 30.0 +/- 2.6 years. Indirect calorimetry was used to measure fuel oxidation during level treadmill walking from 3.2 to 7.2 km h -1 progressively increased at increments of 0.8 km h -1 and 10.0-min intervals. Corroborating many previous reports, the O2 cost of transport (mL O2 kg -1 km -1 ) was numerically lowest at 4.8 km h -1 , near PWS, but was not significantly different than 5.6 km h -1 . The impact of walking speed on the fuel selection of skeletal muscle was much more dramatic. At speeds less than or equal to PWS, muscle carbohydrate (CHO) oxidation rates were quite low, in the range that could be matched by gluconeogenesis. Above 4.8 km h -1 , CHO oxidation rate increased abruptly and tracked the perception of effort (RPE). Stepwise linear regression revealed that CHO oxidation explained 70% of the variance in RPE, and speed provided an additional 4%. In contrast, the other variables included in the analysis, fat oxidation rate, heart rate, and O2 cost of transport, contributed no additional explained variance in RPE. We conclude that PWS is just below a threshold speed, above which CHO oxidation abruptly increases. The central nervous system may be guided by the perception of effort in selecting a PWS that minimizes dependence on CHO oxidation. We further conclude that skeletal muscle metabolic control is an important factor to be taken into account by the central nervous system motor control of human locomotion.