The energy expenditure of level walking was measured in 260 normal male and female subjects walking around a 60.5m-circular outdoor track. Subjects were divided into four age groups (children, 6-12 years; teens; young adults, 20-59 years; and senior adults, 60-80 years). Oxygen consumption was measured with a modified Douglas Bag technique during the fourth and fifth minutes of each trial. Standard tables according to age and sex were derived for the average energy expenditure (rate of oxygen uptake, energy cost per meter, and heart rate) and for the gait characteristics (speed, cadence, stride length) at the subjects' customary slow, normal, and fast walking speeds. Statistical analysis was performed to determine the energy-speed relationship for the different age groups to derive normative tables for the rate of oxygen uptake throughout the range of customary walking velocities.