Energy cost of walking at any given speed is higher for heavier people than for lighter ones. We compared adolescents that were matched for total body mass but had different body composition. Nine pairs of boys (16.37 ± 1.57 years in the lean group and 12.90 ± 1.49 years in the obese group) participated. Metabolic energy expenditure (EE) was compared at three walking speeds and moments and powers at the hip and ankle at push off were analyzed. Assessment of fat mass and distribution was performed using whole body dualenergy x-ray absorptiometry. A repeated measure ANOVA was performed when matched pairs were compared. Based on multiple regression, pooling all subjects together, body mass was the main predictor of EE. Variance explained by adiposity increased with increasing speed. Obese subjects tended to expend more energy than their lean pairs at the two fastest walking speeds (5 and 6kph). There was a significant difference between the pairs in EE (kJ/min) at 6kph (p<0.05). Ventilation showed the same pattern as V02net (exercise V02 minus resting V02), increasing with increasing speed and showing differences between the pairs at the fastest speed. Heart rate was consistently higher in the obese subjects. Stride length, stride rate, progression velocity and moments and powers at the hip and ankle at push off were not correlated with body fat. No relationship between V02net. total amount of body fat, or segment fat content was found. Total amount of fat in the body and the amount of fat in the legs had no influence on gait parameters. In conclusion, excess body fat does not influence the energy cost of walking at low speeds but does so at 6kph. Obese subjects demonstrated higher effort at all speeds. Amount of fat distributed in body segments does not influence either energy cost of walking or mechanical gait parameters. / Thesis / Master of Science (MSc)