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Edward T. Howley

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Substrate Metabolism during Exercise in Children and the “Crossover Concept”

Glen E. Duncan and Edward T. Howley

This review addresses issues related to substrate metabolism in children and how this information compares and contrasts to that of adults. The relative percent of fat and carbohydrate (CHO) utilized by an individual can be estimated from respiratory exchange ratio (RER) values between 0.7 (100% fat, 0% CHO) and 1.0 (100% CHO, 0% fat). The rise in RER towards 1.0 in relation to increased exercise intensity demonstrates the augmented role of CHO as an energy source for muscle; however, fat oxidation also represents a major source of energy during exercise of moderate-to-heavy intensity. Preliminary reports suggest that children demonstrate patterns of fat and CHO use in response to exercise intensity similar to those of adults and also show a reduction in RER at submaximal exercise intensities after training. The use of the “crossover concept" may simplify the presentation of how metabolism is affected by exercise intensity and training.

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Metabolic and Perceptual Responses to Short-Term Cycle Training in Children

Glen E. Duncan and Edward T. Howley

Metabolic and perceptual responses to cycle training were investigated in children in a training group (TG, N = 10) and control group (CG, N = 13). Prior to training, aerobic power (VO2peak) was assessed, and children performed submaximal exercise at graded power outputs. Substrate use was calculated for each level using the respiratory exchange ratio (RER) and metabolic rate, and ratings of perceived exertion (RPE) were obtained to estimate perceptual effort. Training consisted of 12 sessions (three 10-min work bouts 3 times/week, 50% VO2peak) on a cycle ergometer. After 4 weeks, RER and RPE were reevaluated at the same absolute intensities. Overall difference scores indicated a decrease in RER and RPE in the TG and an increase in RER with * no change in RPE in the CG. These data demonstrate that short-term cycle training in children results in enhanced fat use and diminished perception of effort during submaximal exercise.