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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.

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Plateau in Oxygen Uptake at Maximal Exercise in Male Children

Glen E. Duncan, Anthony D. Mahon, Cheryl A. Howe, and Pedro Del Corral

This study examined the influence of test duration and anaerobic capacity on VO2max and the occurrence of a VO2 plateau during treadmill exercise in 25 boys (10.4 ± 0.8 years). Protocols with 1-min (P1) and 2-min (P2) stages, but identical speed and grade changes, were used to manipulate test duration. On separate days, VO2max was measured on P1 and P2, and 200-m run time was assessed. At maximal exercise, VO2, heart rate (HR), and pulmonary ventilation (VE) were similar between protocols, however, respiratory exchange ratio (RER) and treadmill elevation were higher (p < .05) on P1 than on P2. Plateau achievement was not significantly different. On P1, there were no differences between plateau achievers and nonachievers. On P2, test duration and 200-m run time were superior (p < .05), and relative VO2max tended to be higher (p < .10) in plateau achievers. Indices of aerobic and anaerobic capacity may influence plateau achievement on long, but not short duration tests.

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Physiological and Perceptual Responses to Graded Treadmill and Cycle Exercise in Male Children

Glen E. Duncan, Anthony D. Mahon, Julie A. Gay, and Jennifer J. Sherwood

Physiological and perceptual responses at ventilatory threshold (VT) and V̇O2 peak were examined in 10 male children (10.2 ± 1.3 yrs) during graded treadmill and cycle exercise. Treadmill V̇O2peak (57.9 ± 6.7 ml · kg−1 · min−1) was higher (p < .05) than the cycle (51.7 ± 7.7 ml · kg−1 · min−1). Ventilation and heart rate (HR) were higher (p < .05) on the treadmill, while respiratory exchange ratio (RER), rating of perceived exertion (RPE), capillary blood lactate, and test duration were similar between tests. The V̇O2 at VT was higher (p < .05) on the treadmill (36.7 ± 4.6 ml · kg−1 · min−1) than the cycle (32.5 ± 4.4 ml · kg−1 · min−1). When VT was expressed as a percentage of V̇O2 peak, there was no difference (p > .05) between tests. The RPE at VT, HR at VT, and VT expressed as a percentage of HRpeak were also similar (p > .05) between tests. Similar to V̇O2 peak, the V̇O2 at VT is dependent on the mode of exercise. However, when VT is expressed as a percentage of V̇O2 peak, it is independent of testing modality. The RPE at VT appears to be linked to a percentage of V̇O2 peak rather than an absolute V̇O2.