Although body size and muscle mass increase considerably during growth in children, certain aerobic responses to exercise appear to be regulated so that the delivery of oxygen to muscle is maintained at optimized levels. We proposed that the relationship between oxygen uptake, (VO2) and heart rate (HR) was one of the regulated responses. We further hypothesized that the increase in VO2 per increase in HR during progressive exercise would differ in subjects of different size, but when normalized to body weight would be constant since changes in muscle mass are highly correlated to changes in body mass. To test this, we performed a cross-sectional study of 107 normal children, 50 girls and 57 boys ranging in age from 6 to 17 years. The protocol consisted of a continuously increasing work rate on a cycle ergometer, to the limit of the child's tolerance (ramp forcing function). Gas exchange was measured breath-by-breath for the determination of VO2, and heart rate was measured beat-by-beat. We used linear regression techniques to determine M, the slope, and B, the y intercept of the equation: VO2 = M X HR - B. In both boys and girls, M increased significantly with body weight, but when normalized for body weight (M/kg), there was no systematic change with increasing weight or age, the mean value being 0.33 +/- 0.10 ml/min/kg (SD). The mean value for the boys was 0.37 +/- 0.10 which was significantly greater than that of the girls (0.29 +/- 0.08, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

The purpose of this study was to determine how ventilation (VE) and CO2 production (VCO2) in response to exercise change during the growth process in children and teenagers. Dynamic gas exchange responses were measured in two types of studies: 128 healthy children ranging in age from 6 to 18 yr performed progressive exercise tests ("ramp" type protocol) for measurement of the slope of the relationship between VE and VCO2--delta VE/delta VCO2; and the response characteristics of VE and VCO2 in the transition between rest and exercise were measured in 11 teenagers and 11 younger children. Gas exchange was measured breath by breath. We found a small but significant decrease in delta VE/delta VCO2 with increasing body weight (r = -0.46, p less than 0.05), height, or age (mean slope of 27 in the youngest in 21 in the oldest subjects). The response characteristics of VE and VCO2 (measured as the time constant of the best-fit exponential response) were longer than for VO2 in both younger children and teenagers; but the time constants for VE and VCO2 were each approximately 30% faster in younger children compared to teenagers. In addition, end-tidal PCO2 during exercise was significantly lower in the younger subjects (mean value of 39.6 torr) compared to the teenagers (mean value of 43.5 torr). The results suggest that the process of respiratory control in exercise matures to a small degree during childhood in that PCO2 may be regulated at lower levels in younger children and there may be growth-related differences in the relative amounts of CO2 that can be stored in tissues.