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Physical activity in children and adolescents is frequently assessed using heart rate monitoring. Different approaches have been used to identify target heart rates corresponding to moderate and vigorous intensity. One such approach is the use of absolute heart rates of 140 bpm. and 160 bpm. However, this approach does not take individual differences in gender, body composition and fitness level into account. The purpose of the present study was to identify absolute and relative oxygen uptakes corresponding to heart rates of 120 bpm. , 140 bpm. , and 160 bpm and to examine a possible effect of fitness and fatness on the relationship between heart rate and oxygen uptake. The subjects were 60 boys and 67 girls with a mean age of 14. 8 ± 0. 3 years. Heart rate and oxygen uptake was measured during an incremental exercise test to exhaustion on a treadmill. Linear regressions were performed for the VO2 - HR relationship using absolute and relative % peak oxygen uptake data for each individual. Average oxygen uptakes ml/kg/min corresponding to 120 bpm, 140 bpm and 160 bpm were 22 ± 5 40 ± 7%, 30 ± 5 56 ± 7%, 38 ± 6 71 ± 7% and 18 ± 4 40 ± 8%, 24 ± 4 56 ± 6%, 31 ± 4 71 ± 6% for boys and girls respectively. An analysis of covariance showed a significant fitness effect P < 0. 001 for predicted oxygen uptake at all studied heart rates. The results suggest that the use of absolute heart rates to define intensity levels when assessing young people's physical activity using heart rate monitoring detract from the validity of the data.
Ekelund et al. (Tue,) studied this question.