Heart rate as an indicator of the intensity of physical activity in human adolescents

Abstract.

The aims of this study were, in a group of adolescents, firstly to identify the absolute heart rates (HR) and the percentages of maximal heart rates (HR_max) corresponding to 40%, 60% and 80% of peak oxygen uptake (${\rm P}\dot V{\rm O}_2$ ), secondly to identify absolute and relative ($\% {\rm P}\dot V{\rm O}_2$ ) oxygen uptakes ($\dot V{\rm O}_2$ ) corresponding to HR of 120, 140 and 160 beats·min^–1, and thirdly to examine a possible effect of fatness and fitness on the relationship between HR and $\dot V{\rm O}_2$ . The subjects were 127 (60 boys, 67 girls) adolescents with a mean age of 14.8 (SD 0.3) years. The HR and $\dot V{\rm O}_2$ were measured by means of an incremental exercise test to exhaustion. Linear regressions were performed for the ${\rm HR - }\dot V{\rm O}_2$ and $\dot V{\rm O}_2 {\rm - HR}$ relationships using absolute and relative (%HR_max, $\% {\rm P}\dot V{\rm O}_2$ ) data for each individual. From these regressions, target HR and $\dot V{\rm O}_2$ were computed. Average target HR corresponding to 40%, 60% and 80% of ${\rm P}\dot V{\rm O}_2$ were: 119 (SD 9), 145 (SD 9), 171 (SD 8), and 120 (SD 10), 146 (SD 8), 172 (SD 8) beats·min^–1 for boys and girls, respectively. Average $\dot V{\rm O}_2$ corresponding to HR of 120, 140 and 160 beats·min^–1 were: 22 (SD 5), 30 (SD 5), 38 (SD 6) and 18 (SD 4), 24 (SD 4), 31 (SD 4) mlO₂·kg^–1·min^–1for boys and girls, respectively. An analysis of covariance showed a significant fitness effect (P<0.001) for predicted $\dot V{\rm O}_2$ at all HR studied. The results suggest that the use of absolute HR to define exercise intensity levels when assessing young people's physical activity using HR monitoring detracts from the validity of the interpretation of the data.