Heart rate and gas exchange dynamic responses to multiple brief exercise bouts (MBEB) in early- and late-pubertal boys and girls

Abstract

Natural patterns of physical activity in youth are characterized by brief periods of exercise of varying intensity interspersed with rest. To better understand systemic physiologic response mechanisms in children and adolescents, we examined five responses [heart rate (HR), respiratory rate (RR), oxygen uptake (V̇O₂ ), carbon dioxide production (V̇CO₂ ), and minute ventilation (V̇E), measured breath-by-breath] to multiple brief exercise bouts (MBEB). Two groups of healthy participants (early pubertal: 17 female, 20 male; late-pubertal: 23 female, 21 male) performed five consecutive 2-min bouts of constant work rate cycle-ergometer exercise interspersed with 1-min of rest during separate sessions of low- or high-intensity (~40% or 80% peak work, respectively). For each 2-min on-transient and 1-min off-transient we calculated the average value of each cardiopulmonary exercise testing (CPET) variable (Y̅). There were significant MBEB changes in 67 of 80 on- and off-transients. Y̅ increased bout-to-bout for all CPET variables, and the magnitude of increase was greater in the high-intensity exercise. We measured the metabolic cost of MBEB, scaled to work performed, for the entire 15 min and found significantly higher V̇O₂ , V̇CO₂ , and V̇E costs in the early-pubertal participants for both low- and high-intensity MBEB. To reduce breath-by-breath variability in estimation of CPET variable kinetics, we time-interpolated (second-by-second), superimposed, and averaged responses. Reasonable estimates of τ (<20% coefficient of variation) were found only for on-transients of HR and V̇O₂ . There was a remarkable reduction in τHR following the first exercise bout in all groups. Natural patterns of physical activity shape cardiorespiratory responses in healthy children and adolescents. Protocols that measure the effect of a previous bout on the kinetics of subsequent bouts may aid in the clinical utility of CPET.

Keywords: adolescents; cardiopulmonary exercise testing; children; puberty.

FIGURE 1

Theoretical effect of differences in time constant and gain on CPET variables in the first five bouts of MBEB (shaded areas). The modeled responses used the equations for the on‐ and off‐transients shown in the text. We calculated the effects of a few possible changes in τ and gain on the mean values (Y̅) of during on‐transients (closed circles) and off‐transients (open circles). Panel (a) shows the effect of a CPETvariable with τ = 25 sec and a constant (arbitrary) gain; panel (b), an increased τ and a constant gain; and panel (c), τ and gain increase with each bout. These theoretical data suggest that bout‐to‐bout changes in response characteristics might influence the mean values of CPET variables during bouts of exercise as occurs naturally in children and adolescents.

FIGURE 2

Overview of the study.

FIGURE 3

Model‐agnostic CPET analysis.

FIGURE 4

Time‐interpolated group mean average to determine τ and amplitude.

FIGURE 5

Heat map of linear contrast t‐ratio statistic of the selected CPET variables in the on‐ and off‐transients of the eight groups. The linear contrast values indicate the relative statistical strength of the trend. A higher value (dark red) indicates a strong positive linear trend from Bout 1 to Bout 5, while values <0 indicate a negative trend (blue).

FIGURE 6

GMA bout‐to‐bout HR responses to the MBEB protocol in all eight study groups.

FIGURE 7

Summary of mean bout‐to‐bout HR, RR, V̇O₂, and V̇E during on‐transients; each of these variables increased from bout to bout.

FIGURE 8

Off‐transient mean HR values. The consistently higher off‐transient HR values correspond to the GMA data shown in Figure 6. In the early‐pubertal participants, HR returned to baseline much more quickly than in the late‐pubertal participants.

FIGURE 9

Mean MBEB gas exchange variables scaled to work rate calculated over the 15‐min protocol for V̇O₂ (a), V̇CO₂ (b), and V̇E (c). When analyzing low‐intensity MBEB as a group in males and females, early‐pubertal participants had significantly higher values than late‐pubertal participants for V̇O₂ (p < 0.0001), V̇CO₂ (p < 0.0001), and V̇E (p = 0.01). For high‐intensity MBEB, early‐pubertal participants had significantly higher values than late‐pubertal participants for V̇O₂ (p < 0.0001), V̇CO₂ (p < 0.0001), and V̇E (p = 0.0002). When analyzed as a group, females during low‐intensity MBEB had higher values than males for V̇O₂ (p < 0.0081), V̇CO₂ (p < 0.0037), and V̇E (p = 0.0044); females during high‐intensity MBEB had higher values than males for V̇O₂ (p < 0.0032), V̇CO₂ (p < 0.0041), and V̇E (p = 0.0036). We found no significant sex × pubertal stage interaction.

FIGURE 10

On‐transient τ HR, there was a substantial decrease in τ HR for all comparison groups following the first bout.