Muscle phosphocreatine and pulmonary oxygen uptake kinetics in children at the onset and offset of moderate intensity exercise

To further understand the mechanism(s) explaining the faster pulmonary oxygen uptake \((p{\dot{V}}\hbox{O}_{2})\) kinetics found in children compared to adults, this study examined whether the phase II \(p{\dot{V}}\hbox{O}_{2}\) kinetics in children are mechanistically linked to the dynamics of intramuscular PCr, which is known to play a principal role in controlling mitochondrial oxidative phosphorylation during metabolic transitions. On separate days, 18 children completed repeated bouts of moderate intensity constant work-rate exercise for determination of (1) PCr changes every 6 s during prone quadriceps exercise using ³¹P-magnetic resonance spectroscopy, and (2) breath by breath changes in \(p{\dot{V}}\hbox{O}_{2}\) during upright cycle ergometry. Only subjects (n = 12) with 95% confidence intervals ≤±7 s for all estimated time constants were considered for analysis. No differences were found between the PCr and phase II \(p{\dot{V}}\hbox{O}_{2}\) time constants at the onset (PCr 23 ± 5 vs. \(p{\dot{V}}\hbox{O}_{2}\ 23 \pm 4\,\hbox{s}, P=1.000)\) or offset (PCr 28 ± 5 vs. \(p{\dot{V}}\hbox{O}_{2}\ 29 \pm 5\,\hbox{s},\, P=1.000)\) of exercise. The average difference between the PCr and phase II \(p{\dot{V}}\hbox{O}_{2}\) time constants was 4 ± 4 s for the onset and offset responses. Pooling of the exercise onset and offset responses revealed a significant correlation between the PCr and \(p{\dot{V}}\hbox{O}_{2}\) time constants (r = 0.459, P = 0.024). The close kinetic coupling between the \(p{\dot{V}}\hbox{O}_{2}\) and PCr responses at the onset and offset of exercise in children is consistent with our current understanding of metabolic control and suggests that an age-related modulation of the putative phosphate linked controller(s) of mitochondrial oxidative phosphorylation may explain the faster \(p{\dot{V}}\hbox{O}_{2}\) kinetics found in children compared to adults.