Muscle Deoxygenation during Incremental Exercise is Delayed in Children Compared to Young Adults

Abstract

The mechanisms regulating O2 delivery and utilization during endurance exercise differ with maturation and are incompletely understood. This is important to understand the development of cardiovascular and neuromuscular diseases that originate early in life. Time-resolved near-infrared spectroscopy (TRS-NIRS) provides a quantitative non-invasive measurement of deoxy- hemoglobin + myoglobin concentration ([HHb]) at the muscle, an indicator of muscle deoxygenation during exercise. PURPOSE: To compare [HHb] dynamics during incremental ramp exercise in healthy early pubertal children and young adults. METHODS: 23 early pubertal children (16 F; 7-11 years) and 15 young adults (7 F; 21-35 years) underwent an incremental ramp exercise test on a cycle ergometer. Pulmonary gas exchange was measured breath-by-breath (Vmax, Carefusion). Vastus lateralis [HHb] was quantified by TRS-NIRS (TRS-21, Hamamatsu Photonics). RESULTS: Peak VO2 was not different between children and adults normalized to body mass (46.7±8.1 vs. 48.9±7.7 mL/min/kg). Absolute [HHb] was not different between children and adults during unloaded pedaling (23.5±5.5 vs 26.8±10.0 uM, p=0.20) but adults had a higher peak exercise [HHb] (32.0±8.2 vs 44.4±22.6 uM, p=0.01). Two way RM-ANOVA of relative HHb dynamics revealed an effect of age (F(1,36) = 6.895, p=0.013) and interaction with % peak power (p=0.001, Figure). Post-hoc, relative HHb was lower at 60% peak power in children vs adults (57±10% vs 68±13%, p=0.0006), while VO2 did not differ (33.5±4.9% vs 30.6±6.2%, p=0.12). CONCLUSION: Muscle deoxygenation is both attenuated and delayed during incremental exercise in children compared to adults, despite similar relative VO2. This is consistent with a closer matching of muscle microvascular O2 delivery to utilization over a greater range of aerobic power in children. The mechanisms contributing to this improved regulation of muscle blood flow during exercise in children warrant further study.Figure