Relationship between (non)linear phase II pulmonary oxygen uptake kinetics with skeletal muscle oxygenation and age in 11–15 year olds

Abstract

New Findings: What is the central question of this study? Do the phase II parameters of pulmonary oxygen uptake ((Formula presented.)) kinetics display linear, first-order behaviour in association with alterations in skeletal muscle oxygenation during step cycling of different intensities or when exercise is initiated from an elevated work rate in youths. What is the main finding and its importance? Both linear and non-linear features of phase II (Formula presented.) kinetics may be determined by alterations in the dynamic balance between microvascular O2 delivery and utilization in 11–15 year olds. The recruitment of higher-order (i.e. type II) muscle fibres during ‘work-to-work’ cycling might be responsible for modulating (Formula presented.) kinetics with chronological age. Abstract: This study investigated in 19 male youths (mean age: 13.6 ± 1.1 years, range: 11.7–15.7 years) the relationship between pulmonary oxygen uptake ((Formula presented.)) and muscle deoxygenation kinetics during moderate- and very heavy-intensity ‘step’ cycling initiated from unloaded pedalling (i.e. U → M and U → VH) and moderate to very heavy-intensity step cycling (i.e. M → VH). Pulmonary (Formula presented.) was measured breath-by-breath along with the tissue oxygenation index (TOI) of the vastus lateralis using near-infrared spectroscopy. There were no significant differences in the phase II time constant ((Formula presented.)) between U → M and U → VH (23 ± 6 vs. 25 ± 7 s; P = 0.36); however, the (Formula presented.) was slower during M → VH (42 ± 16 s) compared to other conditions (P < 0.001). Quadriceps TOI decreased with a faster (P < 0.01) mean response time (MRT; i.e. time delay + τ) during U → VH (14 ± 2 s) compared to U → M (22 ± 4 s) and M → VH (20 ± 6 s). The difference (Δ) between the (Formula presented.) and MRT-TOI was greater during U → VH compared to U → M (12 ± 7 vs. 2 ± 7 s, P < 0.001) and during M → VH (23 ± 15 s) compared to other conditions (P < 0.02), suggesting an increased proportional speeding of fractional O2 extraction. The slowing of the (Formula presented.) during M → VH relative to U → M and U → VH correlated positively with chronological age (r = 0.68 and 0.57, respectively, P < 0.01). In youths, ‘work-to-work’ transitions slowed microvascular O2 delivery-to-O2 utilization with alterations in phase II (Formula presented.) dynamics accentuated between the ages of 11 and 15 years.