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

New Findings What is the central question of this study? Do the phase II parameters of pulmonary oxygen uptake (V̇O2) 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 V̇O2 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 V̇O2 kinetics with chronological age. 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 (V̇O2) 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 V̇O2 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 (τV̇O2p) between U → M and U → VH (23 ± 6 vs. 25 ± 7 s; P = 0.36); however, the τV̇O2p was slower during M → VH (42 ± 16 s) compared to other conditions (P