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

This study investigated in nineteen male youth (mean age: 13.6 ± 1.1 y, range: 11.7–15.7 y) the relationship between pulmonary oxygen uptake (V̇o₂) and muscle deoxygenation kinetics during moderate‐ and very heavy‐intensity ‘step’ cycling initiated from unloaded pedaling (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 and 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 s vs. 25 ± 7 s; P = 0.36); however, the τV̇o₂p 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 τV̇o₂p 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 τV̇o₂ p 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 youth, “work‐to‐work” transitions slowed microvascular O₂ delivery‐to‐O₂ utilization with alterations in phase II V̇o₂ dynamics accentuated between the ages of 11 to 15 y.