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

Abstract

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 ( o2) 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 o2 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 (τ o2p) between U→M and U→VH (23 ± 6 s vs. 25 ± 7 s; P = 0.36); however, the τ o2p 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 τ o2p 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 τ o2p 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 o2 dynamics accentuated between the ages of 11 to 15 y.