Influence of thigh activation on the $$\dot{V}$$ V ˙ O2 slow component in boys and men

During constant work rate exercise above the lactate threshold (LT), the initial rapid phase of pulmonary oxygen uptake ( $$\dot{V}$$ O2) kinetics is supplemented by an additional $$\dot{V}$$ O2 slow component ( $$\dot{V}$$ O2Sc) which reduces the efficiency of muscular work. The $$\dot{V}$$ O2Sc amplitude has been shown to increase with maturation but the mechanisms are poorly understood. We utilized the transverse relaxation time (T 2) of muscle protons from magnetic resonance imaging (MRI) to test the hypothesis that a lower $$\dot{V}$$ O2 slow component ( $$\dot{V}$$ O2Sc) amplitude in children would be associated with a reduced muscle recruitment compared to adults. Eight boys (mean age 11.4 ± 0.4) and eight men (mean age 25.3 ± 3.3 years) completed repeated step transitions of unloaded-to-very heavy-intensity (U → VH) exercise on a cycle ergometer. MRI scans of the thigh region were acquired at rest and after VH exercise up to the $$\dot{V}$$ O2Sc time delay (ScTD) and after 6 min. T 2 for each of eight muscles was adjusted in relation to cross-sectional area and then summed to provide the area-weighted ΣT 2 as an index of thigh recruitment. There were no child/adult differences in the relative $$\dot{V}$$ O2Sc amplitude [Boys 14 ± 7 vs. Men 18 ± 3 %, P = 0.15, effect size (ES) = 0.8] during which the change (∆) in area-weighted ΣT 2 between the ScTD and 6 min was not different between groups (Boys 1.6 ± 1.2 vs. Men 2.3 ± 1.1 ms, P = 0.27, ES = 0.6). A positive and strong correlation was found between the relative $$\dot{V}$$ O2Sc amplitude and the magnitude of the area-weighted ∆ΣT 2 in men (r = 0.92, P = 0.001) but not in boys (r = 0.09, P = 0.84). This study provides evidence to show that progressive muscle recruitment (as inferred from T 2 changes) contributes to the development of the $$\dot{V}$$ O2Sc during intense submaximal exercise independent of age.