Influence of thigh activation on the V˙O2 slow component in boys and men

Purpose During constant work rate exercise above the lactate threshold (LT), the initial rapid phase of pulmonary oxygen uptake ( V ˙ O 2 ) kinetics is supplemented by an additional V ˙ O 2 slow component ( V ˙ O 2Sc ) which reduces the efficiency of muscular work. The V ˙ O 2Sc 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 V ˙ O 2 slow component ( V ˙ O 2Sc ) amplitude in children would be associated with a reduced muscle recruitment compared to adults. Methods 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 V ˙ O 2Sc 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 Σ T 2 as an index of thigh recruitment. Results There were no child/adult differences in the relative V ˙ O 2Sc amplitude [Boys 14 ± 7 vs. Men 18 ± 3 %, P = 0.15, effect size (ES) = 0.8] during which the change (∆) in area-weighted Σ T 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 V ˙ O 2Sc amplitude and the magnitude of the area-weighted ∆Σ T 2 in men ( r = 0.92, P = 0.001) but not in boys ( r = 0.09, P = 0.84). Conclusions This study provides evidence to show that progressive muscle recruitment (as inferred from T 2 changes) contributes to the development of the V ˙ O 2Sc during intense submaximal exercise independent of age.