title: Influence of thigh activation on the VO₂ slow component in boys and men.
abstract: OBJECTIVE
  During constant work rate exercise above the lactate threshold (LT), the initial rapid phase of pulmonary oxygen uptake (VO₂) kinetics is supplemented by an additional VO₂ slow component (VO₂Sc) which reduces the efficiency of muscular work. The VO₂Sc amplitude has been shown to increase with maturation but the mechanisms are poorly understood. We utilized the transverse relaxation time (T₂) of muscle protons from magnetic resonance imaging (MRI) to test the hypothesis that a lower VO₂ slow component (VO₂Sc) 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 VO₂Sc time delay (ScTD) and after 6 min. T₂ for each of eight muscles was adjusted in relation to cross-sectional area and then summed to provide the area-weighted ΣT₂ as an index of thigh recruitment.
  
  
  RESULTS
  There were no child/adult differences in the relative VO₂Sc amplitude [Boys 14 ± 7 vs. Men 18 ± 3 %, P = 0.15, effect size (ES) = 0.8] during which the change (∆) in area-weighted ΣT₂ 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 VO₂Sc amplitude and the magnitude of the area-weighted ∆ΣT₂ 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 T₂ changes) contributes to the development of the VO₂Sc during intense submaximal exercise independent of age.
authors.affiliation: Children's Health and Exercise Research Centre, Sport and Health Sciences, College of Life and Environmental Sciences, University of Exeter, St Luke's Campus, Heavitree Road, Exeter, EX1 2LU, UK.
authors.forename: Brynmor C
authors.initials: BC
authors.lastname: Breese
authors.forename: Alan R
authors.initials: AR
authors.lastname: Barker
authors.forename: Neil
authors.initials: N
authors.lastname: Armstrong
authors.forename: Jonathan
authors.initials: J
authors.lastname: Fulford
authors.forename: Craig A
authors.initials: CA
authors.lastname: Williams
mesh_terms: D000328:Adult; D000367:Age Factors; D002648:Child; D015444:Exercise; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000254:growth & development / Q000378:metabolism / Q000502:physiology*; D010101:Oxygen Consumption; D011999:Recruitment, Neurophysiological; D013848:Thigh / Q000254:growth & development / Q000502:physiology*