Work rate domain analysis of V̇O2 kinetics using the second-order simultaneous components model: Challenging the etiology of the "slow component".

PURPOSE The representation of human V̇O2 response to exercise with a second-order simultaneous components (SOSC) model differs from those of a first-order (FOME) model in both time and work rate (WR) domains. This study aims to present and compare the WR domain profiles of SOSC and FOME models, discussing their possible relationship with muscle recruitment patterns and classical exercise tolerance markers.

METHODS Seven well-trained male cyclists performed several step on-transitions at moderate, heavy, and severe domain intensities to characterize V̇O2-WR profiles. Linearity and goodness-of-fit were assessed via standard errors of estimates (SEE).

RESULTS Although not significantly, SOSC nonlinear fit of V̇O2 total amplitudes performed better than FOME's equations for moderate (mean SEE ± SD; 42 ± 33 vs 69 ± 26 ml·min-1; p = 0.06) and moderate plus heavy (48 ± 26 vs 88 ± 33 ml·min-1; p = 0.07) intensities. When heavy WR was added to moderate data, SOSC's fit performance remained unaltered (42 ± 33 vs 48 ± 26 ml·min-1; p = 0.50), whereas FOME's fundamental component worsened its fit (69 ± 26 vs 107 ± 44 ml·min-1; p = 0.03).

CONCLUSION These findings, confirmed by visual inspection of individual and group plots, confront the proclaimed linearity of FOME's fundamental component and corroborate the SOSC's power law representation of V̇O2-WR profiles. Additionally, under SOSC's dynamics, FOME's delayed muscle recruitment is no longer a premise, and alternative insights are offered into indices like the lactate threshold, the maximum lactate steady state, and the critical power.