Biochemical origin of (near-) linear curvature constant (W')- V ˙ O 2 slow component ( Δ V ˙ O 2 sc ) and critical power (CP)- V ˙ O 2 transition time (t0.63) relationship in skeletal muscle.

Purpose: The biochemical background of the (near-)linear direct relationship between the curvature constant (W') of the power-duration curve and the magnitude ( $\Delta {\dot{\text{V}}\text{O}}_{\text{2sc}}$ ) of the slow component of the ${\dot{\text{V}}\text{O}}_2$ on-kinetics ( ${\dot{\text{V}}\text{O}}_{\text{2sc}}$ ) as well as reverse relationship between critical power (CP) and the characteristic transition time (t_0.63, analogous to τ_p) of the primary phase II of the ${\dot{\text{V}}\text{O}}_2$ on-kinetics encountered in experimental studies is studied.

Methods: A computer model of the bioenergetic system in skeletal muscle, involving the each-step-activation mechanism of work transitions and P_i double-threshold mechanism of muscle fatigue, is used.

Results: The activity (rate constant) (k_add) of the additional ATP usage, underlying the slow component, determines to a large extent the (near-)linear direct W'- $\Delta {\dot{\text{V}}\text{O}}_{\text{2sc}}$ relationship, as an increase in k_add increases markedly both W' and $\Delta {\dot{\text{V}}\text{O}}_{\text{2sc}}$ . t_0.63 is a derivative of the changes in metabolite (M = PCr or Cr or P_i) concentrations between rest and the steady-state of the phase II M on-kinetics after the onset of exercise. The oxidative phosphorylation (OXPHOS) activity (k_OX) mostly determines the (near)-linear inverse CP-t_0.63 relationship, as an increase in k_OX markedly decreases ΔM and t_0.63, and elevates CP.

Conclusions: The ${\dot{\text{V}}\text{O}}_2$ on-kinetics (e.g., ${\dot{\text{V}}\text{O}}_{\text{2sc}}$ or t_0.63) cannot cause anything in the system, as it is an emergent property of the system functioning on the biochemical level. Physiological variables: muscle ${\dot{\text{V}}\text{O}}_{\text{2sc}}$ and W' as well as t_0.63 and CP, and relationships between them, are determined by biochemical parameters, mainly k_add and k_OX, respectively.