Bioenergetic Mechanisms Linking V˙O2 Kinetics and Exercise Tolerance

Abstract

We hypothesize that the V˙O2 time constant (τV˙O2) determines exercise tolerance by defining the power output associated with a "critical threshold" of intramuscular metabolite accumulation (e.g., inorganic phosphate), above which muscle fatigue and work inefficiency are apparent. Thereafter, the V˙O2 "slow component" and its consequences (increased pulmonary, circulatory, and neuromuscular demands) determine performance limits.

Figure 1.

Schematic representation of: (A) the kinetic V̇O₂ response to a moderate-intensity constant-power output test with different values of τ_V̇O2; and (B) the corresponding interdependent relationship between ΔV̇O₂ (AṪP) and τ_V̇O2 in determining the size of the O₂ deficit below (solid line) and above (hatched area) a critical threshold of intramuscular metabolite accumulation. The hatched areas above represent the duration and amplitude dependent changes in ATP demand and V̇O₂ for a given constant power exercise. Fast V̇O₂ kinetics (τ_V̇O2 = 15 s) allow high ΔV̇O₂ to be achieved in a steady state (below the O₂ deficit associated with the critical threshold). Slower V̇O₂ kinetics (τ_V̇O2 = 30 s or 60 s) will result in attaining the O2 deficit associated with the critical threshold at a lower ΔV̇O₂ (or power output) (see text for additional details).

Figure 2.

A: Simulated relationship between ATP usage activity (A_UT) (herein termed AṪP) and the tolerable duration of exercise. One A_UT unit corresponds roughly to 3 watts of power output. A hyperbolic fit of the simulated A_UT–duration relationship is also shown. The asymptote of this hyperbola is (Parameter B = 75; the critical ATP usage activity) corresponds approximately to critical power (CP) of 222 W of total mechanical power or 210 W of external power. The curvature constant (Parameter A = 50) corresponds to the total work available above CP before intolerance, i.e. W’. Reproduced with permission from (6). B: Critical power (CP) as a function of the fundamental phase pulmonary oxygen uptake kinetics time constant (τ_V̇O2p) during cycle ergometry across populations differing in aerobic function. The figure is derived from 35 reports published between 1982 and 2008. Reproduced with permission from (3).

Figure 3.

A schematic of the AṪP (A), V̇O₂ (B), PCr (C) and [Pi] responses for participants with fast (τ_V̇O2 = 15 s; solid line) and slow V̇O₂ kinetics (τ_V̇O2 = 60 s; dashed line). For a given AṪP, a steady-state is achieved rapidly in all variables with a small τ_V̇O2. The greater rate of O₂ deficit accumulation associated with a large τ_V̇O2 leads to increased PCr depletion and Pi accumulation, to the extent that Pi accumulation (and other muscle metabolites, not shown) exceeds a critical threshold. Exceeding the critical threshold induces fatigue, which generates inefficiency and increases AṪP for a given power output. This initiates a positive feedback loop where increased AṪP, causes increased metabolite accumulation, which causes increased fatigue, which causes further increased AṪP, and so on until peak limits are reached (indicated by X). Stimulation of oxidative phosphorylation by metabolite accumulation above the critical threshold results in an increase in the O₂ cost of the exercise, development of the V̇O_2SC, and eventual attainment of V̇O_2max.

Figure 4.

V̇O₂ (filled circles, top row), PCr (clear circles, top row) and intramuscular pH (pH_i; bottom row) responses to work:recovery durations of 16:32 s (first column), 32:64 s (second column), 64:128 s (third column) or continuous exercise (fourth column) at an external work rate corresponding to 110% peak incremental test power. Lactate threshold (LT) from the ramp incremental test is shown by the dotted line (top row), as is V̇O₂ max (top row, dashed line) and pH_i at task failure from the continuous exercise protocol (bottom row, dashed line). Note that during the 16:32 s protocol, V̇O₂ never exceeds the LT and the fluctuations in pH_i and PCr are small, i.e. consistent with moderate intensity exercise. The peak V̇O₂ amplitude exceeds the LT in the 32:64 and 64:128 s intermittent protocols and during continuous exercise, and this is accompanied by a metabolic acidosis (decline in pH_i), consistent with a greater metabolic strain in these protocols. Reproduced with permission from (66).