Exertional oxygen uptake kinetics: a stamen of stamina?

The fundamental pulmonary O₂ uptake (o₂) response to moderate, constant-load exercise can be characterized as (do₂/dt)(τ) + Δo₂ (t) = Δo_2SS where Δo_2SS is the steady-state response, and τ is the time constant, with the o₂ kinetics reflecting intramuscular O₂ uptake (o₂) kinetics, to within 10%. The role of phosphocreatine (FCr) turnover in o₂ control can be explored using ³¹P-MR spectroscopy, simultaneously with o₂. Although τo₂ and τPCr vary widely among subjects (approx. 20–65 s), they are not significantly different from each other, either at the on- or off-transient. A caveat to interpreting the ‘well-fit’ exponential is that numerous units of similar Δo_2SS but with a wide τ distribution can also yield a o₂ response with an apparent single τ. This τ is, significantly, inversely correlated with lactate threshold and o_2max (but is poorly predictive; a frail stamen, therefore), consistent with τ not characterizing a compartment with uniform kinetics. At higher intensities, the fundamental kinetics become supplemented with a slowly-developing phase, setting o₂ on a trajectory towards maximum o₂. This slow component is also demonstrable in Δ[PCr]: the decreased efficiency thereby reflecting a predominantly high phosphate-cost of force production rather than a high O₂-cost of phosphate production. We also propose that the O₂-deficit for the slow-component is more likely to reflect shifting Δo_2SS rather than a single one with a single τ.