Pulmonary O2 uptake during exercise: conflating muscular and cardiovascular responses.

For moderate-intensity exercise (below lactate threshold, thetaL), muscle O(2) consumption (VO(2)) kinetics are expressed in a first-order phase 2 (or fundamental) pulmonary O(2) uptake (VO(2)) response: dVO(2)/dt . tau + DeltaVO(2)((t)) = DeltaVO(2)((ss)); where DeltaVO(2)(ss) is the steady-state VO(2) increment, and tau the VO(2) time constant (which is within approximately 10% of tauQVO(2)). A likely source of VO(2) control in this intensity domain is ADP-mediated, for which intramuscular phosphocreatine (PCr) may serve as a proxy variable. Whether, in reality, this behavior reflects the operation of a single homogeneous compartment is unclear, however; a multicompartment structure comprised of units having a similar DeltaVO(2)((ss)) but with widely varying tau can also yield a "well-fit" exponential response with an apparent single tau. In support of this is the inverse (although poorly predictive) correlation between tau and both theta(L) and VO(2max). Above theta(L), the fundamental VO(2) kinetics are supplemented with a delayed, slowly developing component that can set VO(2) on a trajectory towards VO(2max), and that has complex temporal- and intensity-related kinetics. This VO(2) slow component is also demonstrable in [PCr], suggesting that the decreased efficiency above theta(L) predominantly reflects a high phosphate cost of force production rather than a high O(2) cost of phosphate production. In addition, the oxygen deficit for the slow component is more likely to reflect a progressive shifting of DeltaVO(2)((ss)) rather than a single DeltaVO(2)((ss)) having a single tau.