Relationship between pulmonary O2 uptake kinetics and muscle deoxygenation during moderate-intensity exercise

The temporal relationship between the kinetics of phase 2 pulmonary O-2 uptake ((V) over dot O-2p) and deoxygenation of the vastus lateralis muscle was examined during moderate-intensity leg-cycling exercise. Young adults (5 men, 6 women; 23 +/- 3 yr; mean +/- SD) performed repeated transitions on 3 separate days from 20 W to a constant work rate corresponding to 80% of lactate threshold. Breath-by-breath (V) over dot O-2p was measured by mass spectrometer and volume turbine. Deoxyhemoglobin (HHb), oxyhemoglobin, and total hemoglobin and myoglobin were sampled each second by near-infrared spectroscopy (Hamamatsu NIRO-300). (V) over dot O-2p data were filtered, interpolated to 1 s, and averaged to 5-s bins; HHb data were averaged to 5-s bins. Phase 2 (V) over dot O-2p data were fit with a monoexponential model. For HHb, a time delay (TDHHb) from exercise onset to an increase in HHb was determined, and thereafter data were fit with a monoexponential model. The time constant for (V) over dot O-2p (30 +/- 8 s) was slower (< 0.01) than that for HHb (10 +/- 3 s). The TDHHb before an increase in HHb was 13 +/- 2 s. The possible mechanisms of the TDHHb are discussed with reference to metabolic activation and matching of local muscle O-2 delivery and O-2 utilization. After this initial TDHHb, the kinetics of local muscle deoxygenation were faster than those of phase 2 (V) over dot O-2p (and presumably muscle O-2 consumption), reflecting increased O-2 extraction and a mismatch between local muscle O-2 consumption and perfusion.