Relationship between pulmonary O sub(2) uptake kinetics and muscle deoxygenation during moderate-intensity exercise

The temporal relationship between the kinetics of phase 2 pulmonary O sub(2) uptake (Vo sub(2p)) and deoxygenation of the vastus lateralis muscle was examined during moderate-intensity leg-cycling exercise. Young adults (5 men, 6 women; 23 plus or minus 3 yr; mean plus or minus 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 Vo sub(2p) was measured by mass spectrometer and volume turbine. Deoxyhemoglobin (HHb), oxyhemoglobin, and total hemoglobin and myoglobin were sampled each second by nearinfrared spectroscopy (Hamamatsu NIRO-300). Vo sub(2p) data were filtered, interpolated to 1 s, and averaged to 5-s bins; HHb data were averaged to 5-s bins. Phase 2 Vo sub(2p) data were fit with a monoexponential model. For HHb, a time delay (TD sub(HHb)) from exercise onset to an increase in HHb was determined, and thereafter data were fit with a monoexponential model. The time constant for Vo sub(2p) (30 plus or minus 8 s) was slower (P < 0.01) than that for HHb (10 plus or minus 3 s). The TD sub(HHb) before an increase in HHb was 13 plus or minus 2 s. The possible mechanisms of the TD sub(HHb) are discussed with reference to metabolic activation and matching of local muscle O sub(2) delivery and O sub(2) utilization. After this initial TD sub(HHb), the kinetics of local muscle deoxygenation were faster than those of phase 2 Vo sub(2p) (and presumably muscle O sub(2) consumption), reflecting increased O sub(2) extraction and a mismatch between local muscle O sub(2) consumption and perfusion.