Spatial heterogeneity of quadriceps muscle deoxygenation kinetics during cycle exercise
To test the hypothesis that, during exercise, substantial heterogeneity of muscle hemoglobin and myoglobin deoxygenation [deoxy(Hb + Mb)] dynamics exists and to determine whether such heterogeneity is associated with the speed of pulmonary O2 uptake (pV̇o2) kinetics, we adapted multi-optical fibers near-infrared spectroscopy (NIRS) to characterize the spatial distribution of muscle deoxygenation kinetics at exercise onset. Seven subjects performed cycle exercise transitions from unloaded to moderate [<gas exchange threshold (GET)] and heavy (>GET) work rates and the relative changes in deoxy(Hb + Mb), at 10 sites in the quadriceps, were sampled by NIRS. At exercise onset, the time delays in muscle deoxy(Hb + Mb) were spatially inhomogeneous [intersite coefficient of variation (CV), 3∼56% for <GET, 2∼21% for >GET]. The primary component kinetics (time constant) of muscle deoxy(Hb + Mb) reflecting increased O2 extraction were also spatially inhomogeneous (intersite CV, 6∼48% for <GET, 7∼47% for >GET) and faster (P < 0.05) than those of phase 2 pV̇o2. However, the degree of dynamic intersite heterogeneity in muscle deoxygenation did not correlate significantly with phase 2 pV̇o2 kinetics. In conclusion, the dynamics of quadriceps microvascular oxygenation demonstrates substantial spatial heterogeneity that must arise from disparities in the relative kinetics of V̇o2 and O2 delivery increase across the regions sampled.