Muscle microvascular hemoglobin concentration and oxygenation within the contraction–relaxation cycle

Inability to directly measure microvascular oxygen distribution and extraction in striated muscle during a contraction/relaxation cycle limits our understanding of oxygen transport to and utilization by contracting muscle. We examined muscle microvascular hemoglobin concentration (total [Hb/Mb]) and oxygenation within the contraction–relaxation cycle to determine if microvascular RBC volume would be preserved and if oxygen extraction continued during the actual contraction phase. Eight subjects performed dynamic knee extension exercise (40 contractions/min) at moderate (∼30% of peak work rate) and heavy (∼80% of peak) work rates. Total hemoglobin/myoglobin (total [Hb/Mb]) and deoxy-hemoglobin/myoglobin (deoxy-[Hb/Mb]) were measured in the rectus femoris using NIRS to determine if microvascular total [Hb/Mb] would be preserved during the contraction, and to estimate microvascular oxygen extraction, respectively. Mean values during the relaxation (RP) and contractile phases and the peak values during the contractile phase for both moderate and heavy exercise were calculated. Total [Hb/Mb] increased from rest to steady-state exercise (6.36 ± 5.08 μM moderate; 5.72 ± 4.46 μM heavy exercise, both P < 0.05), but did not change significantly within the contraction/relaxation cycle. Muscle contractions were associated with a significant (1.29 ± 0.98 μM moderate; 2.16 ± 2.12 μM heavy exercise, P < 0.05) increase in deoxy-[Hb/Mb] relative to RP. We conclude that (a) microvascular RBC volume is preserved during muscle contractions (i.e., RBCs are present in the capillaries), and (b) the cyclical pattern of deoxygenation/oxygenation during the respective contraction/relaxation phases of the contraction cycle suggests that oxygen extraction is not restricted to the relaxation phase but continues to occur during muscle contractions.