Why do arms extract less oxygen than legs during exercise?

To determine whether conditions for O-2 utilization and O-2 off-loading from the hemoglobin are different in exercising arms and legs, six cross-country skiers participated in this study. Femoral and subclavian vein blood flow and gases were determined during skiing on a treadmill at similar to 76% maximal O-2 uptake (VO2 (max)) and at VO2 (max) with different techniques: diagonal stride (combined arm and leg exercise), double poling ( predominantly arm exercise), and leg skiing ( predominantly leg exercise). The percentage of O-2 extraction was always higher for the legs than for the arms. At maximal exercise (diagonal stride), the corresponding mean values were 93 and 85% (n = 3; P < 0.05). During exercise, mean arm O-2 extraction correlated with the PO2 value that causes hemoglobin to be 50% saturated (P-50: r = 0.93, P < 0.05), but for a given value of P50, O-2 extraction was always higher in the legs than in the arms. Mean capillary muscle O-2 conductance of the arm during double poling was 14.5 (SD 2.6) ml (.) min(-1) (.) mmHg(-1), and mean capillary PO2 was 47.7 (SD 2.6) mmHg. Corresponding values for the legs during maximal exercise were 48.3 (SD 13.0) ml (.) min(-1) (.) mmHg(-1) and 33.8 (SD 2.6) mmHg, respectively. Because conditions for O-2 off-loading from the hemoglobin are similar in leg and arm muscles, the observed differences in maximal arm and leg O-2 extraction should be attributed to other factors, such as a higher heterogeneity in blood flow distribution, shorter mean transit time, smaller diffusing area, and larger diffusing distance, in arms than in legs.