Peak muscle perfusion and oxygen uptake in humans: importance of precise estimates of muscle mass.

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Abstract
The knee extensor exercise model was specifically developed to enable in vivo estimates of peak muscle blood flow and O2 uptake in humans. The original finding, using thermodilution measurements to measure blood flow in relation to muscle mass [P. Andersen and B. Saltin. J. Physiol. (Lond.) 366: 233–249, 1985], was questioned, however, as the measurements were two- to threefold higher than those previously obtained with the133Xe clearance and the plethysmography technique. As thermodilution measurements have now been confirmed by other methods and independent research groups, we aimed to address the impact of muscle mass estimates on the peak values of muscle perfusion and O2 uptake. In the present study, knee extensor volume was determined from multiple measurements with computer tomography along the full length of the muscle. In nine healthy humans, quadriceps muscle volume was 2.36 ± 0.17 (range 1.31–3.27) liters, corresponding to 2.48 ± 0.18 (range 1.37–3.43) kg. Anthropometry overestimated the muscle volume by ∼21–46%, depending on whether quadriceps muscle length was estimated from the patella to either the pubic bone, inguinal ligament, or spina iliaca anterior superior. One-legged, dynamic knee extensor exercise up to peak effort of 67 ± 7 (range 55–100) W rendered peak values for leg blood flow (thermodilution) of 5.99 ± 0.66 (range 4.15–9.52) l/min and leg O2 uptake of 856 ± 109 (range 590–1,521) ml/min. Muscle perfusion and O2 uptake reached peak values of 246 ± 24 (range 149–373) and 35.2 ± 3.7 (range 22.6–59.6) ml ⋅ min−1 ⋅ 100 g muscle−1, respectively. These peak values are ∼19–33% larger than those attained by applying anthropometric muscle mass estimates. In conclusion, the present findings emphasize that peak perfusion and O2 uptake in human skeletal muscle may be up to ∼30% higher than previous anthropometric-based estimates that use equivalent techniques for blood flow measurements.