Maximal O₂ delivery and O₂ uptake (O₂) per 100 g of active muscle mass are far greater during knee extensor (KE) than during cycle exercise: 73 and 60 ml · min¹ · 100 g¹ (2.4 kg of muscle) (R. S. Richardson, D. R. Knight, D. C. Poole, S. S. Kurdak, M. C. Hogan, B. Grassi, and P. D. Wagner. Am. J. Physiol. 268 (Heart Circ. Physiol. 37): H1453-H1461, 1995) and 28 and 25 ml · min¹ · 100 g¹ (7.5 kg of muscle) (D. R. Knight, W. Schaffartzik, H. J. Guy, R. Predilleto, M. C. Hogan, and P. D. Wagner. J. Appl. Physiol. 75: 2586-2593, 1993), respectively. Although this is evidence of muscle O₂ supply dependence in itself, it raises the following question: With such high O₂ delivery in KE, are the quadriceps still O₂ supply dependent at maximal exercise? To answer this question, seven trained subjects performed maximum KE exercise in hypoxia [0.12 inspired O₂ fraction (FI_O2)], normoxia (0.21 FI_O2), and hyperoxia (1.0 FI_O2) in a balanced order. The protocol (after warm-up) was a square wave to a previously determined maximum work rate followed by incremental stages to ensure that a true maximum was achieved under each condition. Direct measures of arterial and venous blood O₂ concentration in combination with a thermodilution blood flow technique allowed the determination of O₂ delivery and muscle O₂. Maximal O₂ delivery increased with inspired O₂: 1.3 ± 0.1, 1.6 ± 0.2, and 1.9 ± 0.2 l/min at 0.12, 0.21, and 1.0 FI_O2, respectively (P < 0.05). Maximal work rate was affected by variations in inspired O₂ (25 and +14% at 0.12 and 1.0 FI_O2, respectively, compared with normoxia, P < 0.05) as was maximal O₂ (O_{2 max}): 1.04 ± 0.13, 1.24 ± 0.16, and 1.45 ± 0.19 l/min at 0.12, 0.21, and 1.0 FI_O2, respectively (P < 0.05). Calculated mean capillary PO₂ also varied with FI_O2 (28.3 ± 1.0, 34.8 ± 2.0, and 40.7 ± 1.9 Torr at 0.12, 0.21, and 1.0 FI_O2, respectively, P < 0.05) and was proportionally related to changes in O_{2 max}, supporting our previous finding that a decrease in O₂ supply will proportionately decrease muscle O_{2 max}. As even in the isolated quadriceps (where normoxic O₂ delivery is the highest recorded in humans) an increase in O₂ supply by hyperoxia allows the achievement of a greater O_{2 max}, we conclude that, in normoxic conditions of isolated KE exercise, KE O_{2 max} in trained subjects is not limited by mitochondrial metabolic rate but, rather, by O₂ supply.

blood flow; oxygen extraction; muscle mass; oxygen transport conductance

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