HYPEROXIA INCREASES VO2max IN THE ISOLATED HUMAN QUADRICEPS1403

Abstract

Under normoxic conditions maximal O2 delivery per 100g of active muscle mass in knee-extensor (KE) exercise is far greater than during cycle exercise (KE = 57 ml/min/100g (2.5 kg of muscle); cycling = 25 ml/min/100g(7.5 kg of muscle)). With such high levels of oxygen delivery KE exercise is an ideal model to test the hypothesis that O2 supply limits VO2 in maximally working muscle. To study this relationship five trained subjects performed maximum KE exercise in normoxia (21% O2), hypoxia (12% O2), and hyperoxia (100% O2) in a balanced order. The protocol was (after warm-up) a square wave to a previously determined maximum work rate followed by incremental stages to ensure a true maximum was achieved under each condition. Direct measures of arterial and venous blood [O2] in combination with a thermodilution blood flow technique allowed the determination of O2 delivery, and muscle VO2. Maximal O2 delivery increased with inspired O2: 12% O2=45 ± 5.21% O2=57 ± 8, and 100% O2 = 63 ± 11 ml/min/100g. Maximal work rate was significantly affected by variations in inspired O2 (-19% in 12% O2 and +14% in 100% O2 compared to normoxia) as was VO2max (12% O2 = 35.6 ± 2.1, 21% O2 = 44.0 ± 2.7, and 100%O2=48.4 ± 3.0 ml/min/100g). Calculated mean capillary PO2 also varied with inspired O2 (12%O2 = 29.5 ± 14.21%O2 = 35.8 ± 2.9, and 100%O2 = 42.0 ± 2.1 mmHg) and was proportionally related to changes in VO2max, supporting our previous finding that a decrease in O2 supply will proportionately decrease muscle VO2max. Because even in the isolated quadriceps (where normoxic O2 delivery is the highest recorded in man) an increase in O2 supply by hyperoxia allows the achievement of a greater VO2max, we conclude that in normoxic conditions of isolated KE exercise, KE VO2max is not limited by mitochondrial metabolic rate, but rather by O2 supply.