Muscle mitochondrial capacity exceeds maximal oxygen delivery in humans

Abstract

Across a wide range of species and body mass a close matching exists between maximal conductive oxygen delivery and mitochondrial respiratory rate. In this study we investigated in humans how closely in-vivo maximal oxygen consumption (VO2 max) is matched to state 3 muscle mitochondrial respiration. High resolution respirometry was used to quantify mitochondrial respiration from the biopsies of arm and leg muscles while in-vivo arm and leg VO2 were determined by the Fick method during leg cycling and arm cranking. We hypothesized that muscle mitochondrial respiratory rate exceeds that of systemic oxygen delivery. The state 3 mitochondrial respiration of the deltoid muscle (4.3 ± 0.4 mmol o2kg− 1 min− 1) was similar to the in-vivo VO2 during maximal arm cranking (4.7 ± 0.5 mmol O2 kg− 1 min− 1) with 6 kg muscle. In contrast, the mitochondrial state 3 of the quadriceps was 6.9 ± 0.5 mmol O2 kg− 1 min− 1, exceeding the in-vivo leg VO2 max (5.0 ± 0.2 mmol O2 kg− 1 min− 1) during leg cycling with 20 kg muscle (P < 0.05). Thus, when half or more of the body muscle mass is engaged during exercise, muscle mitochondrial respiratory capacity surpasses in-vivo VO2 max. The findings reveal an excess capacity of muscle mitochondrial respiratory rate over O2 delivery by the circulation in the cascade defining maximal oxidative rate in humans.