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 (VO 2 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 VO 2 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 o 2kg − 1 min − 1 ) was similar to the in-vivo VO 2 during maximal arm cranking (4.7 ± 0.5 mmol O 2 kg − 1 min − 1 ) with 6 kg muscle. In contrast, the mitochondrial state 3 of the quadriceps was 6.9 ± 0.5 mmol O 2 kg − 1 min − 1 , exceeding the in-vivo leg VO 2 max (5.0 ± 0.2 mmol O 2 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 VO 2 max. The findings reveal an excess capacity of muscle mitochondrial respiratory rate over O 2 delivery by the circulation in the cascade defining maximal oxidative rate in humans.