ABSTRACT
The concept of symmorphosis postulates a matching of structural capacity to functional demand within a defined physiological system, regardless of endurance exercise training status. Whether this concept applies to oxygen (O2 ) supply and demand during maximal skeletal muscle O2 consumption (VÌO2 max ) in humans is unclear. Therefore, in vitro skeletal muscle mitochondrial VÌO2 max (Mito VÌO2 max , mitochondrial respiration of fibres biopsied from vastus lateralis) was compared with in vivo skeletal muscle VÌO2 max during single leg knee extensor exercise (KE VÌO2 max , direct Fick by femoral arterial and venous blood samples and Doppler ultrasound blood flow measurements) and whole-body VÌO2 max during cycling (Body VÌO2 max , indirect calorimetry) in 10 endurance exercise-trained and 10 untrained young males. In untrained subjects, during KE exercise, maximal O2 supply (KE QÌO2max ) exceeded (462 ± 37 ml kg(-1) min(-1) , P < 0.05) and KE VÌO2 max matched (340 ± 22 ml kg(-1) min(-1) , P > 0.05) Mito VÌO2 max (364 ± 16 ml kg(-1) min(-1) ). Conversely, in trained subjects, both KE QÌO2max (557 ± 35 ml kg(-1) min(-1) ) and KE VÌO2 max (458 ± 24 ml kg(-1) min(-1) ) fell far short of Mito VÌO2 max (743 ± 35 ml kg(-1) min(-1) , P < 0.05). Although Mito VÌO2 max was related to KE VÌO2 max (r = 0.69, P < 0.05) and Body VÌO2 max (r = 0.91, P < 0.05) in untrained subjects, these variables were entirely unrelated in trained subjects. Therefore, in untrained subjects, VÌO2 max is limited by mitochondrial O2 demand, with evidence of adequate O2 supply, whereas, in trained subjects, an exercise training-induced mitochondrial reserve results in skeletal muscle VÌO2 max being markedly limited by O2 supply. Taken together, these in vivo and in vitro measures reveal clearly differing limitations and excesses at VÌO2 max in untrained and trained humans and challenge the concept of symmorphosis as it applies to O2 supply and demand in humans.
