Increased oxygen extraction and mitochondrial protein expression after small muscle mass endurance training

Scand J Med Sci Sports. 2020 Sep;30(9):1615-1631. doi: 10.1111/sms.13707. Epub 2020 May 29.


When exercising with a small muscle mass, the mass-specific O2 delivery exceeds the muscle oxidative capacity resulting in a lower O2 extraction compared with whole-body exercise. We elevated the muscle oxidative capacity and tested its impact on O2 extraction during small muscle mass exercise. Nine individuals conducted six weeks of one-legged knee extension (1L-KE) endurance training. After training, the trained leg (TL) displayed 45% higher citrate synthase and COX-IV protein content in vastus lateralis and 15%-22% higher pulmonary oxygen uptake ( V ˙ O 2 peak ) and peak power output ( W ˙ peak ) during 1L-KE than the control leg (CON; all P < .05). Leg O2 extraction (catheters) and blood flow (ultrasound Doppler) were measured while both legs exercised simultaneously during 2L-KE at the same submaximal power outputs (real-time feedback-controlled). TL displayed higher O2 extraction than CON (main effect: 1.7 ± 1.6% points; P = .010; 40%-83% of W ˙ peak ) with the largest between-leg difference at 83% of W ˙ peak (O2 extraction: 3.2 ± 2.2% points; arteriovenous O2 difference: 7.1 ± 4.8 mL· L-1 ; P < .001). At 83% of W ˙ peak , muscle O2 conductance (DM O2 ; Fick law of diffusion) and the equilibration index Y were higher in TL (P < .01), indicating reduced diffusion limitations. The between-leg difference in O2 extraction correlated with the between-leg ratio of citrate synthase and COX-IV (r = .72-.73; P = .03), but not with the difference in the capillary-to-fiber ratio (P = .965). In conclusion, endurance training improves O2 extraction during small muscle mass exercise by elevating the muscle oxidative capacity and the recruitment of DM O2, especially evident during high-intensity exercise exploiting a larger fraction of the muscle oxidative capacity.

Keywords: arteriovenous oxygen difference; blood flow; endurance training; fick method; limitations; maximal oxygen uptake; muscle oxygen diffusion; peripheral adaptations.

MeSH terms

  • Adult
  • Citrate (si)-Synthase / metabolism*
  • Endurance Training / methods*
  • Humans
  • Mitochondria, Muscle / metabolism*
  • Mitochondrial Proteins / metabolism*
  • Oxygen Consumption / physiology*
  • Quadriceps Muscle / physiology*
  • Regional Blood Flow / physiology*
  • Young Adult


  • Mitochondrial Proteins
  • Citrate (si)-Synthase