Increased oxygen extraction and mitochondrial protein expression after small muscle mass endurance training.
Level 3 - non-randomized controlled study
Within-subject controlled physiological trial (single-leg training vs. contralateral control leg)
PubMed 32403173 · doi:10.1111/sms.13707
What was done
Nine individuals completed six weeks of one-legged knee extension (1L-KE) endurance training, with the contralateral limb serving as an untrained control (CON). After training, vastus lateralis biopsies were analyzed for citrate synthase and COX-IV protein content. Pulmonary peak oxygen uptake and peak power output were measured during 1L-KE. In addition, leg oxygen extraction using catheters and blood flow using Doppler ultrasound were measured while both legs exercised simultaneously at identical submaximal power outputs ranging from 40% to 83% of peak power output.
What was found
Compared to CON, the trained leg showed 45% higher citrate synthase and COX-IV protein content, and 15% to 22% higher peak oxygen uptake and peak power output during 1L-KE (all P < .05). During bilateral exercise, the trained leg showed higher oxygen extraction (main effect across 40% to 83% of peak power: 1.7 +/- 1.6 percentage points, P = .010). The largest difference occurred at 83% of peak power, where oxygen extraction was 3.2 +/- 2.2 percentage points higher and arteriovenous oxygen difference was 7.1 +/- 4.8 mL/L higher (P < .001). Muscle oxygen conductance and the equilibration index Y were higher in the trained leg at 83% of peak power (P < .01). The between-leg difference in oxygen extraction correlated with the ratio of citrate synthase and COX-IV (r = .72 to .73, P = .03), but not with differences in the capillary-to-fiber ratio (P = .965).
Why it matters
The study shows that elevating muscle oxidative capacity directly improves local oxygen extraction and reduces peripheral diffusion limitations during small muscle mass exercise, independent of capillary density changes.
Limits
The sample size was very small (n = 9). The abstract does not disclose participant demographics such as age, sex, or baseline fitness levels. The single-leg knee-extension model isolates peripheral muscle adaptations and does not capture whole-body cardiovascular constraints.
Cited by
- supports Sub-elite individuals can increase VO2 max by improving muscle oxygen extraction, increasing muscle oxidative enzymes, increasing mitochondrial size and number, and expanding the arteriovenous oxygen difference.