Haematological rather than skeletal muscle adaptations contribute to the increase in peak oxygen uptake induced by moderate endurance training.
Level 4 - case-series / case-control
Single-arm pre-post interventional physiological study without a parallel control group.
PubMed 26282186 · doi:10.1113/JP270250
What was done
Sixteen untrained healthy male volunteers (mean age 25 ± 4 years, baseline peak oxygen uptake 3.5 ± 0.5 l/min) completed 6 weeks of supervised endurance training (3–4 sessions per week). Peak oxygen uptake (V̇O2peak), peak cardiac output (Q̇peak), haemoglobin mass (Hbmass), and blood volumes were measured before and after training. Muscle biopsies were evaluated for mitochondrial volume density, capillarity, fibre types, and oxidative phosphorylation capacity (OXPHOS). Following post-training testing, phlebotomy was performed to return red blood cell volume to pre-training levels, and V̇O2peak and Q̇peak were re-measured to isolate haematological from peripheral adaptations.
What was found
Endurance training significantly increased V̇O2peak by 9 ± 8%, Q̇peak by 7 ± 6%, red blood cell volume by 8 ± 4%, plasma volume by 4 ± 6%, and Hbmass by 6 ± 5% (all P < 0.05). Skeletal muscle capillary-to-fibre ratio increased by 18 ± 16% and mitochondrial volume density increased by 43 ± 30% (both P < 0.05), while OXPHOS remained unaltered. After phlebotomy re-established baseline red cell volume, V̇O2peak decreased by -7 ± 7% and Q̇peak decreased by -10 ± 7% (both P < 0.05). Stepwise multiple regression identified Q̇peak, red blood cell volume, and Hbmass as independent predictors of V̇O2peak.
Why it matters
This study demonstrates that initial aerobic capacity gains from moderate endurance training are primarily driven by central haemodynamic and oxygen-carrying adaptations rather than peripheral skeletal muscle respiratory capacity.
Limits
The sample size was small (n = 16) and restricted to young, healthy untrained males, limiting generalizability to females, older individuals, and trained athletes. The study lacked a non-exercising control group and only examined short-term (6-week) training adaptations.