Haematological rather than skeletal muscle adaptations contribute to the increase in peak oxygen uptake induced by moderate endurance training

Key points This study assessed the respective contributions of haematological and skeletal muscle adaptations to any observed improvement in peak oxygen uptake (V.(O2 peak) ) induced by endurance training (ET). V.(O2 peak) , peak cardiac output (Q. peak ), blood volumes and skeletal muscle biopsies were assessed prior (pre) to and after (post) 6weeks of ET. Following the post-ET assessment, red blood cell volume (RBCV) reverted to the pre-ET level following phlebotomy and V.(O2 peak) and Q. peak were determined again. We speculated that the contribution of skeletal muscle adaptations to an ET-induced increase in V.O2 peak could be identified when offsetting the ET-induced increase in RBCV. V.O2 peak , Q. peak , blood volumes, skeletal muscle mitochondrial volume density and capillarization were increased after ET. Following RBCV normalization, V.O2 peak and Q.peak reverted to pre-ET levels. These results demonstrate the predominant contribution of haematological adaptations to any increase in V.O2 peak induced by ET. It remains unclear whether improvements in peak oxygen uptake (. V O2peak) following endurance training (ET) are primarily determined by central and/ or peripheral adaptations. Herein, we tested the hypothesis that the improvement in. V O2peak following 6 weeks of ET is mainly determined by haematological rather than skeletal muscle adaptations. Sixteen untrained healthy male volunteers (age= 25 +/- 4years,. V O2peak = 3.5 +/- 0.5lmin-1) underwent supervised ET (6weeks, 3-4 sessions perweek).. V-O2peak, peakcardiacoutput (. Q peak), haemoglobin mass (Hbmass) and blood volumeswere assessed prior to and following ET. Skeletalmuscle biopsieswere analysed for mitochondrial volume density (MitoVD), capillarity, fibre types and respiratory capacity (OXPHOS). After the post-ET assessment, red blood cell volume (RBCV) was re-established at the pre-ET level by phlebotomy and. V O2peak and. Q peak were measured again. We speculated that the contribution of skeletal muscle adaptations to the ET-induced increase in. V O2peak would be revealed when controlling for haematological adaptations.. V O2peak and. Q peak were increased (P < 0.05) following ET (9 +/- 8 and 7 +/- 6%, respectively) and decreased (P < 0.05) after phlebotomy (-7 +/- 7 and -10 +/- 7%). RBCV, plasma volume and Hbmass all increased (P < 0.05) after ET (8 +/- 4, 4 +/- 6 and 6 +/- 5%). As for skeletal muscle adaptations, capillary-to-fibre ratio and total MitoVD increased (P < 0.05) following ET (18 +/- 16 and 43 +/- 30%), but OXPHOS remained unaltered. Through stepwise multiple regression analysis,. Q peak, RBCV and Hbmass were found to be independent predictors of. V-O2peak. In conclusion, the improvement in. V-O2peak following 6 weeks of ET is primarily attributed to increases in. Q peak and oxygen-carrying capacity of blood in untrained healthy young subjects.