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

Abstract

This study assessed the respective contributions of haematological and skeletal muscle adaptations to any observed improvement in peak oxygen uptake (VO2peak) induced by endurance training (ET). VO2peak , peak cardiac output (Qpeak), blood volumes and skeletal muscle biopsies were assessed prior (pre) to and after (post) 6 weeks of ET. Following the post-ET assessment, red blood cell volume (RBCV) reverted to the pre-ET level following phlebotomy and VO2peak and Qpeak were determined again. We speculated that the contribution of skeletal muscle adaptations to an ET-induced increase in VO2peak could be identified when offsetting the ET-induced increase in RBCV. VO2peak, Qpeak , blood volumes, skeletal muscle mitochondrial volume density and capillarization were increased after ET. Following RBCV normalization, VO2peak and Qpeak reverted to pre-ET levels. These results demonstrate the predominant contribution of haematological adaptations to any increase in VO2peak induced by ET. It remains unclear whether improvements in peak oxygen uptake (VO2peak) following endurance training (ET) are primarily determined by central and/or peripheral adaptations. Herein, we tested the hypothesis that the improvement in VO2peak following 6 weeks of ET is mainly determined by haematological rather than skeletal muscle adaptations. Sixteen untrained healthy male volunteers (age = 25 ± 4 years, VO2peak = 3.5 ± 0.5 l min-1) underwent supervised ET (6 weeks, 3-4 sessions per week). VO2peak, peak cardiac output (Qpeak), haemoglobin mass (Hbmass) and blood volumes were assessed prior to and following ET. Skeletal muscle biopsies were 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 VO2peak and Qpeak were measured again. We speculated that the contribution of skeletal muscle adaptations to the ET-induced increase in VO2peak would be revealed when controlling for haematological adaptations. VO2peak and Qpeak 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, Qpeak, RBCV and Hbmass were found to be independent predictors of VO2peak. In conclusion, the improvement in VO2peak following 6 weeks of ET is primarily attributed to increases in Qpeak and oxygen-carrying capacity of blood in untrained healthy young subjects.