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

Abstract

Key points: This study assessed the respective contributions of haematological and skeletal muscle adaptations to any observed improvement in peak oxygen uptake () induced by endurance training (ET). , peak cardiac output (), 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 and were determined again. We speculated that the contribution of skeletal muscle adaptations to an ET‐induced increase in could be identified when offsetting the ET‐induced increase in RBCV. , , blood volumes, skeletal muscle mitochondrial volume density and capillarization were increased after ET. Following RBCV normalization, and reverted to pre‐ET levels. These results demonstrate the predominant contribution of haematological adaptations to any increase in induced by ET. It remains unclear whether improvements in peak oxygen uptake () following endurance training (ET) are primarily determined by central and/or peripheral adaptations. Herein, we tested the hypothesis that the improvement in following 6 weeks of ET is mainly determined by haematological rather than skeletal muscle adaptations. Sixteen untrained healthy male volunteers (age = 25 ± 4 years, = 3.5 ± 0.5 l min−1) underwent supervised ET (6 weeks, 3–4 sessions per week). , peak cardiac output (), haemoglobin mass (Hb mass) and blood volumes were assessed prior to and following ET. Skeletal muscle biopsies were analysed for mitochondrial volume density (Mito VD), 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 and were measured again. We speculated that the contribution of skeletal muscle adaptations to the ET‐induced increase in would be revealed when controlling for haematological adaptations. and 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 Hb mass 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 Mito VD increased (P < 0.05) following ET (18 ± 16 and 43 ± 30%), but OXPHOS remained unaltered. Through stepwise multiple regression analysis, , RBCV and Hb mass were found to be independent predictors of . In conclusion, the improvement in following 6 weeks of ET is primarily attributed to increases in and oxygen‐carrying capacity of blood in untrained healthy young subjects.