Relationships between Changes in Hematological Adaptations and Exercise Capacity in Olympic Rowers after a Period of Reduced Training Loads

**Background:** Endurance performance is positively associated with hematological adaptations, with high total hemoglobin mass (tHb-mass) and intravascular volumes commonly observed in elite endurance athletes. However, it remains unclear whether fluctuations in exercise capacity during the annual training cycle are directly linked to changes in hematological adaptations, which appear relatively stable. The authors investigated the relationship between changes in hematological adaptations and exercise capacity in Olympic rowers after a period of reduced training loads, hypothesizing that training-induced changes in maximal and submaximal power would be associated with changes in intravascular volumes.

**Methods:** Ten world-class male rowers (men's coxed eight and substitutes; mean age 26.6 ± 3.3 years; body mass 97.7 ± 5.4 kg; body height 198 ± 6 cm; training experience 11.4 ± 3.3 years; VO2peak 64.2 ± 4.3 ml/kg/min) from the national Olympic team were assessed twice during an annual training cycle: in the competitive phase (July) and the general preparation phase (January). These periods represented a 34% reduction in training volume (7953 min vs. 5283 min over 2 months). Athletes underwent laboratory testing including body mass assessment, blood collection, medical examination, a graded exercise test (GXT) on a rowing ergometer, and measurements of tHb-mass and intravascular volumes using an optimized carbon monoxide rebreathing method. Blood samples were analyzed for hemoglobin concentration (Hb), ferritin, iron, unsaturated iron-binding capacity (UIBC), C-reactive protein (CRP), and soluble transferrin receptor (sTfR). Maximal power output (Pmax) was calculated from the GXT, and submaximal power at lactate threshold was determined using the modified Dmax method and at 4 mmol/l lactate. Statistical analysis used Wilcoxon's matched-pairs test, Cohen's r effect sizes, and Spearman's rank correlation analysis.

**Key Results:** Mean body mass increased from 97.7 ± 5.4 kg to 100.2 ± 6.1 kg (p = 0.022). Relative maximal power (rel.Pmax) decreased from 4.98 ± 0.23 W/kg to 4.78 ± 0.40 W/kg (p = 0.028), representing a −4% change. Lactate concentration 3 minutes post-GXT (La3') decreased from 14.2 ± 1.9 mmol/l to 11.9 ± 2.3 mmol/l (p = 0.005; −16% change). Maximal heart rate (HRmax) decreased from 193 ± 6 bpm to 188 ± 9 bpm (p = 0.017; −2% change). Absolute plasma volume (PV) decreased from 5940 ± 773 ml to 5678 ± 683 ml (p = 0.017; −4% change), and relative PV decreased from 61 ± 5 ml/kg to 57 ± 5 ml/kg (p = 0.005; −7% change). Total hemoglobin mass remained stable (1367 ± 130 g vs. 1361 ± 83 g; p = 0.959), as did hemoglobin concentration (15.5 ± 0.9 g/dl vs. 15.4 ± 0.9 g/dl; p = 0.878) and blood volume (9759 ± 1092 ml vs. 9742 ± 886 ml; p = 0.878). Changes in Pmax were significantly correlated with changes in PV (rS = 0.842, p = 0.002) and BV (rS = 0.818, p = 0.004), but not with changes in tHb-mass (rS = 0.588, p = 0.074) or Hb (rS = −0.188, p = 0.602). Changes in submaximal power (PDmaxM and PAT4) were not significantly correlated with any hematological variable. All iron status and inflammation indices remained within reference ranges at both time points.

**Clinical Implications:** This study demonstrates that in elite endurance athletes, changes in maximal exercise capacity after a period of reduced training loads are closely related to changes in intravascular volumes (PV and BV) rather than to changes in total hemoglobin mass or hemoglobin concentration. These findings suggest that monitoring plasma volume may be more informative than hemoglobin mass for understanding performance fluctuations during training cycles in elite athletes. The relative stability of tHb-mass despite substantial training volume reduction supports the concept that hemoglobin mass is primarily genetically determined with only modest training-induced modifications. The strong correlation between PV changes and maximal power highlights the importance of maintaining intravascular volume for peak performance, though the authors note that environmental temperature differences between summer and winter testing may have influenced PV changes. For coaches and sports medicine practitioners working with elite endurance athletes, these results suggest that strategies to maintain plasma volume during periods of reduced training or in cold environments may help preserve maximal exercise capacity.