Author(s): SKATTEBO, Ø., MARTIN-RINCON, M., RUD, B., BOUSHEL, R., HOLMBERG, H.C., CALBET, J.A.L., HALLÉN, J., Institution: NORWEGIAN SCHOOL OF SPORT SCIENCES, Country: NORWAY, Abstract-ID: 885

INTRODUCTION:
We have recently demonstrated that leg oxygen (O2) extraction and lean mass-normalised cardiac output (Q) and leg blood flow (LBF) at VO2max are similar between highly trained females and males. However, males’ 10% higher arterial O2 content (CaO2) results in 10% higher systemic- and leg O2 delivery and VO2max (Skattebo et al., 2025). Herein, we investigated the interaction between sex and hypoxia during maximal exercise in the same cohort.
METHODS:
10 female and 13 male highly trained cyclists and triathletes (pulmonary VO2max: 53±4 and 64±4 ml∙min-1∙kg-1 body mass) underwent incremental exercise to volitional exhaustion on a cycle ergometer to determine VO2max at 418 m (ambient conditions) and at a simulated altitude of 2,842 m in a hypobaric chamber. Thermistor-tipped catheters were placed in the femoral vein and artery, and Q and LBF were determined by bolus thermodilution. Vascular conductances (VC) were calculated from blood flow and intra-arterial and -venous pressures. Leg O2 extraction was calculated as the fractional difference between O2 content of femoral venous blood and CaO2.
RESULTS:
In hypoxia compared to normoxia, pulmonary and two-leg VO2max decreased by 14±2% and 19±3% (mean±95% CI; P<0.01), respectively, with the reductions being similar across sexes when normalised to lean mass (P=0.14). Two-leg VO2 accounted for 77±3% and 71±3% of the pulmonary VO2max in normoxia and hypoxia, respectively (altitude: P<0.01), and the upper-body VO2 was slightly increased in hypoxia (16±19%; P=0.13). No significant sex x altitude interactions were observed for the following parameters, so results are presented as averages across sexes. The decreased VO2max at altitude was driven by a 17±2% reduction in CaO2 (P<0.01), whereas Q was unchanged (-1±2%; P=0.12). In addition, LBF was reduced by 5±3% (P<0.01), reflecting a larger fraction of Q being directed to the upper body (P=0.04). This was mediated by increased upper-body VC (P<0.01), unaffected two-leg VC (P=0.50), and reduced mean arterial pressure (P<0.01). Hence, O2 delivery to the legs was reduced by 21±3% (P<0.01), but it was reduced by only 9±9% to the upper body (P=0.01). O2 extraction increased both systemically (81±3% to 85±2%), for the legs (92±1% to 94±1%), and for the upper body (58±7 to 70±7) (all P<0.01).
CONCLUSION:
The impact of hypoxia on haemodynamics, O2 delivery and O2 extraction is essentially similar between sexes, with the reduction in pulmonary VO2max driven mainly by reduced CaO2. However, the VO2 for the exercising legs is even more decreased due to hypoxia-stimulated vasodilation and increased vascular conductance in the upper-body. This mechanism aims to preserve O2 delivery to vital organs such as the brain, heart and respiratory muscles; “stealing” blood flow and O2 delivery from the exercising legs.

Skattebo et al. (2025). Determinants of maximal oxygen uptake in highly trained females and males: a mechanistic study of sex differences using advanced invasive methods. J Physiol.