Determinants of maximal oxygen uptake in highly trained females and males: a mechanistic study of sex differences using advanced invasive methods

Females typically have lower body mass-normalised maximal oxygen uptake ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mspace></mml:mspace> <mml:mi>max</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${\dot V_{{{\mathrm{O}}_2}\!\max }}$</mml:annotation></mml:semantics> </mml:math> ) than males. However, whether this difference is solely due to body composition or also reflects sex-based differences in cardiovascular and muscular capacities for O₂ delivery and O₂ extraction remains unclear. This study examined sex differences in the O₂ transport chain when normalised to lean body mass (LBM). Twenty-three highly trained cyclists and triathletes (10 females; 29 ± 6 years) performed incremental cycling to exhaustion on an ergometer with simultaneous assessment of cardiac output, leg blood flow (thermodilution), O₂ delivery, and leg O₂ extraction (arterial and femoral venous catheters). Mitochondrial (TEM) and capillary (immunohistochemistry) densities were assessed in the vastus lateralis. Maximal cardiac output was 26% lower in females than males (22 ± 3 vs. 30 ± 3 l min^-1; P < 0.001). However, this difference disappeared when normalised to LBM (P = 0.375). Two-leg blood flow was similar after normalisation to leg lean mass (LLM; P = 0.327). However, females had 10% lower haemoglobin concentration and arterial O₂ content (177 ± 10 vs. 194 ± 15 ml l^-1; P = 0.004), resulting in 11%-14% lower lean mass-normalised systemic and leg O₂ delivery. Leg O₂ extraction (91 ± 3 vs. 92 ± 3%; P = 0.204) and mitochondria, cristae, and capillary densities were similar between sexes. Therefore, proportional to sex differences in O₂ delivery, females had lower lean mass-normalised pulmonary (63 ± 8 vs. 73 ± 4 ml min^-1 kg_LBM ^-1; P = 0.003) and leg (135 ± 14 vs. 160 ± 14 ml min^-1 kg_LLM ^-1; P = 0.002) <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mspace></mml:mspace> <mml:mi>max</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${\dot V_{{{\mathrm{O}}_2}\!\max }}$</mml:annotation></mml:semantics> </mml:math> . These findings demonstrate that highly trained females and males have similar muscle O₂ extraction and perfusion per kg LBM. However, females' 10% lower haemoglobin concentration results in lower LBM-normalised O₂ delivery and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>̇</mml:mo></mml:mover> <mml:mrow><mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:mspace></mml:mspace> <mml:mi>max</mml:mi></mml:mrow> </mml:msub> <mml:annotation>${\dot V_{{{\mathrm{O}}_2}\!\max }}$</mml:annotation></mml:semantics> </mml:math> . KEY POINTS: Females and males differ substantially in body size and composition, with males having greater skeletal muscle mass and females a higher body fat percentage. During maximal exercise, the active skeletal muscles consume most of the body's oxygen uptake. Consequently, males exhibit higher absolute and body-mass-normalised maximal oxygen uptakes. Here, we show that the heart's capacity to pump blood and perfuse the exercising muscles is similar between sexes when scaled to muscle mass. Despite similar perfusion, oxygen delivery per exercising muscle mass is approximately 10% lower in females than males, caused by a 10% lower blood haemoglobin concentration and oxygen-carrying capacity. Conversely, the fractional oxygen extraction by the skeletal muscles, along with their mitochondria and capillary densities, are similar between sexes. These findings demonstrate that sex differences in body composition and haemoglobin concentration are the primary mechanisms underpinning the lower body-mass normalised maximal oxygen uptake in females compared to males.