Impact of Fluctuating Sex Hormones on VO₂Kinetics during Heavy and Severe Domain Exercise

Menstrual cycle phase does not appear to impact sustainable exercise intensities, nor the power associated with the maximal metabolic steady state. However, the potential impact of fluctuating sex hormones on pulmonary oxygen uptake ([Formula: see text]O 2 ) kinetics is unknown. The goal was to explore the impact of sex hormone profile on [Formula: see text]O 2 kinetics, including the [Formula: see text]O 2 slow component, during exercise spanning the heavy and severe intensity domains. We hypothesized that estradiol would not impact [Formula: see text]O 2 kinetics, but progesterone would quicken [Formula: see text]O 2 kinetics. Twelve endurance trained women (female assigned at birth; 9 IUD users & 3 naturally cycling; Age: 29 ± 7 years) completed six visits. Maximal oxygen uptake and gas exchange parameters were determined via graded exercise test on a cycle ergometer. Participants then completed four weekly cycling protocols consisting of three, 4-minute stages spanning intensity domains (Stage 1: 78 ± 5 %, Stage 2: 86 ± 5 %, and Stage 3: 94 ± 5 % [Formula: see text]O 2MAX ). Serum sex hormone concentration for estradiol and progesterone was determined at the beginning of each visit. Menstrual cycle phase did not dictate visit order. The time constant of the [Formula: see text]O 2 kinetics response (τ[Formula: see text]O 2 ), [Formula: see text]O 2 amplitude, and percent change in the slow component (%SC) over the last 2-min were determined at each stage. For analysis, visits were ordered beginning with the lowest hormone concentration (early follicular) visit first, followed by remaining visits in sequential order. Relationships between hormone concentration, τ[Formula: see text]O 2 , [Formula: see text]O 2 amplitude, and %SC across the 4 visits were analyzed via linear mixed models. As expected, τ[Formula: see text]O 2 increased with exercise intensity (P ≤ 0.045). τ[Formula: see text]O 2 was not impacted by fluctuations in estradiol concentration (P = 0.957). However, when controlling for estradiol concentration, τ[Formula: see text]O 2 was positively correlated with progesterone concentration (0.14 sec / 1 ng/mL progesterone; P < 0.001). Within-participants, progesterone was positively correlated to [Formula: see text]O 2 amplitude (P = 0.005) however, an estradiol-progesterone interaction (P = 0.002) suggests the effect of progesterone was reduced at higher estradiol concentrations. Although, across participants, this effect was weak. The %SC in [Formula: see text]O 2 was not associated with estradiol nor progesterone concentration (P ≥ 0.198). Independent of estradiol concentration, high progesterone concentrations appear to slow the rapid onset [Formula: see text]O 2 kinetics at the beginning of exercise. Additionally, an estradiol - progesterone interaction appears to influence the overall amplitude of the [Formula: see text]O 2 response. Wu Tsai Human Performance Alliance at the University of Oregon This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.