Fluctuations in female sex hormones, particularly estrogen (E2) and progesterone (P4), create distinct hormone profiles across the menstrual cycle that may impact exercise performance. Existing data is conflicting, with studies reporting either no impact or variable effects of sex hormones on performance. However, the influence of hormone profile on performance in highly trained female middle-distance runners has not been explored. Our goal was to determine whether sex hormone profile affects the rate of fatigue progression and running performance in the severe domain. We hypothesized that a high hormone profile, as defined by P4 concentration, would increase the rate of fatigue and decrease time to exhaustion (TTE) relative to a low hormone profile. METHODS: Eleven highly trained female runners (sex assigned at birth; age: 27 ± 5 yrs.; VO2MAX: 59.9 ± 6.3 mL·kg - ¹·min - ¹) completed four laboratory visits. Maximal oxygen consumption (VO2MAX) and lactate turn-point (LTP) were determined via a graded treadmill exercise test. A familiarization TTE visit was followed by two identical experimental visits scheduled 13-15 days apart to align with a low or high hormone profile. The TTE protocol consisted of a constant-speed trial designed to elicit exhaustion in four minutes based on the participant’s critical velocity and LTP. Experimental visits included a venous blood draw for serum sex hormones (E2 and P4), the TTE trial, and determination of hemoglobin mass (HbMASS), red blood cell volume (RBCV), plasma volume (PV), and blood volume (BV), via carbon monoxide rebreathe. Paired t-tests evaluated differences in TTE duration between hormone profiles. Pearson correlations were used to examine relationships with hormone profile, performance, and hematological variables. RESULTS: Nine women demonstrated distinct low (E2: 91 ± 61, P4: 0.1 ± 0) and high (E2: 138 ± 55, P4: 8.8 ± 4.8) hormone profiles between experimental visits. TTE duration was not different between the low and high hormone visits (243.8 ± 20.1 sec vs. 236.4 ± 20.2 sec, respectively; P = 0.64). HbMASS, RBCV, PV, and BV did not differ between hormone profiles (P ≥ 0.699). However, RBCV (r = 0.87, P = 0.002) and PV (r = –0.80, P = 0.01) were strongly associated with TTE duration, independent of hormone profile. CONCLUSION: Fatigue progression during middle-distance running in the severe domain does not appear to be significantly affected by sex hormone profile. However, the responses were highly variable. Variations in RBCV and PV appear to have a strong impact on performance, independent of sex hormone fluctuations. Funding: Nike Inc. This abstract was presented at the American Physiology Summit 2026 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.
Richardson et al. (Fri,) studied this question.