Author(s): HAYASHI, N., MÜNDEL, T., LEI, T.H., BADENHORST, C., Institution: HIROSHIMA UNIVERSITY , Country: JAPAN, Abstract-ID: 2717

INTRODUCTION:
Although respiratory responses to exercise under heat stress across the menstrual cycle have been investigated in eumenorrheic women, the effect of exogenous hormonal profiles remains underexplored. Oral contraceptive use is characterised by minimal endogenous hormonal fluctuation (Elliott-Sale et al., 2020). The present study performed a secondary analysis of previous data (Lei et al., 2017, 2019) to compare exercise-induced respiratory responses under heat stress between the early follicular (EF)/quasi-EF and mid-luteal (ML)/quasi-ML phases in naturally menstruating and oral contraceptive pill (OCP)-using female athletes. Additionally, the study aimed to examine the associations between respiratory responses, thermoregulation, and hormonal profiles within each group.
METHODS:
Sixteen endurance-trained female athletes participated: eight with a natural menstrual cycle (NM group) and eight using OCP (OCP group). Each participant completed four trials, comprising fixed-intensity cycling (at 125 and 150 W) followed by a 30-min self-paced cycling session (with maximal effort; OCP ≈144–151 W, NM ≈138–148 W). Trials were conducted during the EF (days 3–6) /quasi-EF (days 3–5) and the ML (days 18–21)/quasi-ML (days 18–20) phases under two WBGT-matched heat-stress environments: hot, dry (≈34 ℃, ≈41 % RH) and warm, humid (≈29 ℃, ≈81 % RH). Expired respiratory gases were collected using a breath-by-breath system at rest and during exercise. Thermoregulatory response was also assessed.
RESULTS:
Respiratory parameters did not differ significantly between the menstrual/OCP phases. However, during both exercises in the heat conditions, the OCP group exhibited a significantly lower percentage of maximal oxygen uptake (VO2max) during self-paced cycling compared to the NM group (p = 0.003–0.014). despite no significant differences in VO2max or workload between the groups. A simple linear regression analysis showed that, in the OCP group, changes in mean body temperature during fixed-intensity exercise were associated with the ventilatory equivalent for oxygen uptake (125 W: b = 52.285, 95 %CI [12.346, 97.483], p = 0.011, R2 = 0.629; 150 W: b = 35.369, 95 %CI [26.688, 65.231], p = 0.001, R2 = 0.811 and Vt (125 W: b = 1.600, 95 %CI [0.959, 3.276], p < 0.001, R2 = 0.813; 150 W: b = 0.825, 95 %CI [0.201, 1.457], p = 0.040, R2 = 0.582).
CONCLUSION:
No significant menstrual/OCP phase differences were observed in ventilatory responses to exercise under the heat stress in female endurance athletes. In addition, the OCP group exhibited lower %VO2max or VO2 during both fixed-intensity and self-paced maximal exercise compared with the NM group. These results suggest that external factors, such as exogenous hormones and environmental heat stress, may exert a greater influence on respiratory responses during exercise in female athletes than endogenous hormonal fluctuations.