A review of the sports medicine literature reveals a clear underrepresentation of female athletes in research. In the current era of precision medicine, increasing attention has been directed toward the regulatory roles of estrogen and progesterone in athletic performance and health optimization. Regular fluctuations in estrogen and progesterone across menstrual cycle phases (i.e., early follicular, late follicular, and mid-luteal phases) may influence strength performance, hydration status, body composition, and energy balance. However, few studies have examined these outcomes using hormonal confirmation of menstrual cycle phases. Monophasic oral contraceptive use also represents a highly relevant hormonal condition among female athletes, as exogenous hormones suppress endogenous ovarian fluctuations and create distinct hormonal profiles across active pill-consumption and withdrawal phases. Nevertheless, the influence of oral contraceptive phases on strength-related outcomes, hydration markers, body water regulation, body composition, and energy balance remains insufficiently characterized, particularly in comparison with naturally menstruating athletes. In response to these gaps, this longitudinal observational study primarily aims to examine variations in strength-related outcomes across three distinct menstrual cycle phases (early follicular, late follicular, and mid-luteal) in eumenorrheic athletes. Secondary objectives include: (i) conducting within- and between-group comparisons of hydration status, energy balance, and strength outcomes (maximal, endurance, and explosive torque) in naturally menstruating athletes and oral contraceptive users; (ii) testing the reliability of methods used to assess body water, energy expenditure, and body composition across the menstrual cycle; (iii) exploring associations between energy availability, resting energy expenditure, and sex hormone concentrations across menstrual cycle phases; and (iv) testing, validating, and proposing methodological recommendations for the use of bioelectrical impedance analysis in tracking fluid-related changes across hormonal phases. To achieve these goals, the study will use a longitudinal observational design involving 40 female athletes, including 24 naturally menstruating athletes and 16 oral contraceptive users. Naturally menstruating athletes will be assessed during the early follicular, late follicular, and mid-luteal phases of the menstrual cycle, while oral contraceptive users will be assessed across pill-consumption and withdrawal phases. Measurements will be conducted across the three menstrual cycle phases and across oral contraceptive use phases, and will include: i) maximal voluntary isometric strength assessed using handgrip dynamometry, bench press, and leg press; ii) serum estrogen and progesterone; iii) body water and its compartments, and water turnover by dilution techniques; iv) hydration status by plasma osmolality, sodium, and vasopressin; v) energy balance by doubly labeled water and body composition changes; vi) resting energy expenditure by indirect calorimetry.