This study will investigate the effect of menstrual cycle (MC) and oral contraceptive pill phase on aspects of exercise physiology and athletic performance in female athletes. Specifically, this study intends to investigate the effects of circulating fluctuations in oestradiol and progesterone, experienced during the menstrual cycle and oral contraceptive pill use, on aspects of exercise physiology and athletic performance in female athletes. This research will help researchers determine if a particular hormonal profile affects physiological functioning such as muscle strength, maximum oxygen uptake and athletic performance in female athletes.

The menstrual cycle is the body's way of preparing for pregnancy. A "normal" or "healthy" cycle lasts an average of 21-35 days, which starts on the first day of bleeding. Throughout each cycle, levels of the hormones oestrogen and progesterone vary. The oral contraceptive pill is designed to prevent pregnancy; one way it does this is by lowering the levels of oestrogen and progesterone. The main purpose of oestrogen and progesterone is for reproduction. These hormones have also been shown to affect other bodily functions. The effects of oestrogen and progesterone on a woman's capacity to undertake exercise is an important factor for active women to consider to optimise training schedules and athletic performances. Emerging research has suggested that menstrual cycle phase or oral contraceptive use is associated with changes in sport and exercise performance. However, research on this topic is lacking (both in quality and quantity), and currently is quite contradictory. Much more high-quality research is required to explore the impact of both natural and supplemented oestrogen and progesterone on exercise-related outcomes to give convincing advice to active women. This research must follow strict procedural guidelines and include accurate hormonal measurement (through blood samples) to increase its usefulness to exercising women. Such research is needed to establish a cause-and-effect relationship for any differences in exercise physiology and athletic performance between the different phases of the menstrual cycle, between phases of oral contraceptive pill taking and not-taking, and between women using or not-using oral contraceptives.

Inclusion Criteria:

• Cis gender women
• 18-40 years of age
• Body mass index ≥ 19.5 and ≤ 25 kg∙-2; an exception can be made in the case of some athletes where they have a higher BMI, but are considered more lean/muscular than fat.
• Must be defined as healthy, i.e., not having any medical condition or taking any medication known to affect any of the outcome measures. Examples include diagnosed hyperandrogenism from polycystic ovary syndrome, congenital adrenal hyperplasia, or other differences in sexual development.
• Cannot smoke
• Considered to be physically active. Participants training status will be classified using the criteria outlined by McKay et al. (In press). For this study, participants must be Tier 2 athletes or above (Table 1). In addition, participants will be considered on the basis of relative maximal oxygen consumption (Decroix et al., 2016); needing to be Performance Level 2 or above (Table 2). Participants must provide details on gravidity and parity and must not be in the postpartum period (i.e., within 12 months of childbirth), currently breastfeeding, or pregnant. Participants will be recruited based on their endogenous hormonal profile (i.e., eumenorrheic or OCP users) and these profiles (including inclusion/exclusion criteria) are described below in detail.

Group Specific Inclusion Criteria:

Group 1. Eumenorrheic [EUM]

• Must have menstrual cycle lengths ≥ 21 days and ≤ 35 days resulting in 9 or more consecutive periods per year
• Must provide evidence of a luteinising hormone (LH) surge
• Must demonstrate the required hormonal profile as defined in Table 3
• Cannot have used any type of hormonal contraceptives (HC) for a minimum of 3 months, but ideally 6 months, prior to enrolment

Group 2. Oral contraceptive pill [OCP] users

• Must have been taking their OCP ≥ 3 months prior to recruitment
• Must be taking a combined, monophasic, second generation OCP (see Table 4 for permitted brands)
• Must demonstrate the correct hormonal profile as defined in Table 5

Exclusion Criteria:

-Cannot speak, read and understand English

The experimenter will be blind to the intended testing timepoint. Experimenter blinding will be achieved by having an independent party schedule the testing session, based on the participant's hormonal profile, without disclosing this to the experimenter who will conduct the testing session. On completion of the study, the blinded experimenters will be asked to comment if blinding was achieved and maintained.