# Oxygen, Hormones, and Performance: A Case Study of
## 1 Introduction  
The menstrual cycle (MC) is a complex physiological process involving interactions between the hypothalamus, pituitary gland, ovaries, and uterus to prepare the body for gestation. It is divided into the ovarian cycle, which consists of the follicular and luteal phases, and the endometrial cycle, which comprises the proliferative, secretory, and menstrual phases [1]. The complex feedback system that regulates the MC involves key hormones such as oestrogen (E2), progesterone (P4), pituitary gonadotrophins (follicle-stimulating hormone, FSH; luteinising hormone, LH) and the hypothalamic gonadotrophinreleasing hormone (GnRH) [2]. As mentioned above, the MC itself is divided into the ovarian cycle, which is related to the maturation and release of the mature oocyte from the ovaries [2]; and the endometrial cycle, which parallels the ovarian cycle and describes the process of the endometrium throughout the cycle. MC [1] consists of four hormnally regulated phases (Figure 1). It begins with the follicular phase (day 1-14, varying individually), where menstruation occurs, and GnRH stimulates FSH and LH release, promoting follicle growth. The follicles produce E2, thickening the endometrium. Around day 14, a dominant follicle triggers ovulation,releasing an egg. In the luteal phase, the corpus luteum forms, releasing E2 and P4, which suppress FSH and LH, while the endometrium matures into the secretory phase. If pregnancy does not occur, the hormone levels drop, the corpus luteum regresses, and the menstrual phase begins with the shedding of the endometrium, restarting the cycle. The study of the MC in female athletes requires important methodological considerations. One of them is to verify the phase of the MC during the study [3]. The scientific literature often lacks objective verification of the menstrual cycle, frequently relying on subjective methods that do not account for ovulation or potential luteal phase deficiencies. The occurrence of non-ovulatory cycles and luteal phase deficiencies is often overlooked due to the reliance on calendar-based tracking and the first day of menstrual bleeding as primary indicators. However, it is crucial to accurately determine the phase of the cycle, as the presence of regular menstrual bleeding does not necessarily indicate normal hormone concentrations. Throughout the menstrual cycle, women experience hormonal fluctuations that can influence physical, psychological, physiological, and biomechanical factors. Reproductive hormones play a significant role in metabolism, ventilation, immune function, and cardiovascular regulation [4]. These hormones vary in concentration throughout life and the MC. This unique characteristic means that a woman, over the years, may experience numerous hormonal profiles and is therefore a variable that requires complex methodological considerations to study [4]. In terms of motivation and subjective perception [5], E2 levels have been recognised as playing a protective role against low-mood states. Consequently, during phases of lower E2 concentration, negative emotions may be amplified. Additionally, female athletes often associate the menstrual phase with sensations of discomfort, pain, and mood disturbances. Fluctuations in hormones during MCs seem to be linked to a heightened vulnerability to injuries, particularly during ovulation [6]. Additionally, the levels of laxity, neuromuscular  
Figure 1: Representation of the different phases of the normal menstrual cycle, illustrating the variation in hormone levels of oestrogen (E2), luteinising hormone (LH), follicle-stimulating hormone (FSH) and progesterone (P4). Figure made with the influence of [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON]. lar control, and strength vary throughout the MC, reaching their peaks during the ovulatory phase and aligning with a peak in E2 levels [6]. However, scientific evidence to date has yielded inconclusive results regarding the effects of the MC on physical exercise.