# Oxygen, Hormones, and Performance: A Case Study of
## 4 Discussion  
The aim of this study was to examine the potential influence of the menstrual cycle (MC) on variations in body composition, performance, and fatigue in female athletes. Notably, this research introduces a novel area of study in the field of sports science and women's physiology, as no prior studies have investigated the relationship between muscle oxygen saturation (SmO\({}_{2}\)) and the MC. Furthermore, the application of near-infrared spectroscopy (NIRS; MOXY, Fortiori Design LLC, Hutchinson, MN, USA) offers a valuable tool for analysing oxygen consumption and recovery curves in various contexts [22], thereby contributing to the optimisation of training strategies for female athletes. This study identified an increase in muscle mass and a reduction in fat mass during the follicular and ovulatory phases. Previous research suggests that elevated levels of oestrogen (E2) and progesterone (P4) in female athletes may contribute to increased muscle mass and muscle diameter [23; 24], as well as enhanced muscle strength [25]. In one of the scarce interventions where hormonal milieu was taken into consideration [26], higher trainability of strength was found regarding the phase of the cycle, showing a wide inter individual variability in all subjects. This effect is mainly dependent of the variations in steroid hormone levels during the cycle and their potential impact on protein synthesis. In fact, the significant decrease in muscle strength occurs during the perimenopausal and postmenopausal stages (24), when hormones must be replaced to maintain strength levels. Notably, the highest peak of repetitions observed in this study occurred during the ovulatory phase, coinciding with the peak levels of both hormones. Furthermore, this phase was associated with greater muscle mass in both legs and a higher total muscle percentage. Additionally, increased levels of motivation have been reported during ovulation compared to the follicular and luteal phases, whereas the most negative sensations were recorded in the late luteal phase, corresponding to the premenstrual period [5]. This association may partly explain the greater capacity for exertion and, consequently, the increased strength output observed. However, a recent study has indicated that there are no significant differences in the rate of perceived exertion (RPE) in strength-related tasks across different menstrual cycle phases [27]. A deep reflection on the behaviour of both legs during the BSS test shows that both muscles work in a similar manner throughout the cycle, except during the luteal phase. In this phase, the hamstrings exert a similar effort to that of the other phases, whereas the quadriceps barely lose saturation. This could suggest that they are scarcely engaged during the effort and, consequently, the agonist muscles would be more exposed to the load, potentially leading to greater fatigue. This occurrence correlates with studies that observe asymmetries during the luteal phase due to increased laxity [28; 29]. The rise in progesterone may lead to poorer performance in maximal efforts, resulting in an increased risk of injuries. The non-dominant leg demonstrates greater variability in recovery during the menstruation phase, whereas the dominant leg exhibits more pronounced fluctuations during the luteal phase. The poorest performance is expected to occur in the late luteal and early follicular phases due to significant variations in E2 and P4 levels [30]. This may be associated with a slight reduction in total muscle mass and an increase in fat mass. However, muscle strength production is primarily dependent on the recruitment and firing capacity of motor units. On the other hand, there are also studies mentioning that the muscle exhibits a higher fatigue ratio during menstruation compared to the follicular and luteal phases [31].