## 4 Discussion  
This study provides novel research about the influence of the menstrual cycle and OC on knee laxity and ACL injuries. The menstrual cycle seems to influence knee laxity in women, the latter being higher during the ovulatory phase. However, current knowledge cannot yield conclusions about the influence of the menstrual cycle on the risk of ACL injury. OC may decrease the possibility of an ACL tear. As demonstrated by [PERSON] et al., women with an ACL tear show a decrease in the concentration of testosterone, 17B estradiol and progesterone. This hormonal drop could lead to ACL damage and could be considered a risk factor. This, therefore, raises questions about the protective action or lack thereof of hormones. Indeed, during the ovulatory phase, when estradiol concentration is maximal, joint stability would be negatively affected [35]. This estradiol peak during the ovulatory phase is confirmed in the study carried out by [PERSON] et al. since the concentration of 17B estradiol significantly increases during ovulation, compared to the follicular phase [28]. These findings are in accordance with the results of the study performed by [PERSON] et al. [31]. Regarding knee laxity, an increase in TAT could be observed during the ovulatory phase compared to the follicular phase [28]. In this line, [PERSON] et al. observed an increase in GJL during the menstrual cycle phase where estradiol is higher [31]. Therefore, it can be assumed that the increase in estrogen concentration would induce an increase in knee laxity characterized by TAT. In concordance, the review performed by [PERSON] et al. in 2017 identified a higher ACL laxity during the ovulatory phase compared to the follicular phase [24]. The role of OC in regulating the menstrual cycle has been investigated in some studies [27; 29; 30]. [PERSON] et al. did not detect an increased risk of ACL injury in OC or intrauterine device (IUD) users, not allowing a protective effect to be established. Nevertheless, the control group (IUD) had exogenous hormones, so it would be interesting to study the difference with eumenorrheic women (women with a regular cycle and not taking OC). According to [PERSON] et al. and [PERSON] et al., OC users have decreased ACL laxity [29] due to a lower relaxin concentration [30], and therefore may have a lower risk of an ACL tear. Consistent with these findings, the study carried out by [PERSON] et al. suggests that the regulatory effects of OC on estrogen levels may reduce the overall risk of ACL injury in young women by reducing the risk factor for induced ACL laxity by estrogen [26]. Despite the limited clinical evidence on this protective aspect, [PERSON] et al. state that it would be advisable to consider the use of OC in elite high school and college-aged athletes, especially those who are at risk of ACL tear [25]. Future research is needed to confirm this protective association. Some aspects should be considered when interpreting the results of this review. The age range of the participants is very wide, between 13 and 49 years old. In addition, the selected studies do not specify whether the participants had been pregnant before. During pregnancy, women suffer an important hormonal imbalance [36], so it would be wise to carry out studies on women who have never been pregnant to observe the true impact of a menstrual cycle without previous upheaval. It can be assumed that after pregnancy the hormonal levels are modified, in particular those of relaxin, so it would be interesting to observe whether there is an increased risk of ACL injury in postpartum women. Another important aspect to consider is that the hormonal profile of each woman may differ during each cycle even in women with a regular menstrual cycle [37]. For this reason, measurement tools should be predefined identically in each study to avoid any bias in the evaluation of data, allowing researchers to perform a meta-analysis on this subject.