The underrepresentation of female athletes in sports research: considerations for cardiovascular health

The proportion of female athletes in competitive sporting events—including the World Championships and Olympic games—has increased to nearly half of all athletes, and competing athletes increasingly include those who are peripartum. While participation in sports is closely implicated in cardiovascular health and disease, females remain underrepresented in recreational and performance sports research. This underrepresentation is marked during pregnancy, a unique biological state with substantial hemodynamic changes that have implications on athletic performance and on any underlying cardiovascular conditions. The evidence to guide recreational and performance sports is derived mainly from research in males, which has limited generalizability in females. This is a call for researchers to include female participants, including consenting pregnant and lactating women, in sports physiology research and for guideline committees to include sex-specific and peripartum-specific cardiovascular recommendations for athletes. Females are under-enrolled in both recreational and performance sports research, and this parallels the underrepresentation of females in research across health and disease states.1 When female athletes are included, their sample sizes are frequently smaller than their male counterparts.2 Among 12 511 386 participants in 5261 manuscripts published in six sport and exercise science journals between 2014 and 2020, females accounted for 34% of the study population; in addition, only 6% of total publications were conducted exclusively on females. The underrepresentation female athletes in research translates to knowledge gaps about sex differences in cardiovascular physiology and sports performance,2,3 and there is limited knowledge to guide policy for training and participation in elite events. There is an urgent need for more females to be included in sports research and for more female-only research, which looks at the effects of sex-specific factors, such as the menstrual cycle, hormonal contraceptive use, pregnancy, and menopause on sports physiology, sports performance, and cardiovascular health.3,4 Findings of sports science research in males do not address the biological factors that are unique to females and impact their cardiovascular health.5 Females have unique health considerations, including different metabolism of medications, lower haemoglobin levels, and smaller cardiac volumes than males. Females are prone to iron deficiency anaemia related to menstruation and pregnancy. Hormonal fluctuations during different stages of the menstrual cycle may influence training and physical performance. For example, relaxin and oestrogen concentrations peak during the luteal phase of the menstrual cycle and are associated with an increased risk of injuries.5 Oestrogen and progesterone fluctuations during the menstrual cycle may also affect temperature regulation, central nervous system fatigue, basal metabolism, which all contribute to exercise performance and cardiovascular health. Aesthetic sports and caloric restriction may contribute to energy deficiency, with detrimental consequences on sports performance and cardiovascular health.5 Given the cardiovascular and cerebrovascular adaptations that occur during stages of pregnancy, recommendations for elite level exercise during the postpartum phase should be informed by evidence for maternal and foetal outcomes. Maternal cardiac output increases to ensure adequate perfusion to the uterus, placenta, and maternal organs.6 Maternal resting heart rate increases by, on average, 20 b.p.m. and stroke volume increases by ∼40% during gestation.6 Reversible chamber enlargement has also been observed, with atrial diameters expanding up to 40% secondary to the expanded blood volume of pregnancy.6 Blood pressure normally decreases during pregnancy due to progesterone, which promotes vascular smooth muscle relaxation and increased nitric oxide.6 Decreased cerebral blood flow has also been reported with no apparent changes to cerebral autoregulation during pregnancy.6 These pregnancy-related cardiovascular changes typically resolve after the first 4–12 postpartum weeks.6 Current guidelines indicate that it is safe for female athletes who were physically active before pregnancy to engage in recreational exercise during and after pregnancy, but recommendations regarding performance sports and knowledge regarding maternal and foetal outcomes are lacking.6 Inactivity during pregnancy has been associated with excessive weight gain, hypertensive disorders of pregnancy, and gestational diabetes—known risk factors for cardiovascular disease and foetal complications.7 The intensity and duration of physical activity during pregnancy can influence the degree of change in foetal cardiac autonomic control, but evidence is conflicting. In one study, females engaging in high intensity exercise were found to have foetuses with lower, more variable heart rate.7 However, studies on resistance training found that increased frequency, intensity, and duration of resistance training were associated with lower rates of foetal complications.7 The developing foetal cardiovascular system is thought to respond differently to various types of maternal exercise, but there are knowledge gaps pertaining to foetal outcomes that require further research.7 The impact of elite training on post-partum physiology and lactation remains unclear. Findings from an observational study with 16 post-partum participants showed that females who engaged in vigorous exercise for ≥45 min/day tended to produce higher milk volume and energy output.8 Another observational study in 41 top competitive athletes reported that elite female athletes can benefit substantially from training at high frequencies (i.e. 6-day exercise routine) during an uncomplicated pregnancy as this approach would facilitate a more rapid return to competitive sports in the post-partum period.9 However, current research is largely limited to observational studies with small sample sizes and further high-quality studies are needed. There are several strategies to close the sex-specific knowledge gaps in sports training (Figure 1). First, investigating the cardiovascular physiology and health outcomes of female athletes requires representative inclusion of female athletes in the respective sport. Second, pregnant and lactating females should be included in sports research to close knowledge gaps and guide policy specific to this population. It is also essential to include those at different stages of pregnancy, as each is associated with distinct cardiovascular physiological changes. In addition, there is a need to change the ubiquitous male-directed branding and imagery in peer-reviewed scientific publications on sports physiology or performance sports in favour of gender-equal imagery. Sex-specific knowledge gaps pertaining to sports performance, implications for cardiovascular health, and strategies to increase the inclusion of female athletes in sports research. CV, cardiovascular; SV, stroke volume; HR, heart rate; SVR, systemic vascular resistance; BP, blood pressure; BMI, body mass index. Sex-specific knowledge gaps pertaining to sports performance, implications for cardiovascular health, and strategies to increase the inclusion of female athletes in sports research. CV, cardiovascular; SV, stroke volume; HR, heart rate; SVR, systemic vascular resistance; BP, blood pressure; BMI, body mass index. Strategies to increase the enrolment of females as research participants should include the recruitment, retention, and advancement of women sports researchers. Recruitment of female participants has been previously associated with trial leadership by women, who are largely underrepresented in leadership in sport and cardiovascular research.3,10 In fact, women accounted for ∼25% of first and <20% of senior authorship positions among >4800 randomized controlled trials published from January 2000 to September 2020 in high-impact sport sciences journals.5 The underrepresentation of women in research leadership positions may lead to lower prioritization of research questions specific to females and less emphasis on research representativeness. The critical gaps in knowledge pertaining to elite training in female athletes can only be closed by committing to transformative changes in the way research is conducted. The systematic exclusion of pregnant and post-partum athletes, and the underrepresentation of females in general, must no longer be considered acceptable and efforts must be made at multiple levels to ensure that research participants are representative of the population to which findings apply. Conflict of interest: The authors hereby declare no conflicts of interest. There are no new data associated with this article.

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