Are Women Naturally Designed for Ultra-Endurance Sports?

1984 marks the year when women were allowed to compete in the marathon distance at the Olympics. Previously, it was believed endurance exercise harmed women (11). To this day, women's participation in ultra-endurance events is often less than 10% (1). Low participation numbers can be attributed to social norms, rules, and roles as primary caregivers. The lack of female role models competing likely compounds the issue. There’s ample room for participation and female endurance performances to grow.

Females Outperforming Males

Men's and women's sprint to marathon finish times differ by about 10-12% (1). In ultra-marathon events, such as Badwater (135 miles) and the Spartathlon (153 miles), the differences are around 20% (1). Nonetheless, there have been numerous occasions where female ultra runners win outright. In endurance swimming, female athletes blaze past men. In 1926, 19-year-old Gertrude Ederle swam the English Channel in 14 hours and 31 minutes, two hours faster than any man. In 1967, cyclist Beryl Burton set the world record for the 12-hour time trial, surpassing the men’s record for two years. Performances like these occur but always beg the question, what if more women were participating in endurance sports?

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With fewer female competitors at endurance events, there is less competition to drive performances to the female limit. Scientific studies are confronted with standardizing training regimes and matching fitness; deciphering why and how females close the gap is challenging. This review of endurance athlete characteristics will highlight differences between the biological sexes and how females can train smarter for endurance sports.

Body Composition

Generally, female athletes are smaller but have more body fat and less muscle. Body fat is used for energy and is an active organ that regulates physiological, metabolic, and hormonal responses (9). An individual's health can affect the type of fat they are prone to creating, subcutaneous or visceral. More visceral fat is linked to diseases (9).

Takeaways

• Female body composition has not correlated with ultramarathon performance outcomes as it has for men (2).

• Subcutaneous fat is needed for hormone production. Hormone imbalances can lead to inflammation, reduced adipose tissue vascularization, and risk of metabolic disorders and diseases (9).

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Biomechanics

Women have a wider pelvis, which creates a greater angle from the upper femur towards the knee, resulting in a slight knock-knee effect or greater hip adduction and internal rotation (1). Less rear foot external rotation can increase knee abduction as the body compensates for larger hip adduction (1).

Takeaways

• Utilize strength training regimes to activate and strengthen hip abductors and external rotators to prevent overuse or compensatory injuries.

Heart and Blood Physiology

Women have smaller chests, hearts, and lungs, which causes a lower cardiac output or blood stroke volume (10). To compensate, heart rate tends to be, on average, 5-8 beats higher than that of male proponents (10). Max heart rate is similar, but heart rate variability is lower in females (10). Female hormones also affect the rhythm and health of the heart (10). The female heart is also shaped differently, with the left ventricle disproportionately smaller (10).

Females have less circulating blood, fewer red blood cells, lower hematocrit levels, and ultimately, lower oxygen-carrying capacity (1). This decreases VO2max or the body's ability to use oxygen (1). In women who have born children and semi-maintained exercise during pregnancy, their post-pregnancy circulating blood volume can remain higher than pre-pregnancy by 33%, and blood plasma volume by 1000mL (5). Altitude and heat training can also increase oxygen-carrying capacity and blood plasma volume.

Takeaways

• Female hearts are not smaller versions of male hearts (10). Devices such as smartwatches that provide recovery recommendations based on biomarkers such as heart rate variability may be inaccurate for women unless they consider hormonal shifts.

• Women’s heart rate thresholds should not be prescribed interchangeably as though a male athlete. The individual athlete must be considered. The rate of perceived exertion, blood lactate, and power may be better tools for prescribing intensity (6).

• Pregnancy may elevate VO2 max post-partum for some time (5). More research is needed to determine how long these effects last.

• Follow female-specific heat and altitude protocols (3).

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Muscle

Women have more type I or slow twitch muscle fibers with higher mitochondria activity, supporting fat oxidation (7). However, for shorter events that require more power or anaerobic capacity, type II muscle fiber types can be increased through specific training (7). Even during endurance events, you likely surge up hills or need type II muscles to help you power past a competitor during a race. Be aware that during the luteal phase, if protein intake is inadequate, muscle is broken down (7).

Takeaways

• More type I muscle fibers make women designed for endurance.

• Type II muscle fibers can be strengthened and increased through high-intensity intervals and strength training.

• Ingest 1.4-2.4 grams of protein per kg of body weight daily, with emphasis during the luteal phase (7).

Pacing and Psyche

Studies reviewing pacing strategies have conflicting results. Some demonstrate a more even pace for women in ultramarathon events, while others find uneven pacing, especially in mountainous races such as Ultra-Trail du Mont Blanc (2). This may be due to women's lower muscle mass leading to fatigue with numerous climbing and descending. Female athletes often experience higher flow psyche ratings during endurance events, leading to better emotional regulation and pacing (2).

Takeaways

• More research is needed to determine whether women generally pace consistently depending on the terrain.

• Regardless of gender, athletes with a more consistent pace perform better (2).

• Mental flow state may be correlated to consistent pacing (2).

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Sweat and Muscle Cramping

Females have more efficient heat dissipation through sweat evaporation, even with fewer sweat glands (2). Women require at least nine days of heat exposure to increase the number of sweat glands, whereas men require less (3). Some studies have shown that men are equally or more likely to overheat and succumb to muscle cramping (2).

Takeaways

• Use heat training protocols specific to women (3).

Metabolic Flexibility

Due to greater innate metabolic flexibility, women readily use fatty acids for energy during submaximal or endurance exercise (11).

Takeaways

• Women shouldn’t restrict carbohydrates or use fasted training to improve fat burning as fuel. Women are already fat-adapted and can be harmed by low carbohydrate training protocols (7).

Fatigue Resistance

Women demonstrate less fatigue following races, which could be due to muscle fiber type and less metabolite accumulation (2). Additionally, women have been found to incur less exercise-induced heart fatigue, which may be due to estrogen’s heart-protective effects (2).

Takeaways

• Women may recover faster from endurance events.

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There’s More to Unravel

In 1996, a study matched similar-level athletes, male to female; all females outperformed their matched male athlete during a 90 km effort, even though they had lower VO2max and higher percent body fat (8). The women could sustain exercise at a significantly greater fraction of their VO2max than the men (8). Interestingly, a research study compared the 1,881,070 runners over 38,860 trail running races (4). The gap between men and women decreased as distance increased, even with less female participation in ultra events (4).

Women have a unique physiology that, when worked with, can culminate into amazing feats of endurance. Science may not have answers yet, nor is female participation anywhere close to 50% in most ultra-endurance sports. We have yet to unravel female endurance athletes' potential.

References

1. Besson, T., et al. (2022). Sex Differences in Endurance Running. Sports Medicine, 52:1235-1257

2. Kelly, P. M. (2023). Is There Evidence for the Development of Sex-Specific Guidelines for Ultramarathon Coaches and Athletes? A Systematic Review.

   Sports Medicine - Open, 9. https://doi.org/10.1186/s40798-022-00533-9

3. Kirby, N. et al. (2019). Nine-, but Not Four-Days Heat Acclimation Improves Self-Paced Endurance Performance in Females. Frontiers in Physiology, 10. doi:10.3389/fphys.2019.00539

4. Le Mat, F., et al. (2023). Running Endurance in Women Compared to Men: Retrospective Analysis of Matched Real-World Big Data. Sports Medicine, 53, 917-926.

5. Pivarnik, J. M., et al. (2017). Can Pregnancy be an Ergogenic Aid to Athletic Performance?: A Review. Women in Sport and Physical Activity Journal, 25(2), 111–117. doi:10.1123/wspaj.2016-0018

6. Rascon, J., et al. (2020) Differences between Males and Females in Determining Exercise Intensity. Int J Exerc Sci, 13(4):1305-1316. PMID: 33042374

7. Sims, S., et al. (2023) International society of sports nutrition position stand: nutritional concerns of the female athlete, Journal of the International Society of Sports Nutrition, 20:1, DOI: 10.1080/15502783.2023.2204066

8. Speechly, D., et al. (1996). Differences in Ultra-Endurance Exercise n Performance-Matched Male and Female Runners.

   Medicine & Science in Sports & Exercise, 28(3) 359-365.

9. Steiner, B. M., Berry, D, C. (2022). The Regulation of Adipose Tissue Health by Estrogens.

   Front. Endocrinol. 13. https://doi.org/10.3389/fendo.2022.889923

10. St. Pierre, S., et al. (2022). Sex Matters: A Comprehensive Comparison of Female and Male Hearts. Frontiers in Physiology. 13. doi:10.3389/fphys.2022.831179

11. Tarnopolsky M,A. (2008). Sex differences in exercise metabolism and the role of 17-beta estradiol. Med Sci Sports Exerc. 40(4):648-54. doi: 10.1249/MSS.0b013e31816212ff. PMID: 18317381