An interesting review (1) was published recently re-igniting the discussion whether women will ever outperform men in ultra-endurance events. Males tend to outperform females in a number of sports. For example in the marathon the difference is around 10%. However, this difference is much smaller in ultra-endurance sports, such as in ultra-marathon (with a difference of 4%), or in extreme ultra endurance events where the difference is negligible. It appears that the longer the event, the smaller the difference. In fact, in ultra-distance swimming females tend to perform better than males!
In the 1990s, the rate of improvement in female sports performance was greater than that of male sport performance which was likely due to greater involvement of females in high level sport. Although some suggested at that time that females might one day outpace males in some sports, this has not manifested and is unlikely to occur for a number of reasons.
It appears that the longer the event, the smaller the difference.
In a famous article by Whipp and Ward in Nature in 1992, this was illustrated:
Males have physiological advantages that manifest in many events:
Aerobic capacity
Aerobic capacity and maximal oxygen uptake (V̇O2max) are major factors in explaining endurance exercise performance, as these determine the intensity of the exercise that can be sustained aerobically. Males have higher V̇O2max values than females, which is partly explicable by higher haemoglobin and haematocrit levels, allowing greater oxygen carrying capacity in the blood. Although some of the differences in V̇O2max can be removed by normalising for muscle mass (which is higher in males), this does not completely remove the difference.
Body composition
Males tend to have lower levels of body fat and larger muscle mass than females, which means a smaller amount of non-metabolically active tissue needs to be carried, improving power:weight ratio.
Testosterone
Testosterone (the male sex hormone) leads to higher haemoglobin levels, improved recovery and greater muscle mass, and is 15-20 times higher in males than females after puberty.
Although some of the differences in V̇O2max can be removed by normalising for muscle mass (which is higher in males), this does not completely remove the difference.
Why are there smaller performance differences in ultra-endurance sports?
There are a variety of differences in female physiology that can explain the smaller disparity between males and females in the ultra-endurance sports mentioned already:
Muscle composition
Females appear to have reduced muscle fatiguability after ultra-endurance exercise, which culminates in their muscles being more able to produce force during and after such exercise. There are a number of possible reasons for this...
Females have more Type I (‘slow twitch’ or oxidative) muscle fibres than males. These produce less force than their larger Type II counterparts but are much more fatigue resistant and have better oxygen delivery.
Males have more Type II (‘fast twitch’ or glycolytic) fibres, which in part explains why males are much better at sports requiring high power output such as sprinting. These large fibres can actually restrict blood flow during exercise by putting pressure on arteries, reducing oxygen delivery in extended exercise.
Pacing
Females have been shown to have a more even running speed in endurance events, resulting in better pacing. Although they start at a slower speed than males, they tend to maintain this speed better and have faster finishing times than males, who tend to slow as the race progresses. This could be due to better fatigue resistance, but may also be psychological factors, such as higher risk-taking traits and overconfidence in males.
Females have been shown to have a more even running speed in endurance events, resulting in better pacing.
Substrate utilisation
Substrate utilisation (the use of different fuels for exercise) is different between males and females. Fat and carbohydrate are the main energy sources in endurance and ultra-endurance exercise, which contribute similarly to energy requirements.
Because glycogen stores are limited, there may be a benefit in endurance and ultra-endurance exercise to oxidising more fat, leading to glycogen sparing. The review discusses that females consistently oxidise more fat than males in endurance exercise, and males have greater glycogen depletion for the same exercise. We discussed this in a recent blog as well.
Fat oxidation
Females have more of the metabolic machinery required for oxidising fat, as well as gene expression that favours it (which is in part due to their greater Type I fibre proportion). This benefit is likely quite small, especially in ultra-endurance exercise where rates of glycogen depletion are slower than in classical endurance exercise. However, one possible benefit of this is that females have attenuated caloric requirements, meaning less exogenous carbohydrate (e.g. carbohydrate drinks, gels or bars) needs to be consumed, which can cause gastrointestinal distress.
Oxygen economy and energy efficiency appear to be similar in males and females, despite the higher body fat levels of females, which might be expected to reduce it. Interestingly, the higher body fat of females might serve to explain the better performance in females in long distance swimming events, as body fat provides buoyancy. The smaller body size of females also contributes to smaller drag in the water, which likely provides some benefit.
Females have attenuated caloric requirements, meaning less exogenous carbohydrate (e.g. carbohydrate drinks, gels or bars) needs to be consumed
Gastrointestinal distress
Gastrointestinal distress (including symptoms such as bloating, abdominal pain, diarrhoea, vomiting) are very common in endurance sports, and the relationship between presence of symptoms and poorer performance is well established.
Females report higher incidence of gastrointestinal symptoms during and around training and competition, especially with ingestion of exogenous carbohydrate. These symptoms may be due to slower intestinal transit time in females as well as smaller stomach sizes, though they often coincide with menstruation in some female athletes. Although the review paper does not discuss this, it is also possible that these issues are related to the general lower food intake and lower carbohydrate intake in female athletes.
Training the gut to handle appropriate amounts of carbohydrate during exercise may be particularly important for females, who tend to be less accustomed to such carbohydrate feeding. However, it is worth noting that due to slower glycogen depletion in females and the nature of ultra-endurance exercise using less glycogen due to its lower intensity, that high rates of carbohydrate ingestion are not necessarily required for all athletes.
Lack of research
Many of the studies done in the field of sports and exercise sciences preferentially involve male subjects, with female subjects making up just a 39% of subjects in such studies. Rarer still are direct comparisons of males and females within the same study. Although the discipline is changing to recruit more female participants, the existing literature is heavily biased towards studies on male subjects.
Summary
There are clearly many factors that contribute to performance in endurance and ultra-endurance events beyond those discussed by the review, including psychological factors and some other physiological characteristics such as thermoregulation and nutritional and training regimens. However, the physiological characteristics discussed here do provide possible explanations for why the performance gap between the sexes is smaller in ultra-endurance events. Although it seems unlikely that females will one day outpace males en masse, these differences are notable and are worthy of further research.
References
Tiller NB, Elliott-Sale KJ, Knechtle B, Wilson PB, Roberts JD, Millet GY. Do Sex Differences in Physiology Confer a Female Advantage in Ultra-Endurance Sport? Sports Med. 2021.
Costello JT, Bieuzen F, Bleakley CM. Where are all the female participants in Sports and Exercise Medicine research? Eur J Sport Sci. 14(8):847-51, 2014
de Oliveira EP, Burini RC. Carbohydrate-Dependent, Exercise-Induced Gastrointestinal Distress. Nutrients. 6(10):4191-9, 2014
Deaner RO, Carter RE, Joyner MJ, Hunter SK. Men are more likely than women to slow in the marathon. Med Sci Sports Exerc. 47(3):607-16, 2015
