Every week in the world of sport brings new controversy, as transgender athletes born male vie to compete in female categories. New Zealand weightlifter Laurel Hubbard, amid much debate, became the first openly transgender woman to compete in the Olympics, in Tokyo in 2021. Since then, the issue has been raised in boxing, rowing, and more recently, athletics.
In January, World Athletics, the international governing body, announced that its current “preferred option” was to allow transgender female athletes to compete – but to halve the maximum testosterone limit for qualification to 2.5 nMol/l (nanomoles per litre) consistently for 24 months (compared with the current rule of below 5 nMol/l for 12 months). According to the NHS, an ideal level for women is 0.3 to 2.4 nMol.
Three team GB runners voiced their opposition to this policy. Then, last week, Eilish McColgan, the long-distance runner and 2022 Commonwealth Games 10,000 metres champion, added her thoughts. “Hormones aside, what we go through every single month within the menstrual cycle; we are not just reduced-testosterone humans,” she said. “I think there is a lot more work to be done with regards to finding out, ‘Is there an advantage?’ because, even if it is a one per cent advantage, then it is too much.”
So, is it true that athletes born male will always have the edge over those born female, despite the current level of testosterone in their body? Here, we take a look at the science.
Genetics and hormones
In general, a world-class male athlete will always beat a world-class female athlete, says Dr Richard Bruce, senior lecturer in cardiorespiratory physiology at King’s College London. “The world record for the men tends to be about 10 per cent faster for pretty much every event from 100 metres up to the marathon,” he says. The same goes for swimming. In field events, such as long jump and discus, there is about a 20 per cent difference and in weightlifting it’s 30 per cent.
This male advantage is mostly down to genetics. Your genes control the production of hormones as well as the receptors that interact with the hormones. “You can affect that by taking drugs to dampen it down, but the actual gene is still there,” says Professor Paul Lee, a Harley Street specialist in sports, regenerative and orthopaedic surgery.
Although testosterone exists and functions similarly in women and men, men have markedly higher average testosterone than women. “Men tend to build a lot more muscle than women because every cell in their body is preprogrammed to testosterone,” says Prof Lee. This leads to key advantages in strength and power.
However, experts are unclear whether having grown up as a male for several years (and having enjoyed a full complement of testosterone during puberty) conveys a lingering advantage on athletes who now wish to compete with women. Long-term studies are still needed.
Cardiorespiratory (lung capacity and cardiac output)
As men tend to be taller than women, the volume of their lungs is typically 20 per cent bigger. “Therefore they can take deeper breaths,” says Prof Lee. “This tends to give men a slight advantage because they are able to get more air into their body, leading to greater oxygenation via the bloodstream – and therefore more endurance.”
Dr Bruce also feels an advantage in performance is conferred by heart size and haemoglobin levels. Haemoglobin is essential for transferring oxygen in your blood from the lungs to the tissues. Though equal in childhood, in women haemoglobin levels plateau during early puberty, while in men it continues to rise, leading to a 10 to 20 per cent difference in haemoglobin concentration.
Higher volume of red blood cells equals higher athletic performance. Dr Bruce says: “In any running event of upwards of 5K where there is aerobic exercise associated with longer-distance events, haemoglobin levels are important.”
Fat and muscle composition
Men’s naturally higher levels of testosterone mean they tend to be leaner than women. “Testosterone is related to metabolic rate and means that men tend not to put on fat as easily; they put on muscle,” explains Prof Lee. Even with the same training regime, a man will put on 20 per cent more muscle, he says: “For the female it will not be as easy, they will have to work harder to get the same muscles as men.”
“The proportion of slow-twitch (endurance) and fast-twitch (power) fibres are fairly similar across genders,” says Dr Paula Vickerton, director of anatomical studies at Queen Mary University of London, “but males have larger fast-twitch fibres, which allows them to generate higher force.”.
This combination of power and strength is part of why men have an advantage in short and middle distance running events.
Women’s fat composition may help in some endurance sports. In 2019, British ultrarunner Jasmin Paris made headlines after becoming the first woman to win the 268-mile Montane Spine Race, knocking 12 hours off the previous record for the ultra-marathon. “Female composition helps with long distances as they have reserves to fall back on,” says Prof Lee.
The science is pretty simple on this one: the more muscle mass you have, the more power you can produce. “That is a really big deal,” says Prof Lee. Therefore, combined with height, this multiplies the male advantage.
The longer the lever arm (the distance between the point of application of a force and the axis), the more force a person can produce. “If you have inherently more muscle mass in the body and you’ve got a bigger skeleton, whatever force you can produce will multiply very quickly,” says Prof Lee. For instance, studies have consistently shown that men have a higher football-kicking velocity than women. “When we try to kick a football we usually can produce about seven times our body weight to go through that ball,” says Prof Lee. “The longer the femur, the bigger the lever arm is.”
“If you’ve got a stronger muscle attachment to the bone, the bone gets thicker and stronger in response,” says Dr Vickerton, “Testosterone drives muscle growth, which in turn drives bone mass.” Women’s muscles are smaller, so their bones are weaker, particularly after menopause. Prof Lee agrees: “When we assess bone strength via scans, the male line is higher than the female.”
Weaker bone density will play a role in the prevalence of stress fractures in female athletes.
Whether you can do pancake splits or struggle with a lunge, the key is again, hormones. “Oestrogen tends to increase flexibility, and men don’t really have a huge reserve of oestrogen,” says Prof Lee. “That’s why premenopausal women are typically more flexible.”
Gymnastics is an example of an integrated sport that accounts for physiological differences between men and women. “It allows men and women to compete in slightly different but similar disciplines that really show off their physiology,” explains Dr Bruce. “So, men do the rings, which require a lot of upper body strength, and women do the balance beam that requires balance and flexibility.” The downside of this is an increased risk of damage to tendons and ligaments, such as injuries to the anterior cruciate ligament (ACL), as seen in the Lionesses Chloe Kelly and Beth Mead.
Lower body anatomy
The anatomy of the pelvis and knees reveals the clearest differences between men and women, says Dr Vickerton: “Females have a wider pelvis, which means their femurs are at a sharper angle then in men, which loads their knees quite differently.” This can lead to an increased risk of ACL tears, which are up to six times more likely for female footballers, as well as a higher chance of dislocation.
These differences mean that in general women have ‘knock knees’ whereas men are more likely to have bow legs. “Footballers have very bow legs,” says Prof Lee. “The more you bow your legs, the better you are at football, because you can go very low and keep your centre of gravity and hit the ball horizontally. Knock knees are very bad for football, you will just collapse. It’s just more difficult to be good.”
Overall joint size affects the force that can be generated. Men’s hip sockets tend to be 65mm in diameter, bigger than women by a centimetre, meaning they will have a greater muscle mass, “which will probably translate to about double the force that they can generate on each individual kick, say, of a ball,” explains Prof Lee.
Upper body anatomy
The shoulder girdle (the set of bones which connect the arm on each side) doesn’t have the same clear anatomical differences between males and females as in the lower limbs. Rather, it is overall body size that contributes to men having greater upper body strength.
“A man being taller means he will have a bigger chest wall and can therefore produce a lot more power,” says Prof Lee. “When you see the track and field athletes, such as javelin throwers, they are really big and tall, male or female. You can generate a lot more power from the upper limb with a slightly bigger skeleton.”
The absence of breasts also confers a male advantage. “When females try to build that pectoralis major muscle it’s more restricted, which makes it more difficult to build,” he says. “They have less muscle to start with, and less anchorage, with extra load on top, which means the muscle has to work harder.”
However, women’s elbows also have a slightly higher carrying angle. “Which means when you have your arms by your sides they’re not perfectly straight. It means the lever arm is shorter and not as powerful,” says Prof Lee.