Testosterone is the major androgenic (male) hormone and one of the most common doping agents. Athletes who participate in strength and power-based sports, including bodybuilding, athletics, wrestling and cycling, have used testosterone for decades for its muscle-building properties.
Contemporary anti-doping tests can detect and distinguish between the presence of pharmaceutical (“exogenous”) testosterone and natural (“endogenous”) testosterone with a high level of certainty. The presence of exogenous testosterone is essential to return a positive result.
Meanwhile, some people, males and females, present with high levels of natural testosterone without having ever taken androgenic hormones. These people are called “hyperandrogenic”.
The common perception is that total testosterone levels directly determine athletic performance. But our new research challenges the assumption that naturally high testosterone levels are associated with stronger athletic performance in females.
How does testosterone enhance performance?
Testosterone acts on muscle cells by binding to a specific receptor protein, the androgen receptor. Upon testosterone binding, the androgen receptor signals to the muscle cell to activate the pathways that trigger an increase in muscle mass, called muscle hypertrophy. As a result, the muscle grows and becomes stronger.
But let’s look at what happens when testosterone can’t perform its job in the muscle. “Androgen receptor knockout mice” are genetically modified mice that do not produce this receptor. When compared to normal male mice, male androgen receptor knockout mice lose up to 20% of their muscle mass and strength. This makes sense since testosterone doesn’t have a receptor to bind to anymore.
Surprisingly though, this doesn’t happen in female mice. Female androgen receptor knockout mice are as strong and muscular as their control counterparts. This suggests testosterone may not be necessary to reach peak muscle mass and strength in females.
Our new human data align with this hypothesis. We used a large, publicly available database and showed total testosterone levels were not associated with muscle mass or strength in 716 pre-menopausal females.
This is in contrast to males, where higher testosterone concentrations are associated with increased muscle mass and strength.
We’re also doing experimental research on this topic. We’ve recruited 14 young female volunteers with natural testosterone levels along a spectrum from low to hyperandrogenic.
Although this part of our research is not yet published in a peer-reviewed journal, our results so far appear to confirm the findings from the epidemiological data. We’ve found testosterone levels don’t correlate with thigh muscle size, strength and power even after 12 weeks of resistance training aimed at maximising muscle mass and building strength.
Our laboratory-based study allows us to tightly control for external factors that may influence muscle mass and strength, such as diet, sleep, training status and menstrual cycle.
Why mightn’t testosterone enhance athletic performance in females?
Previous research suggests the female sex hormones oestrogen and progesterone may take over some of the muscle-building role of testosterone in young females.
Another important consideration is natural testosterone exists in two forms: “free” within the bloodstream, or “bound” to a protein that reduces its capacity to signal to the muscle. Our research suggests “free” testosterone has the greater role in regulating female muscle mass and performance.
It’s important to acknowledge that past a certain threshold, testosterone may have a different effect on female muscle physiology.
A recent study measuring the effects of pharmaceutical testosterone on physical performance in females found after ten weeks the volunteers receiving testosterone had gained more muscle mass and could run for longer on a treadmill before becoming exhausted compared to the volunteers who didn’t receive testosterone.
Surprisingly though, there was no between-group difference in muscle power, muscle strength, explosive power (sprinting) and the maximum rate of oxygen consumption measured during exercise, which is the best indicator of cardiorespiratory fitness.
These findings support our hypothesis that total testosterone isn’t a direct determinant of muscle strength and performance in females.
Correction: this article previously discussed the World Athletics eligibility regulations for female athletes. In correspondence to The Conversation, World Athletics has said its regulations relate to females with 46-XY chromosomes. The subjects of the author’s research are females with 46-XX chromosomes. To account for this, all references to World Athletics regulations have been removed from this article.