Testosterone has a well-documented effect on athletic performance, operating through several distinct biological pathways that influence muscle size, strength, oxygen-carrying capacity, and even the speed at which muscles contract. The relationship is not as straightforward as “more testosterone equals better athlete,” though. Injecting supraphysiologic doses clearly boosts performance, but among healthy athletes competing within normal hormonal ranges, circulating testosterone is a surprisingly poor predictor of who comes out on top.
How Testosterone Builds Muscle
The most familiar effect of testosterone on performance runs through skeletal muscle. Testosterone drives muscle growth by ramping up protein synthesis within muscle fibers, making them thicker and capable of producing more force. When existing cell nuclei can no longer keep up with the demand for new protein, testosterone activates satellite cells, which are essentially reserve cells sitting on the surface of muscle fibers, ready to donate their nuclei and allow the fiber to grow even larger.1PubMed Central. Cellular and molecular mechanisms responsible for the action of testosterone on human skeletal muscle. A basis for illegal performance enhancement This process underpins the long-term gains in lean mass that testosterone promotes.
There is also a faster, less widely discussed pathway. Beyond the slow process of building new protein over weeks, testosterone appears to have a rapid, direct effect on the contractile machinery inside muscle cells. Research in rodent muscle has shown that testosterone can trigger quick calcium release from internal stores within the muscle cell, which affects how forcefully and explosively the muscle contracts. This mechanism operates on a timescale of seconds to minutes rather than weeks, and it works independently of the traditional gene-activation pathway that drives muscle growth.2Journal of Sports Science and Medicine. Evidence for a Non-Genomic Action of Testosterone in Skeletal Muscle Which may Improve Athletic Performance: Implications for the Female Athlete If these findings translate fully to humans, testosterone may influence explosive power output in ways that go well beyond simply having bigger muscles.
More Red Blood Cells, More Oxygen
Testosterone does not just work on muscle. It also raises hemoglobin and hematocrit, the measures of how many red blood cells you have and how much oxygen your blood can carry. This happens because testosterone stimulates the kidneys to produce more erythropoietin (the hormone that tells bone marrow to make red blood cells) while simultaneously suppressing hepcidin, a liver protein that limits how much iron gets absorbed and made available for new red blood cells.3PubMed Central. Testosterone induces erythrocytosis via increased erythropoietin and suppressed hepcidin: evidence for a new erythropoietin/hemoglobin set point In clinical studies, testosterone administration produced roughly a 7% to 10% increase in hemoglobin and hematocrit. Animal studies have confirmed the mechanism, showing increases in reticulocyte counts (newly formed red blood cells) and serum iron alongside the drop in hepcidin.4PubMed Central. Testosterone administration inhibits hepcidin transcription and is associated with increased iron incorporation into red blood cells
For endurance athletes, this matters enormously. A higher hemoglobin concentration means more oxygen delivered per heartbeat, which directly raises the ceiling on sustained aerobic output. It is the same mechanism that makes altitude training and blood doping effective. The wide gap in circulating testosterone between men and women is a major reason men typically have higher hemoglobin levels, and this difference alone accounts for a meaningful chunk of the sex gap in endurance performance.5PubMed Central. Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance
What Supraphysiologic Doses Actually Do
The clearest evidence that testosterone enhances performance comes from studies using doses well above the natural range. A landmark trial gave young men weekly injections of 600 mg of testosterone enanthate, far above anything the body produces on its own, for ten weeks. Some participants also did strength training; others did not. The men who received testosterone without exercising at all gained more muscle and strength than the men who exercised on placebo. Testosterone alone increased bench press strength by about 9 kg and squat strength by about 16 kg, while the placebo-plus-exercise group gained only about 3 kg on the squat and lost a kilogram on the bench press.6PubMed. The effects of supraphysiologic doses of testosterone on muscle size and strength in normal men The combination of testosterone and exercise produced the biggest gains of all.
A later review of this and related research confirmed the pattern. When supraphysiologic testosterone was combined with strength training, lean body mass increased by about 6.1 kg over ten weeks. Testosterone alone (without exercise) added about 3.2 kg, exercise alone added about 1.9 kg, and the placebo-only group gained less than a kilogram.7PubMed Central. Comparing the Impacts of Testosterone and Exercise on Lean Body Mass, Strength and Aerobic Fitness in Aging Men These numbers make it clear that exogenous testosterone at high doses is a powerful performance enhancer on its own and amplifies the effect of training.
Why Your Natural Testosterone Level Does Not Predict Who Wins
Here is where the story gets complicated. If testosterone drives muscle and performance, you might expect the athletes with the highest natural levels to be the strongest and fastest. Among healthy young men who train, that is not what researchers find. A study tracking resistance-trained men through a 12-week program found that none of the hormones measured, including testosterone, predicted who gained the most muscle. The men who responded best to training had higher androgen receptor content in their muscles, but their blood hormone levels were no different from those who responded poorly.8PubMed Central. Muscle Androgen Receptor Content but Not Systemic Hormones Is Associated With Resistance Training-Induced Skeletal Muscle Hypertrophy in Healthy, Young Men
In elite athletes, a similar disconnect appears. An analysis of endocrine profiles in elite sportspeople found no significant relationship between circulating testosterone and performance in women. Among men, while testosterone levels differed between sports, the link to individual performance was not straightforward either.9PubMed Central. Why do endocrine profiles in elite athletes differ between sports? The likely explanation is that within the normal physiological range, other factors, including androgen receptor density, genetic variation in receptor sensitivity, training history, and neuromuscular coordination, matter at least as much as the raw hormone level in your blood.
Genetic variation in the androgen receptor itself illustrates this point. Research has found that men with a particular version of a repeat sequence in the androgen receptor gene had meaningfully different fat-free mass, with differences of roughly 1 to 3 kg between groups, even after accounting for other variables.10Journal of Applied Physiology. Androgen receptor CAG repeat polymorphism is associated with fat-free mass in men In other words, two men with identical testosterone levels could end up with substantially different amounts of muscle because their receptors respond differently to the same hormone signal.
Testosterone and the Performance Gap Between Men and Women
The sex difference in circulating testosterone is enormous, roughly 10- to 20-fold on average, and this gap is central to why men and women perform differently in most sports. A comprehensive review estimated that testosterone-driven differences in muscle mass, strength, and hemoglobin concentration together produce at least an 8% to 12% performance advantage for men.5PubMed Central. Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance That range holds fairly consistently across events from sprinting to endurance to throwing.
Among women specifically, testosterone still plays a role. Even within the much narrower female range, higher testosterone levels are associated with greater muscle mass and better performance. Women with hyperandrogenism, a condition involving naturally elevated testosterone, tend to have measurable advantages in lean mass, strength, and oxygen delivery.11PubMed Central. Female hyperandrogenism and elite sport This is one of the reasons sports governing bodies have periodically attempted, with considerable controversy, to impose testosterone thresholds for eligibility in women’s events.
What Testosterone Suppression Studies Show
Research on transgender women who undergo testosterone suppression has provided a unique window into how reversible testosterone’s effects on the body actually are. A systematic review looking at the impact of hormone therapy found that after 12 months of testosterone suppression, measures of strength, lean body mass, and muscle area all decreased significantly, but remained above those of cisgender women. After as long as 36 months, the gap had narrowed further but still had not fully closed.12PubMed Central. How does hormone transition in transgender women change body composition, muscle strength and haemoglobin? Systematic review with a focus on the implications for sport participation Hemoglobin levels, by contrast, dropped to typical female ranges within about four months.
Another review put a finer point on the muscle side. The loss of lean body mass, muscle area, and strength after 12 months of testosterone suppression amounted to roughly 5%, described as “very modest” relative to the starting advantage.13PubMed Central. Transgender Women in the Female Category of Sport: Perspectives on Testosterone Suppression and Performance Advantage These findings suggest that while testosterone is essential for building the original muscle mass and strength, once those adaptations are established, they persist for a long time even when the hormonal signal is removed. The extra myonuclei that testosterone helped create may be part of the explanation for that persistence.
When Training Itself Lowers Testosterone
One of the stranger twists in the testosterone-performance relationship is that extremely heavy training can actually suppress testosterone production. This has been most clearly documented in male endurance athletes, particularly marathon runners and cyclists who train at high volumes. These men can develop persistently low resting testosterone and low libido, a pattern researchers have termed the “exercise-hypogonadal male condition.”14PubMed. Effects of endurance exercise on the reproductive system of men: the “exercise-hypogonadal male condition”
The underlying cause appears to be chronic energy deficit. When calorie intake does not keep pace with training expenditure, the body begins suppressing reproductive hormone production as a way to conserve energy, similar to what happens in female athletes who lose their menstrual cycle. Evidence suggests that alterations in testosterone and even sperm production are among the first physiological adaptations to insufficient energy intake. When the negative energy balance is chronic, the reproductive axis can stay suppressed indefinitely.15PubMed Central. Hungry runners – low energy availability in male endurance athletes and its impact on performance and testosterone: mini-review This creates a paradox: the athletes training the hardest may end up with the lowest testosterone levels, potentially undermining their recovery and adaptation to training.
Sleep Deprivation and the Hormonal Cost
Poor sleep is another reliable way to tank your testosterone. Sleep disruptions reduce anabolic hormones including testosterone and growth hormone while simultaneously raising cortisol, a catabolic stress hormone that works against muscle repair and growth.16PubMed Central. Sleep and Athletic Performance: A Multidimensional Review of Physiological and Molecular Mechanisms The practical consequences for athletes include impaired muscular strength, reduced power output, diminished endurance, and compromised cognitive function.
Studies of U.S. Army Rangers undergoing training that included severe sleep restriction confirmed the connection in a high-stress, high-exertion population. Sleep loss drove testosterone levels down, and the researchers noted that soldiers attempting to exercise after a night of lost sleep would likely perform worse, experience less muscle growth, and recover more slowly due to lower-than-normal testosterone.17International Journal of Sports and Exercise Medicine. Sleep Loss During Military Training Reduces Testosterone in U.S. Army Rangers: A Two-Study Series For athletes in any sport, this is a practical reminder that optimizing sleep is, in effect, optimizing your hormonal environment for adaptation.
Tendon Risk and the Tissue Mismatch Problem
Testosterone’s ability to rapidly increase muscle mass and strength creates a potential vulnerability. Tendons, the connective tissue bands that anchor muscle to bone, adapt to increased loading much more slowly than the muscle itself does. When testosterone therapy or abuse produces quick gains in muscle force, the tendons may not keep up, and the mismatch can lead to injury. Clinical data has identified associations between testosterone therapy and injuries to the distal biceps tendon, rotator cuff, quadriceps tendon, patellar tendon, and Achilles tendon.18PubMed Central. Testosterone Therapy and Tendon Injury: Clinical Evidence, Biological Mechanisms, and Sports Medicine Implications The evidence remains observational and is substantially confounded, but a small increase in tendon injury risk is considered plausible, particularly in highly active individuals who rapidly increase their training loads.
On the recovery side, there is reason to think physiological testosterone levels support tissue repair. Data from hypogonadal men and aging populations suggests that restoring testosterone to normal levels may reduce disuse atrophy and improve recovery from injury. However, prospective evidence in injured athletes who actually compete is still limited.19PubMed Central. Testosterone Replacement Therapy in Athletes: Implications for Injury Recovery and Musculoskeletal Performance The picture, then, is that testosterone can both help you build performance capacity and, if the dose outpaces your connective tissue adaptation, increase the chance of breaking down.
Testosterone, Aging, and the Motor Unit
As men age, testosterone levels decline gradually, and this drop tracks with losses in muscle mass, bone density, and physical function. Clinical trials of testosterone replacement in older men with confirmed deficiency have shown benefits in physical performance, sexual function, and bone density, without significantly increasing the risk of cardiovascular events or prostate cancer.20PubMed. Testosterone Replacement Therapy for Testosterone Deficiency in Older Men
One area where testosterone’s effect on older adults is particularly interesting involves motor units, the bundles of muscle fibers controlled by a single nerve. In a study of both untrained and highly active older men, higher testosterone levels were associated with healthier-looking motor unit signals, specifically lower complexity, which reflects more organized neuromuscular function. This relationship held even after adjusting for whether the men were athletes or sedentary.21PubMed Central. Circulating testosterone and dehydroepiandrosterone are associated with individual motor unit features in untrained and highly active older men In practical terms, testosterone may help preserve not just muscle size but also the quality of the nerve-muscle connections that determine how well an aging person can actually use their muscles.
The Power of Believing You Got the Good Stuff
An underappreciated factor in testosterone-related performance is expectation. A pilot study told one group of trainees they had been placed in an “intervention group” receiving a performance-enhancing supplement (they had not), while a control group was told nothing. The group that believed they were getting an edge improved their squat one-rep max by about 5.7%, compared to just 0.9% in the control group, a statistically significant difference that persisted even after controlling for the fact that the “intervention” group trained slightly more consistently.22PubMed Central. The effects of being told you are in the intervention group on training results: a pilot study The finding is from a small study, but it raises a real question about how much of the perceived benefit of testosterone boosters, legal supplements marketed with testosterone-enhancing claims, comes from the substance itself versus the user’s confidence that they have an advantage.
How Anti-Doping Labs Catch Synthetic Testosterone
Because testosterone is a naturally occurring hormone, detecting its abuse is harder than catching a purely synthetic drug. The traditional screening method uses the ratio of testosterone to epitestosterone in urine, but this ratio varies naturally between individuals and across populations. The more definitive method relies on carbon isotope ratio testing. Synthetic testosterone is manufactured from plant-derived precursors whose carbon isotope signature differs slightly from the testosterone your body produces. By measuring the ratio of carbon-13 to carbon-12 in urinary steroid metabolites, labs can distinguish pharmaceutical testosterone from the endogenous kind with high reliability.23PubMed Central. Detection of testosterone administration based on the carbon isotope ratio profiling of endogenous steroids: international reference populations of professional soccer players24Clinical Chemistry. Performance Characteristics of a Carbon Isotope Ratio Method for Detecting Doping with Testosterone Based on Urine Diols: Controls and Athletes with Elevated Testosterone/Epitestosterone Ratios
This method was a significant advance for anti-doping because athletes had previously been able to beat the T/E ratio screen by co-administering epitestosterone or by micro-dosing testosterone to keep the ratio within acceptable limits. Carbon isotope ratio testing closed that loophole, making testosterone one of the more reliably detectable doping agents, even when used in small amounts. It remains a cornerstone of the anti-doping testing arsenal used by agencies worldwide.