Men run roughly 10 to 12 percent faster than women across standard racing distances, from the 100-meter dash to the marathon, and the gap has held remarkably steady for decades after narrowing sharply through the twentieth century.1PubMed Central. Expanding the Gap: An Updated Look Into Sex Differences in Running Performance The explanation is not one single thing but a stack of physiological differences, most of them set in motion by hormones during puberty. Each layer on its own contributes a modest edge; together they compound into a consistent and measurable speed advantage.
The Size of the Gap
Across all standard running events, the average performance difference between the fastest men and fastest women sits at about 11 percent.2PubMed. The Unlikeliness of an Imminent Sub-2-Hour Marathon: Historical Trends of the Gender Gap in Running Events That figure is an average. At the very top of the sport, the gap is narrower for world records and world-leading times, then widens slightly among lower-ranked athletes.1PubMed Central. Expanding the Gap: An Updated Look Into Sex Differences in Running Performance The gap also shifted over time: women’s performances improved dramatically in the second half of the 1900s as participation barriers fell and training infrastructure caught up, but the convergence plateaued and the difference has been relatively stable since. What drives that stubborn 10-to-12 percent is biology, and the trail leads back almost entirely to puberty.
Testosterone Sets the Table
Before puberty, boys and girls perform about the same in running events, and their circulating testosterone levels are essentially identical. Then the testes begin producing roughly 30 times more testosterone than they did before puberty, pushing adult male levels to about 10 to 15 times those of adult women.3PubMed Central. Transwoman Elite Athletes: Their Extra Percentage Relative to Female Physiology That hormonal surge triggers a cascade of changes: more muscle mass, greater bone density, higher hemoglobin concentration in the blood, and shifts in fat distribution. Research has found a clear dose-response relationship between circulating testosterone and both muscle mass and hemoglobin, and these dichotomies account for at least an 8 to 12 percent advantage in men.4PubMed Central. Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance
The neatest natural experiment confirming testosterone’s role comes from studies of transgender athletes. When transmen begin testosterone therapy, their 1.5-mile run times improve and catch up to those of cisgender men within about a year. When transwomen begin estrogen therapy, their run times get slower but remain faster than those of cisgender women even after two years of hormone treatment.5British Journal of Sports Medicine. Effect of gender affirming hormones on athletic performance in transwomen and transmen: implications for sporting organisations and legislators This residual advantage in transwomen suggests that some of the structural changes testosterone induces, particularly in the skeleton and possibly in residual muscle architecture, are not fully reversed by lowering testosterone later in life.
Muscle Fiber Composition and Size
Testosterone doesn’t just make muscles bigger; it shapes how they are built. Large meta-analyses of biopsy data consistently find that men have bigger muscle fibers across the board. Their Type II fibers, the ones that produce fast, powerful contractions, are particularly enlarged compared to women’s. Men also carry a higher proportion of Type II fibers overall, giving their muscles a composition tilted toward explosive force.6PubMed. Sex differences in skeletal muscle fiber types: A meta-analysis Women, conversely, have a greater share of Type I fibers, the slow-twitch variety that resist fatigue and favor endurance.
A more recent systematic review and meta-analysis of skeletal muscle data confirmed the pattern: men had larger fiber cross-sectional areas for both Type I and Type II fibers, and these differences persisted regardless of age, physical activity level, or which muscle group was sampled.7PubMed Central. Sex differences in human skeletal muscle fiber types and the influence of age, physical activity, and muscle group: A systematic review and meta-analysis In practical terms, men have both more contractile tissue and a fiber mix biased toward generating high force quickly. For sprinting, where raw power is everything, this matters enormously. For longer distances, the advantage is smaller but still significant because a bigger engine still produces more work per stride.
The Oxygen Pipeline
Running fast demands oxygen, and men have a larger pipeline to deliver it at every stage. The story begins in the blood: testosterone stimulates the production of red blood cells, so men typically have higher hemoglobin concentrations. More hemoglobin means more oxygen carried per liter of blood. Testosterone’s dose-response relationship with hemoglobin is well established, and the difference contributes meaningfully to the gap in aerobic capacity.4PubMed Central. Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance
Men also have larger hearts and higher stroke volumes, meaning each heartbeat pumps more blood. In one study, stroke volume during submaximal cycling averaged about 110 milliliters per beat in men versus 87 in women, a gap of roughly 25 percent.8PubMed. Changes in stroke volume and maximal aerobic capacity with increased blood volume in men women When blood volume was experimentally expanded in both sexes, both improved their stroke volume by a similar proportion, suggesting that the cardiac reserve capacity itself is comparable. The difference at baseline comes down to structural size: bigger chambers, more blood, more hemoglobin.
VO2 max, the gold-standard measure of aerobic capacity, integrates all of this. It reflects how much oxygen the body can take in, transport, and use per minute. On average, men’s VO2 max exceeds women’s by 15 to 30 percent depending on the population studied, with the gap narrowing when values are adjusted for lean body mass. But even after adjusting, a gap remains, because the hemoglobin and heart-size differences are not just about body weight.
Smaller Lungs, Higher Cost of Breathing
The respiratory system adds another layer. Women have smaller lung volumes and narrower airways than men, even when matched for overall body size. This is not simply a consequence of being shorter on average; a phenomenon called dysanapsis means that women’s conducting airways are disproportionately small relative to their lung tissue.9PubMed. Revisiting dysanapsis: sex-based differences in airways and the mechanics of breathing during exercise The practical consequence is that women use a greater fraction of their total ventilatory capacity during hard exercise and have a higher metabolic cost of breathing at any given ventilation rate.10PubMed. Mechanics of breathing during exercise in men and women: sex versus body size differences?
Female athletes are also more likely to hit a ventilatory ceiling during intense exercise, a condition called expiratory flow limitation, where they physically cannot push air out fast enough. The higher work of breathing in women likely traces to those smaller lung volumes and narrower airways.11PubMed Central. Respiratory mechanics during exercise in endurance-trained men and women More oxygen diverted to the respiratory muscles means less available for the legs. At the ragged edge of performance, this matters.
Tendons, Estrogen, and Running Economy
Running economy, how much energy it takes to hold a given pace, depends partly on how well your tendons store and return elastic energy with each stride. Stiffer tendons act like better springs. Research on trained runners found that men’s Achilles tendons were significantly stiffer than women’s, and the difference held across a range of force conditions.12PubMed Central. Energy cost of running and Achilles tendon stiffness in man and woman trained runners Some of this relates to body mass, since heavier people load their tendons more, but estrogen itself appears to play a direct role. Estrogen reduces the stiffness of tendons and ligaments, which can decrease power output and increase injury risk in women.13PubMed Central. Effect of Estrogen on Musculoskeletal Performance and Injury Risk
This is one of the more counterintuitive pieces of the puzzle. Estrogen does good things for women’s muscle mass and bone density, but its effects on connective tissue cut the other way. A less stiff Achilles tendon returns less energy per stride, which means more metabolic work to maintain the same speed. High estrogen levels have been directly associated with decreases in power and performance.
How Women Run Differently
Biomechanics add yet another dimension. Wider pelvic structure in women changes the geometry of the lower extremity during running. Female recreational runners show greater peak hip adduction, greater hip internal rotation, and greater knee abduction compared with male runners.14PubMed. Gender differences in lower extremity mechanics during running These differences have been confirmed across multiple studies, with women consistently showing roughly 3 to 4 degrees more hip adduction and internal rotation.15PubMed. Sex differences in lower extremity kinematics and patellofemoral kinetics during running
Whether these biomechanical differences significantly affect top speed is debatable. They are more clearly linked to injury patterns: the combination of greater hip adduction and internal rotation is associated with higher rates of patellofemoral pain and anterior cruciate ligament injuries in women. From a pure speed standpoint, the biomechanical differences probably matter less than the muscular and cardiovascular factors described above. But they do influence how women need to train to stay healthy and fast, and they contribute to the overall picture of physiological divergence.
Where Women Have an Edge
The story is not all one-directional. Women’s higher proportion of slow-twitch muscle fibers gives them a meaningful advantage in resisting fatigue. In laboratory tests, women were able to sustain isometric elbow flexion contractions for about 40 percent longer than men before giving out.16PubMed Central. Sex Differences in Fatigue Resistance Are Muscle Group Dependent Electrically stimulated contraction tests, which remove any motivational differences, confirmed the finding: women’s quadriceps fatigued less than men’s, and the difference was not explained by muscle size or blood flow.17PubMed. Sex differences in contractile properties and fatigue resistance of human skeletal muscle Women’s muscles are simply slower to fatigue, likely because of their fiber-type composition.
Women also burn a higher proportion of fat relative to total energy expenditure during both running and cycling, particularly at higher exercise intensities.18PubMed. Fat oxidation in men and women endurance athletes in running and cycling Fat is a denser energy source than carbohydrate, and the body’s carbohydrate stores are limited. In very long efforts, a greater reliance on fat oxidation could help preserve glycogen and delay the wall-hitting depletion that derails so many endurance performances. These advantages don’t override the speed gap in standard events, but they become more relevant as distances stretch beyond the marathon.
The Ultra-Endurance Question
A persistent claim in running culture is that the performance gap closes or even reverses at ultra-marathon distances. The reality is more nuanced. Between 1975 and 2013, women did reduce the gap to men in most timed ultra-marathons, particularly in events lasting 6 hours or longer (up through 240-hour races). But in 24-hour and 48-hour events, the gap actually increased over the same period.19PubMed Central. Do women reduce the gap to men in ultra-marathon running? Much of the narrowing in longer timed events may reflect rising female participation in those events during a period when participation was already high among men, meaning the talent pool was deepening faster on the women’s side.
There is no distance at which women, as a population, have been shown to outperform men. But the convergence at extreme distances aligns with what the physiology predicts: women’s better fatigue resistance and fat-burning capacity become more valuable as the event gets longer, and men’s raw power advantage matters less as pace drops. It is plausible that the gap will continue to narrow in the longest events as women’s participation grows, but a full reversal remains speculative.
Heat, Sweat, and Running in the Real World
Racing doesn’t happen in a lab, so thermoregulation matters. Men produce more heat in absolute terms during a race, which makes sense given their greater metabolic output. In a 15-kilometer road race in cool conditions, men’s whole-body sweat rate was about 60 percent higher than women’s, and their core body temperature rose slightly more.20PubMed. Core Temperature and Sweating in Men and Women During a 15-km Race in Cool Conditions Despite the higher heat production and higher sweat rate, men’s and women’s finishing core temperatures were the same.
The sex difference in sweating has an interesting twist. When cardiovascular fitness, body size, and heat acclimation are matched between men and women, much of the thermoregulatory difference disappears.21PubMed. A review of comparative responses of men and women to heat stress Women’s lower sweat rate is actually an advantage in hot and humid conditions, where sweat that drips off without evaporating is wasted. Some researchers have speculated that women’s thermoregulatory profile might become advantageous in very hot, very long races, though this remains hard to isolate from other variables in field studies.
How the Gap Changes with Age
The performance gap between men and women is not static across the lifespan. Masters athletics data, covering competitors from their mid-twenties through their eighties, shows that both sexes decline with age, but women decline faster. The overall reduction in athletic performance from ages 25 to 89 was about 43 percent for men and 51 percent for women.22Movement & Sport Sciences – Science & Motricité. Are masters athletic performances predictive of human aging in men and women? Translated to per-decade rates, men lost about 6.6 percent per decade while women lost about 7.7 percent. The divergence became most pronounced after age 65.
American Masters road running records tell a similar story: the performance gap between men and women was smallest in the 40-to-54 and 70-to-84 age brackets, but the largest gap appeared in the 85-and-above category.23PubMed Central. American Masters Road Running Records-The Performance Gap Between Female and Male Age Group Runners from 5 Km to 6 Days Running The likely explanation involves the sharper decline in estrogen during menopause, which accelerates losses in muscle mass, bone density, and tendon integrity. Men’s testosterone levels decline too, but more gradually. Combined with the smaller starting pool of very elderly female competitors, this widens the gap in the oldest age groups.
Why the Gap Is Not About Effort or Access
It’s worth being direct about what the evidence does and doesn’t say. The roughly 11 percent gap in performance at the elite level is not a product of unequal effort, inferior coaching, or fewer opportunities, though all of those have historically been real problems and contributed to the much larger gaps seen before the 1970s. The narrowing that occurred as women gained access to competitive sport, funding, and professional training infrastructure brought the gap to its current level, but it has not continued to shrink since. The biological factors outlined here, from testosterone-driven muscle mass to hemoglobin concentration to lung anatomy, explain why a persistent difference remains even when training, nutrition, and competition infrastructure are matched.
None of this means any given man is faster than any given woman. Elite female runners would outpace the vast majority of recreational male runners. The gap describes population-level averages at the top of the sport, where everyone is maximizing their biological potential. At the recreational level, training, consistency, and individual genetics matter far more than sex for determining how fast you personally will run.
Central Activation and the Brain’s Role
One intriguing finding from fatigue research is that part of the sex difference in endurance may live in the brain rather than the muscles. When researchers blocked blood flow to the working muscles during fatiguing exercise, effectively removing any oxygen-delivery advantage, the sex difference in fatigue largely disappeared.24PubMed. Sex differences in human skeletal muscle fatigue are eliminated under ischemic conditions The primary mechanism behind the difference appeared to be a relatively greater impairment of central activation in men, meaning men’s brains seem to dial back the signal to the muscles sooner than women’s during sustained contractions. Whether this reflects something protective, something neurochemical, or simply a side effect of greater force production is still being sorted out. But it suggests that the nervous system, not just the muscles, is tuned differently between sexes in ways that affect performance at the edges of endurance.