The starting point: not a smaller man
For decades, much of sports science studied men and then assumed that simply scaling down the numbers would land on women. Physiologist Stacy Sims summed up the problem in a sentence that became a talk and a book (ROAR, 2016, revised in 2024): women are not small men. Real differences between the sexes exist, and they deserve attention. But the central point of this text is a single one: those differences are about composition and metabolism, not about the capacity to improve with training.
When studies compare men and women with the same fitness level, the female body responds to strength and endurance stimulus in the same proportion. What changes is the starting point, the preferred fuel and some specific risks. Let us take it piece by piece.
VO2max and muscle mass: composition, not an adaptation ceiling
On average, women have lower absolute VO2max and less muscle mass than men. The explanation is largely body composition: men have, on average, higher hemoglobin concentration and total mass (more oxygen transport) and greater fat-free mass. When VO2max is normalized by fat-free mass, the difference between the sexes shrinks considerably and, in samples matched for fitness, it comes close to disappearing.
The practical reading: a smaller absolute number does not mean lower trainability. Reviews with meta-analyses show that the relative gain in muscle mass after strength training is similar between the sexes. In other words, a woman adapts just as well as a man. She simply starts from a different composition.
Fuel: more fat at submaximal intensities
In submaximal efforts, at the same relative intensity, women tend to oxidize more fat and less carbohydrate than men. This shows up as a lower respiratory quotient and as a higher maximal fat oxidation rate. In a classic study with men and women matched for fitness, peak fat oxidation was about 6.6 versus 4.5 mg per kg of fat-free mass per minute, and it occurred at a slightly higher intensity (near 58% of VO2max, versus 50% in men).
Estrogen helps explain this pattern, favoring the use of fatty acids and sparing glycogen. For endurance training, this is a metabolic advantage. But note: using more fat as fuel does not mean eating less. It is precisely the opposite that usually causes trouble, as we will see in the energy section.
Muscle fibers and thermoregulation
A 2024 meta-analysis found, on average, a higher proportion of type I fibers (more oxidative and more resistant to fatigue) in women: about a 51% distribution, against 48% in men. Type I fibers sustain long efforts, which helps explain why, in several tasks, women resist muscle fatigue better. Men, with more type II fibers, tend to generate more force and peak power. These are group tendencies, with enormous individual overlap.
In thermoregulation, women have, on average, a lower sweat rate for the same heat production. Curiously, this happens even though they have, in several regions, a higher density of active sweat glands. In practice, the difference only matters at very high intensities and in extreme heat, and adequate hydration matters for both sexes. It is worth noting that the phases of the menstrual cycle slightly shift basal temperature, something each athlete observes best in her own body.
Estrogen, progesterone and the cycle
Sex hormones modulate metabolism, fluid retention, ligament laxity and perceived effort across the cycle. Science is still building consensus on how (and whether) to periodize training by cycle phase, and findings are heterogeneous. The most honest position today: the cycle is a real, individual variable, and it is worth recording how you feel and perform. That is personal observation, not a fixed rule. Any clinical decision about the cycle, contraception or symptoms belongs to a physician.
ACL: why the risk is higher and what reduces it
Women have a higher incidence of anterior cruciate ligament (ACL) injury. Relative risk varies widely with the sport and the study method, with the literature pointing to something between 2 and 8 times, and especially high values for non-contact injuries. Three groups of factors explain it:
- Anatomical: larger Q angle and differences in the geometry of the knee and tibia.
- Hormonal: fluctuations across the cycle that affect laxity and neuromuscular control.
- Neuromuscular: pointed to as the most important factor and, happily, the most trainable. It includes landing pattern and greater quadriceps dominance over the hamstrings.
The good news: the most decisive factor can be changed. Meta-analyses show that neuromuscular training with strength and plyometrics, with good jumping and landing technique, reduces ACL risk by about 50% in female athletes, with an especially strong effect in adolescents. That is not a detail: it is one of the best-evidenced injury prevention interventions in women's sport.
Enough energy: the most common mistake is eating too little
If there is one place where female physiology charges a high price, it is low energy availability. Training a lot and eating little leads to the condition the International Olympic Committee named RED-S (relative energy deficiency in sport), an evolution of the Female Athlete Triad concept. The consequences go far beyond performance: they affect menstrual function, bone health, immunity, protein synthesis and mood.
The practical message is simple and important: using more fat as fuel does not authorize restricting food. Losing menstruation while training is not a sign of lightness or being in shape; it is a warning sign that calls for assessment. Good nutrition sustains adaptation, bones and continuity.
Elite is not a personal goal
Records help show the ceiling of human performance, but an elite athlete's number does not become an individual target. Those results come from rare genetics, decades of training, professional structure and a context almost nobody has. Your reference is your own baseline: what you did last month, how you slept, how you ate, how your knee responded to strength work. Personal progress, measured honestly, is worth more than any comparison with the peak of the elite.
The conclusion closes where the text began. A woman is not a smaller man. The differences are real, but they point to the same destination: strength training and plyometrics with technique to protect the knee, well-periodized endurance and, above all, enough energy to sustain all of it.
