Men are, on average, significantly stronger than women, and the gap is largest in the upper body. Adult women typically produce 50% to 60% of male upper-body strength and 60% to 70% of male lower-body strength. This isn’t explained by a single factor. It’s the result of several biological systems working together: hormones, muscle fiber size, skeletal structure, body composition, and oxygen-carrying capacity.
Testosterone and Muscle Mass
Testosterone is the primary driver. Men produce roughly 10 to 20 times more testosterone than women, and this hormone directly increases muscle protein synthesis, the process by which the body builds and repairs muscle tissue. Over time, higher rates of synthesis lead to greater muscle mass. Interestingly, studies using labeled amino acids have found no measurable sex difference in the baseline rate of muscle protein breakdown or whole-body protein turnover. The difference isn’t that women break down muscle faster. It’s that testosterone tips the balance toward building more of it in men.
This hormonal gap translates into a meaningful difference in lean body mass. When researchers measured lean mass index (total lean mass adjusted for height) using body scans, men averaged 16.6 kg/m² compared to 13.8 kg/m² in women. That roughly 20% difference in lean tissue is the foundation for nearly every other strength advantage.
Muscle Fiber Size, Not Just Quantity
A large meta-analysis looking at sex differences in muscle fibers found that men and women have a roughly similar distribution of fiber types. Men carry about 51.6% fast-twitch (Type II) fibers compared to 48.3% in women, a small gap. The real difference is in fiber size. Male fast-twitch fibers are about 41% larger in cross-sectional area than female fast-twitch fibers (5,272 μm² vs. 3,483 μm²). Even the slow-twitch fibers used for endurance are about 17% larger in men.
Fast-twitch fibers are the ones responsible for powerful, explosive movements like sprinting, jumping, and lifting heavy loads. Because men have substantially larger versions of these fibers, they can generate more force per contraction. This is one reason the strength gap between sexes is more pronounced during high-intensity, maximal efforts than during lighter, endurance-based activities.
Why the Upper Body Gap Is So Large
The strength difference between men and women isn’t uniform across the body. It’s roughly twice as large in the upper body as in the lower body. By ages 14 to 17, boys already show 50% greater upper-limb strength but only 30% greater lower-limb strength compared to girls, and this pattern carries into adulthood.
Part of the explanation is skeletal. Even when researchers matched men and women for overall body size, men still had larger shoulder blades, longer coracoid processes (the bony hooks where muscles attach on the scapula), thicker cortical bone in the upper arm, and larger humeral heads. These aren’t just cosmetic differences. A wider shaft diameter in the upper arm bone gives it a greater ability to withstand bending forces, and longer bony attachment points give muscles better mechanical leverage, meaning each contraction produces more usable force. Women tend to carry a greater proportion of their muscle mass in the lower body, which partially closes the gap in leg strength.
Grip Strength as a Case Study
Grip strength is one of the most commonly measured benchmarks of raw physical strength, and it illustrates the gap clearly. In a nationally representative U.S. sample, combined grip strength peaked in the 30 to 39 age group at 216.4 pounds for men and 136.5 pounds for women. That means the average woman in her peak years produces about 63% of the average man’s grip force.
The overlap between the two populations is smaller than many people assume. While the strongest women certainly outgrip the weakest men, the distributions are separated enough that grip strength alone can predict a person’s sex with high accuracy. This gap persists across age groups, though it narrows slightly in older adults.
Oxygen Delivery and Sustained Power
Strength isn’t only about muscle. It also depends on how efficiently your body fuels that muscle with oxygen during effort. Men carry higher concentrations of hemoglobin, the protein in red blood cells that binds oxygen. In one large study, average hemoglobin was 14.8 g/L in men versus 12.3 g/L in women, a difference of about 20%. Higher hemoglobin means more oxygen reaches working muscles per heartbeat, which supports both sustained effort and recovery between bouts of exertion.
This is one reason men generally have higher aerobic capacity (VO₂ max) even after adjusting for body size. During repeated heavy lifts or prolonged physical tasks, that extra oxygen-carrying capacity helps maintain force output over time.
What Doesn’t Differ as Much as Expected
Not every tissue contributes to the strength gap. Tendons, the tough cords that transmit muscle force to bone, are surprisingly similar between sexes in their mechanical properties. Research on tendon structure found that while male tendons were slightly larger (about 6% in the Achilles), the actual stiffness, peak load capacity, and fatigue resistance were nearly identical between males and females. The collagen proteins that bear the load were present at similar levels. Some earlier studies had suggested women have less stiff tendons, but the direct mechanical testing paints a more nuanced picture.
Neuromuscular control also differs in ways that don’t simply favor men. When researchers measured how motor units (the nerve-muscle connections that produce contractions) fire during effort, women actually showed higher discharge rates at each intensity level. Women also recruited motor units at higher thresholds, suggesting a different neural strategy rather than an inferior one. Men, meanwhile, showed more asymmetry between their dominant and non-dominant hands in how motor units fired. These differences reflect distinct neural patterns for controlling force, not a clear advantage for either sex.
How the Factors Stack Up Together
No single mechanism explains the full strength gap. Testosterone drives greater muscle mass. That mass is composed of larger individual fibers, especially the fast-twitch fibers that matter most for peak force. The male skeleton provides longer lever arms and thicker bones, particularly in the shoulders and arms. Higher hemoglobin supports the oxygen delivery needed to sustain high-force efforts. Each factor compounds the others.
The result is a consistent, well-documented difference that emerges during puberty and persists throughout life. Before puberty, strength differences between boys and girls are modest. Once testosterone levels diverge in adolescence, the gap widens rapidly and remains stable into middle age before both sexes experience gradual declines. The biological systems responsible are deeply intertwined, which is why the strength difference is one of the most robust physical differences between the sexes.