At the leg press your plates look like everyone else's, and the squat rack takes a bar you load without a second thought. On the back-extension bench, your plate is a size smaller than the room's. Your bench bar goes up almost bare. Arms take the lightest pair.
Comment threads and reels hand you the same explanation, that women are naturally weak up top, that hormones decide it, and that the lag is how female bodies are built. Is it really harder for women to build upper-body strength, or have you been getting something wrong?
Why is it harder for women to build upper-body strength?
Your read on the room holds up, on average. Women's strength usually runs at 60 to 70% of men's in the lower body, roughly 60% in the trunk and 50 to 60% in the upper body, with the bench press usually at 40 to 55%. Those are group averages from a 2023 review written while its author worked at Vitruvian, a company that makes strength-training equipment.
The upper body is where women start furthest behind men. It is also where, with both sexes training on one program, women's strength grew more than men's, each relative to its own start. Across pooled studies of mostly untrained people, that tilt showed up in the upper body, with no significant sex difference in the lower body.
Five hundred and eighty-five untrained adults aged 18 to 40 showed what that looks like, each training one arm for 12 weeks. Women's curl strength rose 64.1% from where it started, and men's rose 39.8%, even though the men added more kilograms.
Starting further back is the likely reason for women's bigger percentages, by that study's own explanation, and across all 585 people, the lower the starting curl, the bigger the percentage gain tended to be. A kilogram added to a small starting weight is simply a bigger share of it. Women's starting weights sit furthest below men's in the upper body.
Women start further behind men in upper-body strength than in lower-body strength (usually 50–60% of men's, against 60–70%), most likely because more of men's muscle sits up top. In trials where mostly untrained women and men followed one program, women's upper-body strength rose as much or more relative to its own start, while men added more kilograms.
— Nuzzo 2023 · J Strength Cond Res · review (author employed at Vitruvian); Roberts et al. 2020 · J Strength Cond Res · 17 studies of upper-body strength; Hubal et al. 2005 · Med Sci Sports Exerc · n=585
Measured in kilograms, the picture tilts back toward men. Women's bigger percentage came to 3.6 kilograms on the curl, on average; men's smaller one came to 4.3. The women's average curl went from 53.0% of men's to 62.3%, by FitChef's arithmetic on the study's averages, so the gap narrowed as a share while it widened a little in kilograms. A big percentage of a small starting weight still arrives as a small step on the rack.
Muscle is the most likely source of the distance. Men carry a bigger share of their muscle in the upper body, and in one study the 2023 review cites, women's muscle mass in the upper body was 60% of men's, against 67% for the lower body. More of men's muscle area is also made of type II fibers, the kind that produce more force. Even with body size taken into account, upper-body strength is much lower in female athletes than in male athletes, unlike the lower body, according to a 2026 position statement on female athletes from a strength and conditioning association.
Women's nervous systems drive their muscles as hard as men's do, since voluntary activation, the lab measure of that drive, showed no sex difference in the studies the 2023 review gathered.
The signal from a woman's brain arrives as strong as a man's, and there is less muscle up top to answer it.
Testosterone enters the story before the first workout. Women have 10 to 20 times less total testosterone than men after puberty, in the figures of a 2024 review of hormones and muscle growth, and the gap that opens then helps give men larger muscle fibers and bigger muscles. The hormone rises after a workout do not appear to drive strength or muscle gains in either sex, the same review concludes, and despite the testosterone gap, women and men make similar gains relative to where they start.
Two habits set everyday gyms apart from the trials, where women and men follow one program, and trial results should not be read as a forecast for everyday training, a caution the 2023 review makes itself. Men spend more time training their upper bodies, going by two student surveys and a third study, which might in part explain the bigger upper-body gap. Men also picked relatively heavier weights on all five exercises in one study where untrained people chose their own loads, though statistics could not rule out chance.
The upper-body result rests mostly on untrained adults aged 18 to 50, and it varied widely from one study to the next. Untrained women may have more room to gain upper-body strength, a possibility the pooled analysis leaves unexplained, and men may simply know lifts like the bench press better, leaving women more to learn. Whether the sex differences from the one-arm study would hold for whole-body or lower-body training is, in its authors' words, unknown. In adults over 50, women's upper-body strength rose about as much as men's relative to where each started, in a separate pooled analysis summarized in the 2023 review, while older women's bigger relative gains appeared in the lower body.
For women who already lift, the evidence thins out, and definitive statements about how trained women adapt are hard to make for lack of studies, in the view of the 2026 position statement. Female athletes are relatively equally adaptive to resistance training as male athletes, it adds, and trained ones keep adapting as they keep training. Strength and muscle size are measured separately, and how women's muscle growth compares with men's has its own pooled answer. Women with years of lifting behind them are the group this research has measured least.