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Ever wonder why workouts often do different things for women and men? Find out with Groupon’s answer.

Mini-Muscles: Why Don't Women Bulk Up When They Lift Weights?

Women's strength-training classes often promise results such as a toned, slim, or defined body—but they almost never promise to add bulk. Yet these very same workouts do just that for men. That’s because although a rigorous strength-training routine can help a woman boost her endurance and lower her body-fat percentage, it won’t help her develop big, bulky muscles (not without anabolic steroids, anyways). Just look at the world of professional weightlifting—successful women will squat many times their own weight, but won’t achieve the same physical proportions as their male counterparts.

One key factor is estrogen. The hormone, which is present in both men and women at different levels, works hard to limit the body's capacity to develop new muscle tissue. This has a practical explanation. Women's bodies have evolved to retain the ample fat stores needed to ensure fertility and successful pregnancy. Men's bodies don't have the same obligation, and so they possess far larger quantities of testosterone—the muscle-building hormone—than women do. This is an example of sexual dimorphism, a physical difference between males and females of the same species, not unlike the male peacock's feathers, the lion's mane, or the gander’s jaunty hat collection.