Researchers envision a day when workouts are tailored for men vs. women.

At the summer Olympics in Beijing, world records likely will be broken, inevitably leading to much speculation about the limits on athletic achievement. Just how much farther can the human body go without steroids or other performance-enhancing substances? And will women ever be able to perform at the level of men.

"I don't think they really are ever going to catch up," says Danielle Day, an assistant professor of exercise and sport performance at the University of New England in Biddeford, Maine, who follows the topic.While women have made tremendous gains in athletic performance, experts generally agree that the world's top female athletes will probably never outperform their male counterparts. Men are bigger and have more muscle and higher levels of that powerhouse hormone testosterone than women.

But as researchers have begun to unravel the differences between the sexes with regard to physical activity, they're learning that there's more to the story. Men and women also differ in energy metabolism, lung function and other factors during exercise.