Running is a sport that both men and women enjoy, whether they're racing in a 5K or a marathon, or competing for a team or their country while speeding around a track. But no matter the venue, it's pretty common to see men clock faster times than women do.

Given that both men and women train equally hard, why is it that men, on average, are faster runners than women? Even the world's fastest man is about a second speedier on the 100-meter dash than the world's fastest woman: Usain Bolt did it in 9.58 seconds, versus the late Florence Griffith Joyner's time of 10.49 seconds.

The answer to this gender bender is multifold, but it has a lot to do with hormones and body size, doctors told Live Science. [Who Are the World's Fastest Man and Woman?]

Before girls and boys hit puberty, their bodies are fairly similar. During puberty, however, boys experience a surge of testosterone. By adulthood, some men have up to 20 times more testosterone than women do, according to HealthLine.

Testosterone plays several roles, including telling the body to create new blood cells, keeping bones and muscles strong and prompting growth spurts, according to the Society of Endocrinology.

"Because [women] produce less testosterone, we are at a disadvantage in terms of muscle," said Dr. Emily Kraus, a primary care sports medicine physician at Stanford Health Care in California. "Males have a greater amount of muscle bulk."

A man's leg is about 80 percent muscle, compared with about 60 percent muscle in a woman's leg, Kraus said. That extra muscle can help men run faster, she said. Also, men's muscles tend to have larger fast-twitch muscle fibers, which help with sprinting, than women do, Kraus said.

In addition, women have more estrogen than men do, which leads them to have a higher percentage of body fat than men have. "That can also lead to a small disadvantage for running performance [for women, in comparison with men]," Kraus said.

Body size is another factor. Women, on average, have smaller lungs than men do, meaning their maximal oxygen consumption (VO2 max) is lower. The VO2 max for a sedentary woman is about 33 milliliters of oxygen per kilogram of body mass per minute, while a young sedentary man's is about 42 ml/kg/min, according to a 1998 study in the journal Medicine and Science in Sports and Exercise.

In elite runners, VO2 max is higher, but men still top women. Basically, "the amount of oxygen produced at maximum exertion is greater in males than in females," Kraus said. This means that women have to work harder to breathe in oxygen that they can deliver to their muscles, she said.

Women's hearts also tend to be smaller than men's, which means they have a smaller stroke volume, or the amount of oxygenated blood that the left ventricle pumps out in one beat.

"Even though [women] have a higher heart rate, it's not enough to counterbalance the lower stroke volume that [women] have," Kraus said. "Each time the heart pumps blood, that amount of blood is less in a female than in a male." That means less blood and less oxygen are delivered to women's muscles, she added.

To top that off, women also have less hemoglobin, the protein in red blood cells that carries oxygen to the body's tissues, including the muscles, Kraus said.

Biomechanics and running

As far as biomechanics, men usually have longer legs than women do, meaning they have more room for muscle, as well as a longer stride length, said Dr. Miho Tanaka, an assistant professor of orthopedic surgery and director of the Women's Sports Medicine Program at Johns Hopkins Medicine.

Moreover, because women tend to have wider hips, their running stance is not as efficient as a man's is, Tanaka said.

"Muscles work efficiently when everything is in line," Tanaka said. "If your hips are very narrow, like a man's, then your quads are running straight from your hips, past your knees. It's in a straight line, so it's acting in the same direction that you're running."

For a runner with wider hips, however, the "muscles almost have to turn a corner, so to speak," Tanaka said. "It's not like the optimized function for the muscle."

This isn't to say that women with wide hips can't run, but it's one of the many factors that explain why women, on average, aren't as fast as men, she said.

To sum it up, women's lungs and hearts have a smaller capacity to breathe in oxygen and pump oxygenated blood, respectively, and they have less hemoglobin in their blood to carry that oxygen. Moreover, women tend to have less lean muscle and shorter legs than men do, as well as wider hips, which makes running less efficient.

"It's quite impressive; even with these disadvantages at baseline, some women are still quite competitive with men," Kraus said.

Original article on Live Science.