Why gymnasts are so flexible.

Gymnasts can do handsprings, splits, and flips with double twists — extraordinary feats of flexibility and strength that make their sport one of the most popular to watch during the Summer Olympics.

But how are these athletes so incredibly flexible? Live Science spoke with Dr. Timothy Miller, an associate professor of orthopedic surgery and sports medicine at The Ohio State University Wexner Medical Center, to learn more.

First off, it's important to note the gymnasts' ages, Miller said. Generations ago, Olympic female gymnasts were in their 20s and 30s, but that changed after 14-year-old Romanian gymnast Nadia Comaneci won three gold medals (along with one silver and one bronze) at the 1976 Montreal Summer Olympics, according to Live Strong.

When a wave of younger gymnasts entered subsequent games, the International Federation of Gymnastics increased the age requirements, mainly because of health concerns for the girls, Live Strong said. Nowadays, female gymnasts must be at least 16 years old in the calendar year in which the Olympics are held to compete in the games. But unlike in the old days, most gymnasts are still quite young, usually between the ages of 16 and 22, Miller said.

This age range is no coincidence. Younger women are more flexible than older women for a number of reasons, he said.

For instance, a woman's main reproductive years are between ages 14 and 30, Miller said. During that time, they have more relaxin hormone, which gives soft tissues, such as ligaments and tendons, increased flexibility.

If a woman becomes pregnant, relaxin helps her soft tissues relax to accommodate the growing baby, Miller said. But it also allows young female gymnasts more agility. "It just allows their bodies to be more limber," Miller told Live Science.

Delayed puberty

Female gymnasts tend to have smaller, more agile bodies because they exercise at intense levels, which tends to delay puberty. For instance, when she was 13 years old, Jordyn Wieber (a now-retired American gymnast who won a gold medal at the 2012 Summer Olympics) trained for 30 hours a week, according to an interview she did with USA-Gymnastics.org.

Puberty is delayed in young athletes, in part, because the body needs a certain amount of fat to go through puberty, which these young athletes don't have because they burn so many calories, said Dr. Sabrina Strickland, a sports medicine surgeon at the Hospital for Special Surgery in New York City. Moreover, many young gymnasts develop eating disorders, which also stunt growth, Live Strong reported.

Puberty in girls leads to a growth spurt and weight gain, both of which are delayed in many gymnasts, Strickland said. She recalled one girl who stopped gymnastics training at age 11, and rapidly grew 3 inches (7.6 centimeters).

But for gymnasts, short stature is an advantage. It gives them a lower center of gravity, which sits at the midpoint of the body. If their center of gravity is lower, that means it's closer their base of support (i.e. the legs), making it easier to balance on beams, according to Live Strong.

Smaller people have a lower center of gravity.
Moreover, gymnasts who have a high strength-to-mass ratio (that is, they don't weigh much, but they're strong for their build) excel at whole-body rotations, according to a 2003 study in the journal Sports Biomechanics.

"Larger gymnasts, while able to produce more power and greater angular momentum, could not match the performance of the smaller ones," the researchers wrote in the study.

Young joints

People's joints typically stiffen with age, often from overuse or from a gradual progression of arthritis, a condition in which the joints become inflamed, Miller said. Sometimes, people with joint damage develop bony growths called bone spurs around their joints, which limit their ability to move, he added.

But younger gymnasts often have fewer injuries — including fractures.