Nearly four years ago, the United States military announced it was opening all combat jobs to women. Since then, more than 30 women have passed difficult training programs with the Army Rangers and the Marines.

Their numbers are small. But their completion of some of the military's most difficult physical and mental courses has raised a scientific question: Who are these "hyperfit" women and what makes them so competitive?

Army medical researchers hope to find answers in a study.

Holly McClung, a scientist at the Army Research Institute of Environmental Medicine in Massachusetts, is working on the study.

"We're really interested in those elite women that are the first to make it through physically demanding training," she said.

Julie Hughes also works on the study. She said, "There's this group of women who made it through the training so we want to ... explore what makes them unique."

The goal, said McClung, is to find the mental and physical factors that help the women succeed. By finding this information, researchers might be able to help other women compete for the same jobs.

The plan for the study is to have the military women come to an Army base in Natick, Massachusetts, in groups of two or three. Having several tested at the same time, McClung said, will make it more interesting and encourage competition.

They will go through many tests over three days.

In one test, the women will run on treadmills while wearing a mask and breathing device connected to a computer. The device will measure how much oxygen is being used at a person's peak exercise rate, known as the VO2 Max score. An average person may have a score of about 30. Top athletes can score in the 80s.

The breathing test, along with others, will be used to determine the women's physical fitness. Written tests and interviews will study their mental toughness.

McClung and Hughes also said they hope to be able to compare the women's results with similar testing on men.

Mark Esper, the Secretary of Defense, said the study will be important to help understand fitness and perhaps reduce injury rates.

"My belief is it's grit — grit gets them through this," he said of the women. "You have to have a certain level of athletic ability, but they take it to an extraordinary level. These women are tough."

I'm John Russell.

Lolita C. Baldor reported on this story for the Associated Press. John Russell adapted her story for Learning English. Hai Do was the editor.

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factor – n. something that helps produce or influence a result : one of the things that cause something to happen

encourage – v. to make (someone) more determined, hopeful, or confident

grit – n. informal : mental toughness and courage

extraordinary – adj. extremely good or impressive

unique – adj. very special or unusual

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