Ask Darby Nelson to flex her biceps and she will gladly oblige. Don’t mistake her confidence for vanity, however — this is a 17-year-old who contentedly wears sweatpants and a T-shirt to class most days to Westfield High in Chantilly. “If I can work out at any time, I feel good,” she says of her attire.

Asking Nelson to display her muscles is like inviting an artist to speak about her craft. It’s a point of pride for her, a 5-foot-6 senior who added an intense workout routine to her life nearly two years ago only to morph into a muscular weight-lifting fiend and All-Met rower.

While her classmates are asleep at 6 a.m., she’s at her gym, tossing around chalk-covered barbells and wiping sweat from her brow. While classmates drink water, juice or soda, she mixes her chocolate whey protein shakes every morning from her usual spot at the front of the class. While some teenage girls her age worry about skinny jeans and looking thin, Nelson concerns herself with improving her squat and deadlift.

She is proud of her powerful physique and all she has poured into building it.

“I’ve just accepted society’s norms and break them when they come across me,” she said.

Nelson’s lifting makes her unique at her school, one of the biggest in the state. She views it as not just a way to prepare for crew, a sport dependent on cardiovascular endurance and leg and arm strength. It’s also preparing her for the Naval Academy, where she was recruited to row, and a career in the military. While society generally rewards men, not women, for appearing muscular, Nelson embraces looking strong.

“I don’t care about being big because I can do things that most people can’t,” she said. “It’s kind of empowering.”

Nelson is a disciple of a growing licensed fitness routine established in the 1980s called CrossFit. It combines elements of gymnastics, cardiovascular exercises and weight lifting, and features anything from climbing rope, running to flipping tires. A recent workout involved doing as many sets of seven pull-ups and seven 65-pound shoulder presses as possible for 14 minutes without a break. The basic tenet of CrossFit is intensity and little rest, and doing as many repetitions as you can in as little time as possible.

Searching for an exercise to help improve her rowing and ease back spasms that had developed from it, Nelson stumbled into CrossFit the summer before her junior year. Her rowing team did very little weight lifting and a family friend suggested the routine. The first day she tried it, she could manage only half of the jump rope and situps required.

By the third day of CrossFit, Nelson lifted 112 pounds over her head in a split jerk routine, where the barbell is thrust from the shoulders into the air while putting one leg forward. It was scary, because she had never put so much weight over her head, but an addicting feeling. She wanted to get better.

“She’s always amazed me just how she pushed the boundaries,” said Sara Blaschke, her mother. “And even more so now how she has taken to this. It wasn’t at all what I expected. She has always done the unexpected to me.”

CrossFit and lifting helped Nelson shave nine seconds from her 2,000-meter time, down to 7 minutes, 24 seconds, within the first four months. Nelson fell so in love with CrossFit that she earned her trainer certification last March and has competed in six competitions across the country.

At her first foray into competitive power lifting in December, she deadlifted 330 pounds — a national record for girls in her age and weight class (deadlifting requires bringing a barbell from the floor to the waist). Nelson has since lifted 350 pounds at a CrossFit competition in March.

During the crew offseason, she works out six days a week; in season, it’s two or three times a week while balancing crew practice.

“I know a lot of guys are scared of her because when she took weight lifting through the school [junior year], she was lifting more than them,” said close friend Anne Culbertson, a senior coxswain on school’s boys’ crew team. “I’m sure they were embarrassed that she was blowing them out of the water.”

According to Hammer Down CrossFit owner Todd Katz, who has owned the gym for three years, Nelson is the strongest woman there.

At school, she’s built her identity as such. Crew teammates come to her for advice on muscle recovery. Recently, a male classmate she barely talked to began chatting with her online, soliciting her advice on Olympic lifting. She’s become a go-to of sorts for workout enthusiasts in her circle — a role she relishes.

“You work hard to develop the body the way that you want to,” said Nelson, the youngest of three girls. “For some people, that’s fashion. They spend hours a day on makeup and their hair and whatever. And for me, I spend hours a day working out. That’s how I’ve developed myself to be. So if you’re going to ask me to flex, then sure.”

Her daily meals consist mainly of a protein shake after her 6 a.m. workouts. Because she attends the junior ROTC program at Chantilly, she goes home for lunch, where she eats a pre-made, pre-ordered meal from a company that adheres to a Paleo diet of lean meats, vegetables and fruits. She snacks on whole almonds, bacon or deli meat, such as salami. Dinner is generally her favorite easy-to-make combination of protein and carbohydrates: scrambled eggs, bacon and sweet potatoes.

But being a bigger, more powerful girl comes with its challenges. High school is all about conforming to the trends, as Nelson put it — and that’s where she differs. Finding prom and homecoming dresses hasn’t been easy — though Culbertson is Nelson’s main advisor on shopping. “It pretty much has got to be backless and strapless because not many girls are built like she is,” Blaschke said.

On a recent afternoon crew practice at Bull Run Marina in Manassas, as she ran to grab a missing piece of equipment from the boathouse, Nelson overheard a boy from an opposing high school ask another: is that a girl or a guy that just ran past you?

Long ago, Nelson learned that no thickness of skin can fully prevent the pain of words. As a freshman, when she started rowing, she was a self-admitted “bigger” kid. Before she started CrossFit, she weighed 160 pounds. She’s the same weight now, exchanging the fat for muscle. Nelson can’t remember the last time she has cried after a teasing remark but it has happened. She didn’t that afternoon, instead she continued on her way.

Later that night, Nelson recounted the event on her Facebook page, closing with the tongue-in-cheek line: “I’m not going to be saving him from the zombie apocalypse.”

It was a telling comment from a teenager who, when she talks about her goals in life, seems as if she wants to be Superwoman, an all-powerful, problem-solving dynamo. She wants to be able to physically and mentally ready for anything life throws at her, from carrying a buddy in combat to moving a couch. She chose the military because she considers soldiers heroes and wants to give back by doing the same.

“Having these skills and abilities in my back pocket whenever I need to, it’s a good feeling,” she said. “I won’t be struggling to do one or two pushups or anything.”

Nelson is soft-spoken and introspective. She also exudes self-confidence in a I’m-comfortable-in-my-own-skin manner. Katz often has to stop himself when running through a list of things for her to do when she babysits his children. “Sometimes I forget I’m talking to a high schooler,” he said.

“The people that like me, they like me for me,” she said. “I appreciate the fact that I don’t have to look nice for people to like me.”

As Nelson rows and pulls the oars back toward her chest, her built triceps bulge. As she tosses around barbells, her hands white from the chalk, the light casts shadows on the curves of her developed forearms. Even among her fellow rowers, her muscularity stands out.

Sometimes, Nelson sneaks a glance in the mirror and notices how her large trapezoid muscles above her shoulders have grown. Looking at her reflection staring back at her, a smile crosses her face.

“That’s awesome,” she says.