"Have you seen Kevin's whiteboards?"

If you spend any time at Under Armour headquarters, you'll hear that question again and again. Founder and CEO Kevin Plank really likes whiteboards, and his favorite use for them is to write out leadership maxims for his team. Inside and outside his office, whole walls of floor-to-ceiling whiteboards contain dozens of curt principles he's scrawled over the years: Expedite the inevitable. Perfection is the enemy of innovation. Respect everyone, fear no one.

These commandments are meant not as simple inspiration or hard rules, he says, but together make up a system of "guardrails" that allow everyone under him to operate as entrepreneurs by channeling his thinking. The Plank principles are drilled into new employees during a weeklong orientation, and they're painted all over the hallways at company headquarters, a former Procter & Gamble factory on the Baltimore waterfront. Think like an entrepreneur. Create like an innovator. Perform like a teammate.

Plank has the affect and intensity of a head coach--direct eye contact, military analogies, the air of someone you do not want to disappoint. "Winning is a part of our culture--it's who we are," he says in his lofty office overlooking the harbor. (The only artwork behind his desk: a giant UA logo, its letters stacked to evoke arms raised in victory.) "And culture is formed on habits." Perhaps the most important guardrail, and the company's official mission, is seeking to "make all athletes better." It has long equaled thinking about clothes as high-performance gear, but recently it's taken on a big new meaning.

Over the past two years, Under Armour has spent close to $1 billion buying and investing in three leading makers of activity- and diet-tracking mobile apps. By doing so, the company has amassed the world's largest digital health-and-fitness community, with 150 million users. Plank envisions all of those users, and their metrics, as a big data engine to drive everything from product development to merchandising to marketing. Many observers, though, balked at the $710 million cost of the acquisitions, questioning whether Under Armour could quickly produce any return on investment--two of the three companies were unprofitable--let alone succeed in a space that shares little with making shirts and shoes. Longtime staffers worried the moves would crimp company performance, affect bonuses, or divert focus from the core business. Plank spent more hours than he cares to count, including a large chunk of his winter vacation last year, in one-on-one conversations to persuade them otherwise. "It was important," he says, "that this not just be my decision."

Plank likes to say that the key to Under Armour's success is that he never focused on all the reasons it couldn't happen. A former Division 1 college football player, Plank famously bootstrapped Under Armour's launch in 1995 armed with one simple insight: The cotton undershirts football players wore under their pads slowed them down when they became soaked with sweat. After prototyping a moisture-wicking, formfitting alternative--made of fabric for women's undergarments--and testing it on ex-teammates, Plank set up shop in his grandmother's basement and, just before he went broke, scored his first big sale, to Georgia Tech. The company went on to create a whole new market for performance apparel, IPO'd in 2005, and now sponsors some of the world's greatest athletes, including Jordan Spieth, Stephen Curry, and Lindsey Vonn.

Today, Under Armour has 13,500 employees around the world and nearly $4 billion in revenue. But Plank is still every bit the entrepreneur, chasing audacious dreams--chief among them overtaking Nike as the world's largest sportswear maker. Under Armour leapfrogged the longtime number two, Adidas, in the U.S. sportswear market in 2014, but worldwide it's still third. And Nike remains far larger, with more than $30 billion in revenue in 2015 Which is part of why Plank wants to move so aggressively. Nike has about a fifth as many users on its Nike+ platform as Under Armour does on its apps, and in 2014 the shoe giant shut down its FuelBand fitness-tracker business.

The real work is only beginning, though, as Plank has adopted the kind of world-changing ambitions more common to a Google or Facebook. He envisions that Under Armour Connected Fitness will "fundamentally affect global health." This month--doubters be damned--the company will start selling a pair of biometric fitness devices and a smart scale made in partnership with the Taiwanese smartphone company HTC. The move will put Plank in direct competition with Fitbit and Apple in the fast-growing wearables market. It's a bold, characteristically Plankian bet--and a "very risky" one, says Morningstar retail analyst Paul Swinand. (Morningstar and Inc. are both owned by Joe Mansueto.)

"Under Armour has been a phenomenal success story," Swinand says. Its stock has risen steadily--almost 2,000 percent in the decade since its IPO. "But when you're hitting a home run every quarter on the core apparel business, why mess around with a moon shot?"

Plank rarely admits to much uncertainty or doubt, so it's telling that he echoes Swinand in describing Connected Fitness's ambitions as a "moon shot." But another of his whiteboard sayings comes to mind, this one courtesy of his friend and former U.S. Special Operations commander Admiral Eric Olson: Nobody ever won a horserace by yelling "Whoa!"

Robin Thurston, co-founder and then CEO of Austin-based app maker MapMy­Fitness, got his first taste of Plank's high-speed force-of-will approach when the Under Armour founder cold-called him in July 2013. Plank explained that he loved Thurston's app MapMyRun. "I run five miles three times a week, I log everything, I look up routes when I travel," Plank began. "What are you doing with the company?"

Thurston replied that he was about to raise more venture capital to pursue ambitious expansion plans: The company had bought several hundred domains based on every physical activity, and planned to launch new products for each. Thurston and his investors saw MapMyFitness as poised to become the leading digital health-and-fitness network.

"Don't do that," Plank shot back. "Come talk to me instead."

A couple of weeks later, Plank and three key lieutenants showed up early at the New York City offices of Allen & Company, where Thurston and his team were huddling with their bankers. The MapMyFitness team got about 20 minutes into a detailed PowerPoint presentation when Plank interrupted. "This is awesome," he said, "but I want to stop you and go talk to Robin myself for a few minutes"--without any bankers running interference. Forty minutes later, Plank and Thurston returned, and Plank asked the MapMyFitness team if they'd like to go to Baltimore, right away, to check out the Under Armour campus.

It wasn't 11 a.m. when the group--along with NFL veteran and sports­caster Boomer Esiason, who'd been waiting at the airport to hitch a ride on Plank's jet--pulled up at Under Armour headquarters. Former Washington Redskin LaVar Arrington opened Thurston's door, and offered a tour of the campus, as well as some oatmeal cookies, to the stunned app makers. Within two weeks, the parties had agreed that Under Armour would acquire the startup for $150 million, and Thurston would remain atop MapMy­Fitness and become Under Armour's chief digital officer.

Thurston, a onetime professional cyclist who maintained MapMyFitness's position as a top fitness app from the iPhone's earliest days, tells the story in his new office in downtown Austin, in a brand-new building where giant images of Under Armour athletes adorn the walls (amid, of course, motivational mantras) and several hundred new engineers and other tech employees work. At first, Thurston says, Under Armour's interest was a puzzler. He'd entertained partnering with insurance companies and media companies, but he always worried they'd exploit all the data MapMyFitness gathers about people's personal habits in ways that would violate the trust he'd built with the community. Under Armour had simply never occurred to him as a home for his company.

But the first thing Plank did in that private meeting in New York was pull up a concept video Under Armour had created earlier that year called "Future Girl." It showed a young woman starting a morning workout in clothes that were touch-sensitive and could call up data displays and even change color with the tap of a finger. "I made this for you," Plank said to Thurston. (In truth, it had run as a TV commercial; Plank told me it was made for someone like Robin even though "I didn't know who Robin would be.") He wanted to be sure that Thurston wouldn't bolt after the sale, but would instead see an exciting opportunity and lead it. Under Armour had always been a tech company, in its way, Plank explained--but it had struggled with digital.

None of the products in the "Future Girl" video existed then--and a variation of one is hitting the market now--but merging performance products with performance data and inter­active technology was a top Under Armour priority, given Plank's instinct that that's where the world was going. Plank had directed a team several years earlier to create an "electric" product, and they'd come up with the E39 compression shirt, which had sensors embedded in the fabric to track an athlete's heart rate. The shirt launched at the 2011 NFL training combine to much fanfare, but a simplified consumer version--a sensor-equipped chest band­--had only niche appeal. That experience made Plank realize Under Armour couldn't compete with hardware companies that employ thousands of engineers and constantly turn out incremental innovations.

"It's absurd that you know more about your car than you know about your body," says Plank. He's betting athletes' personal data will turbocharge their fitness and Under Armour's future.

"It's very normal for a product company--which is really what Under Armour is--to have gone down the path of trying to create hardware," says Thurston. "They know the distribution channels, they know how to sell products, they know how to market them. But as they started doing their homework on what was happening in the space, they realized that the strength [of digital fitness] was actually in the community."

Plank also knew it would take years to build a community like Thurston's. "It wasn't that I didn't know the right answers to be seeking from engineers. I didn't even know the right questions to ask," Plank admits. "I'm a sporting goods guy."

After the MapMyFitness acquisition closed in late 2013, Plank and Thurston proceeded uncharacteristically slowly, taking time to set priorities for Under Armour's digital transformation. Thurston identified four key pillars of health--sleep, fitness, activity, and nutrition--that he based on Plank's "make all athletes better" mission. Once that vision snapped into focus, Plank saw an opportunity not just to be a collector of human activity data but also to be the central processor that turns that data--regardless of whose device or app collected it--into useful insights. "OK. Let's do it," he told Thurston one day in late 2014. By the following March, they had spent more than half a billion dollars acquiring two more companies: San Francisco-based MyFitnessPal, a nutrition-tracking system for people to log their meals, and Copenhagen-based Endomondo, a personal-training program whose users are almost entirely outside the U.S. Under Armour suddenly had not only the world's largest digital fitness community but hundreds of engineers and reams of user data as well.

Just one big question loomed: How would any of that help Under Armour chip away at Nike's dominance, or at least sell a lot more workout shirts?

Across the railroad tracks from the Under Armour campus, a low redbrick building houses the company's innovation lab, where president of product and innovation Kevin Haley leads a team of biomechanists, designers, engineers, and a psychologist to develop shoe and apparel concepts. There are weather chambers to re-create different exercise scenarios, devices that stretch and compress materials, gait-analysis systems, washers and dryers, 3-D printers, laser cutters, and countless other machines. The deeper you go into the long, narrow lab space, the more secretive the operations. The prototyping room is locked down from all but a few select employees and executives, who must pass a biometric scanner to enter.

Before taking over the innovation lab, Haley created the Under Armour consumer insights department. Early on, "the secret of our success was that we were the consumer," Haley says. "Kevin was a football player. He just knew. But slowly, we got older than our consumer." The company stopped bragging about not using focus groups and started tapping its sponsored athletes for product insights, sending researchers to look in people's closets, and running online surveys.

What Under Armour didn't know with much precision, though, was how people used its products after buying them. "You just know if a person swipes a credit card or not," as Haley puts it--and even that only happens a couple of times a year for any customer. "We call something a basketball shirt, but is the guy wearing it to football practice? Is the boyfriend shirt he gives to his girlfriend something she wears as pajamas?"

But armed with data from Connected Fitness apps, Haley says, he can take design cues from 150 million people who, having downloaded a fitness app, are exactly the target audience: "There's unbelievable data in there. You know their running pace, how far they go, how often they go. You literally know what brand of Greek yogurt they use."

It's too early to see many new products as a result of all the new data--developing a piece of gear typically takes 18 months--but Haley points to one. The company learned from MapMyFitness data that the average run is 3.1 miles--"not one or two miles, not five miles, but 3.1," Haley says. So when it came to making the Speedform Gemini running shoe, which was released last January to largely rave reviews, the company added "charged foam" padding tailored to that kind of run.

"The toughest question for us is not, Are there cool technologies out there?" says Haley. "It's, What do you want me to work on? This gives us unbelievable insight that's both incredibly broad and deep, with the same group of people we're marketing toward." That could be especially helpful in the two huge growth opportunities for Under Armour. More than 60 percent of Connected Fitness's users are women, who account for just 30 percent of Under Armour's apparel sales. And while only about 11 percent of its sales are international, 35 percent of the Connected community is outside the U.S.

Still, the high-stakes bet on Connected Fitness will be slow to pay off. Under Armour recently increased its projections for the next two years, estimating that it would nearly double net revenue by 2018, to $7.5 billion (up from a previous estimate of $6.8 billion). Only $200 million--a paltry 2.7 percent--will come from Connected Fitness. But Thurston likens his digital community to "having a Super Bowl-size audience every day," and one of the most immediately practical moves will be using those apps as a marketing channel. A feature called Gear Tracker, for instance, allows MapMyFitness users to log the shoes they use every time they go running, and get a reminder when their mileage suggests it's time to buy new ones. A partnership with Zappos makes ordering replacements easy.

"Imagine you're traveling in Chicago for work," Plank says. "You went for a run one morning, and you had a cold the day before. It's 7 degrees in Chicago, so I know your nose was probably running the whole time. Well, we make this great run glove--we call it the snot finger glove, because it's got basically a microfiber personal Kleenex attached to it so you can rub your nose. Imagine if I could send you an ad that says, 'Hey, are you going to be in Chicago for another day? Would you like us to send you a pair of gloves?'" Chris Glode, a key digital executive for the company, told a conference last year that the company has learned that consumers are 83 percent more receptive to fitness messages after a workout--so an ad could hit you right as you log your run. The average Underarmour.com order coming via a Connected Fitness app is 26 percent higher than those from other external sources, the company says, so one big priority is to build e-commerce into the apps.

If it all sounds eerily like those ads that, because of your browsing history, follow you around the internet, that's exactly the point--except Under Armour is tracking real behavior and the data is more specific. Everyone in the company says personal data mining will give customers better fitness insights and better shirts and shoes--so they become better athletes. Another way to look at it is that making people better athletes makes them need more gear. As Plank told analysts last July: "Ultimately, the more people exercise, the more athletic footwear and apparel they will buy."

"I love Monopoly," Plank tells me. "You know why? When I play Monopoly with you, I'm going to buy everything from Baltic Avenue to Marvin Gardens. If you get to my side of the board, you'd better roll boxcars or you're going to pay rent."

He's trying to describe why buying MapMyFitness was never going to be enough; the real opportunities would come only if he controlled every part of the digital health experience, even if nutrition, say, has only tangential relevance to the sportswear business. If you're trying to truly understand athletes, you need to see what they do 24 hours a day. "It's absurd that you know more about your car than you know about your body," says Plank.

Hence an app called UA Record, a kind of overall health dashboard that relaunches this month in conjunction with the co-branded HTC devices--a Fitbit-like wrist strap, a chest-worn heart-rate monitor, and a connected scale, all sleek black and knobby red plastic, with scoreboard-inspired readouts. Record is an open platform for people to process their fitness data from any device, and Record exec Glode calls it "the ultimate digital expression of Under Armour." Plank gets especially animated talking about the relaunch of Record and the related devices, which will be sold together under the name Health Box. It'll be the first time users get to experience his full vision, and it's where he sees the potential to "affect global health." Roughly one in five Americans has downloaded one of Plank's apps, Plank has said, so he wants not only to help users keep track of their running times or weight loss but also to compare their data with that of millions of other people like them and offer valuable insights.

Whether the system sounds ingeniously simple or a bit too clever for its own good may depend on your dedication to fitness. To Plank, it's the former, of course: "It's like, I wake up in the morning, my wearable device tells me how long I slept, and data point one beams to the cloud. I go to the bathroom and step on the scale, and data point two beams to the cloud. I'm going to exercise, so I put on my heart-rate strap, and data point three beams to the cloud. And as I walk around all day, the fitness tracker beams how many steps I take. Finally, what did I eat for the day? If I want to go deep into MyFitnessPal and track everything, great, but if not, I just answer if I had a light or average or heavy day."

I am the opposite of a moderate man reads one key Plankism scrawled on a company whiteboard.

"This is where it gets really exciting," Plank says, and launches into a series of scenarios. If you are ill one day in October, you might learn that you get sick around the same time every year and that it correlates with your sleep or diet patterns or any number of patterns that you share with others your age and of similar height and weight. If you go to the doctor, all that information might be a lot more useful than the hand-scrawled note the physician has from your last visit 24 months ago and a couple of basic measurements the nurse took a few minutes earlier. "Nobody owns this," says Plank. "And I'm sitting here thinking, who should? Humana? CVS? You're going to trust them with your data? Why not us?"

Fair enough, says Morningstar's Swinand, but he questions whether Under Armour can win that battle when the competition includes Fitbit, Apple, and even Google. "The way tech works is you have four companies, three end up zeros, and the one that wins wins everything." He raises the possibility that Under Armour could end up the MySpace of fitness tech--and expresses concern about the recent departure of longtime COO and CFO Brad Dickerson. "I think Brad was the voice of reason, and Kevin is the flamboyant entrepreneur," Swinand says.