Wes Welker is in the pool—but not in a chill way. The Broncos wideout squats in 4 feet of water and then explodes up like a deranged salmon. He does it over and over again as Adam Boily, his trainer, paces the pool's edge barking encouragement.

Welker arrived at 7:30 a.m. at this random swimming pool behind a random time-share complex in Florida. He'll do an hourlong workout before the pool opens and the resident retirees settle into their chaises.

All this month, Welker is prepping for his 11th NFL season by working on "the fast twitch," as he says. "I'm teaching my muscles to move as fast as they can. I need to get off the line of scrimmage and burst, and get in and out of cuts." The water adds resistance, he says, and gets him "off the joints" for a better recovery.

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From the pool, Welker hops into his black Yukon and drives to Boily's gym, Bommarito Performance Systems. The facility's program is big on something called MAT—muscle activation techniques. The two men move around the gym as Welker busily switches it up from box jumps (he leaps with both feet from a dead rest onto a 50-inch wooden box) to below-knee clean pulls (he jerks a 240-pound loaded barbell—and both feet leave the ground) to single-leg deadlifts (he lifts a 66-pound hand weight while standing on the opposite leg).

This is how Welker prepares for Broncos training camp, which is almost a month away. "Camp should be easy," he says. "Football should be easy. The training should be hard." He's been at it long enough to know that if he's not in peak physical condition, or if he's babying an injury, his mental game suffers as much as his performance does. After jumping out of the pool this morning, he made an offhand observation to Boily: "If you feel good, you can think about playing good."

Wes Welker has always been a hard-charging guy who plays spectacular football. But for years, football didn't exactly love him back. In high school in Oklahoma, he was the star receiver—and rusher and kicker and punt returner. But when the time came for scholarships, he had to wait for a last-minute offer from Texas Tech University. Why? Because he was 5'9"? No matter. He'd show 'em. In his last year at Texas Tech, he set a Big 12 record for receptions per game and NCAA career records for punt returns and punt-return yardage. Surely those stats would guarantee him a spot on an NFL squad.

He went undrafted.

He had failed the "measurables." Maybe the scouts didn't like his height, or his speed in the 40-yard dash, but clearly they were not measuring the right things.

There were 256 supposedly hot prospects drafted by NFL teams in 2004, including 31 wide receivers. Where are those 31 receivers now? They're mostly gone from the game. Gone and not remembered. The only big name still in the league is Larry Fitzgerald Jr.

So it goes. Wes Welker—today a five-time Pro Bowl pick, a guy with a record five seasons of 105 receptions, a guy with the most receiving yards in a six-season span in NFL history—Wes Welker had to free-agent his way onto the San Diego Chargers back in 2004. He got cut, landed with the Dolphins, and then in 2007 joined the New England Patriots, where he clicked with Tom Brady. Last season he joined Peyton Manning in Denver. His contract pays him $6 million a year, which is a comfortable payday for most people but actually well below the salaries for the NFL's top-paid wide receivers. As the Bleacher Report said of him earlier this year: "He is one of the greatest steals—at any position—in NFL history."

You have to wonder: How did the chowderheads at the Scouting Combine miss this guy? And how was he able to keep believing in himself? Despite the setbacks, he has persevered. He's what his coach, John Fox, has described as "very gritty."

Grit. Where did Welker find it? Can you and I have it too?

Let's define the term. Grit has nothing to do with Southern breakfast food; no, it is not a single serving of grits. It is, however, an authentically American concept—a slang word dating back to the early 1800s. If you have grit, you have the toughness and tenacity to see a goal through, with an added dash of resourcefulness and pluck to help overcome setbacks. You have stamina and persistence. You have bravery and backbone. You're someone who can git 'er done.

Grit is the theme of some recent stories, like the movie Gravity, and one of world's oldest stories, The Odyssey. The word is often preceded by the word "true," most notably in Charles Portis's 1968 novel True Grit, which focused on a 14-year-old girl with more of it than you or I or John Wayne will ever have. There's a climactic scene where she's fallen in a snake pit and has to prop herself up with a corpse bone to keep from plunging into a cave below; bats are brushing against her legs, and a rattler bites her hand ... I'll stop there.

Grit is not to be confused with talent. In fact, grit is what you're left with when you don't have talent. If your parents have no money and you have neither a standout skill nor a high IQ, well, there's always grit. It's the great equalizer.

Whatever your gift or aptitude or advantage, grit is the stuff that will help you make full use of it. Grit turns potential into accomplishment. When you look back, grit will be what led you to fulfill your "early promise." You achieve your goals—even if nobody else understands that you will be a Hall of Fame wide receiver someday.

Here's the key question: How many times have you regretted not sticking with something, not hanging in there until your efforts bore fruit? And its flip side: How many times have you regretted sticking with something for far too long, throwing time and money into a bad bet? Most men will say that their regrets are piled up on the side of quitting too soon. If you're still young and don't feel that yet, you will—unless you score well on the "grit test."

Something beyond I!—that's what inspired Angela Duckworth to begin testing for grit. A research psychologist, she got her Ph.D. at the University of Pennsylvania, where she studied under Martin E.P. Seligman, Ph.D., founder of the positive psychology movement. She joined a group of researchers who were exploring the character traits that abound in happy, productive, mentally healthy people. In a 2007 paper, she noted that intelligence was the best-documented predictor of achievement but then asked, "Why do some individuals accomplish more than others of equal intelligence?" Her conclusion: They possess more grit.

She came by that conclusion after surveying 1,218 freshman cadets who entered West Point in July 2004. Upon arrival, the cadets were given a brief questionnaire that asked for their reactions to statements like "I often set a goal but later choose to pursue a different one." You and I would call it a quiz; psychologists call it a "self-report measure." Duckworth and her colleagues had devised it to assess grit (or, more accurately, a person's perception of his or her own grit), which they defined as "perseverance and passion for long-term goals." Here, take the test yourself: See upenn.app.box.com/12itemgrit. I scored a middling 3.5 out of 5.

But back to West Point: About one in 20 cadets drop out during that first grueling summer, known as "Beast Barracks." Notably, the cadets who'd done well on Duckworth's Grit Scale were disproportionately not among them. Cadets with higher-than-average scores were over 60 percent more likely to complete the summer than cadets who didn't score as well. Two years later, Duckworth repeated the experiment on 1,308 members of the entering class of 2010; again, their grit scores helped predict their retention.

Duckworth also gave the Grit Scale test to 138 of her own undergrads. Those who scored high in grit did better academically than their peers, even though the researchers noted a connection between higher grit scores and lower SAT scores. "Among relatively intelligent individuals," Duckworth says, "those who are less bright than their peers compensate by working harder and with more determination."

So what's the bottom line? True grit is a real thing, not just a nostalgic old-school ideal, and people who have more of it go farther in this world. In an interview last year, Duckworth said, "Grit predicts success. Grit is not the only determinant of success; opportunity and talent matter too. But on average, grittier people turn out to be more successful than others, particularly in very challenging situations."

At the Duckworth Lab, scientists like Claire Robertson-Kraft, Ph.D.(c), are at a turning point. "Over the past decade, the research has shown that grit is very predictive of success in a variety of challenging fields," she says. "We have a solid base of research on the importance of grit and are now transitioning into research on how to build grit."

Although gritty research is a long way from reaching conclusions, the early wisdom is tilting toward these avenues of advice:

Set a goal.

Goal setting is second nature to gritty people; they make plans to accomplish what is most important to them. And they don't obsess over the difficulty; if anything, they underplay the work involved. And they begin their pursuit without fuss or delay. Early in the novel True Grit, Mattie Ross travels to Fort Smith to retrieve her father's body. She arranges for an undertaker to ship his coffin home for burial; he asks if she would like to kiss her father goodbye. "No," she replies gruffly, "put the lid on it." She has already set her sights on rough justice.

Another study of West Point cadets suggests you'll be more successful in reaching your goal—say, learning to play guitar—if your motives are primarily internal ("... because I like rock music") rather than as a means to an end ("... I want chicks to dig me").

Practice, practice, practice.

That's the message of ongoing research out of Duckworth's lab. "Students think talent is all that matters," one of her studies notes. "You rarely see other people practice, but nearly all famous people say that practice is what led to their success."

What K. Anders Ericsson, Ph.D., calls "deliberate practice" is not play or performance time but rather activities designed to improve specific aspects of performance. It means working on your weaknesses, working that sweet spot at the edge of your abilities. It involves frustration, concentration, repetition, and expert feedback. And it looks exactly like Welker's morning with his trainer.

Learn to be optimistic.

Gritty people are optimistic people. When an optimistic guy suffers a setback, he thinks of it as temporary and limited in scope. He thinks that with just a bit more effort, he can get over the hump. He may blame someone else for his misfortune. A pessimist, on the other hand, attributes bad events to big, overpowering causes that have now ruined everything forever and ever. He "catastrophizes." And whether it's his own fault or not, he tends to blame himself.

"It's easy to go to that place," Welker says. "You've got to change your thought process: You're tougher than that. Let's go. Come on. You've got to talk to yourself."

Expect difficulty.

We have a Pollyanna problem in American culture—we want to believe that positive thinking alone will carry us to our goal. Office cubicles and school hallways throughout the nation are emblazoned with sayings like "Dream it, believe it, achieve it!" The trouble is, this is exactly the wrong sort of motivation for children and adults alike. "Wishful thinking is, alas, exactly that," concludes a recent Duckworth study of 77 fifth-graders at an urban middle school. The "positive thinking" approach was tested against a more nuanced program in which children were prodded to consider obstacles that would stand in their way and then to make a plan to circumvent those obstacles. Those children went on to improve their grades, attendance, and conduct significantly more than the children who were encouraged to indulge in best outcome fantasies. In other studies, empty positivity has been shown to produce only greater distress, dissatisfaction, and dysfunction.

Don't become distracted. We live in what Internet entrepreneur Joe Kraus has called a "culture of distraction." Can you imagine Mattie Ross in today's world? Teenagers in the United States average 3,300 texts a month. That doesn't leave much focus for chasing outlaws.

In a Duckworth Lab study of more than 1,300 seniors in urban high schools across the country, students sat at a computer and were given the choice between solving incredibly boring math problems, which were displayed on the left side of the screen, or watching entertaining videos or playing a game, displayed on the right side. As it turned out, those students who were most dedicated to completing the boring tasks were 67 percent more likely to be enrolled in college a year later.

After the Bommartio workout, Welker invites me out for gluten-free pancakes. We grab a booth at the local pancake joint and start talking about his grit. Would he like to take Duckworth's Grit Scale? I slide it across the table; of course he's game. He gets a 4 out of 5, which makes me wonder more about the Grit Scale than about Welker. This guy should be off the charts. Maybe grit doesn't explain everything.

Make no mistake: Wes Welker has skills. He is not the football hero who got where he is by sheer willpower. His athletic abilities are awesome to behold; on the field he has brains, focus, and eye-blink reflexes. In the gym he moves through the stations with fiery dispatch. He has, and always had, deep reserves of physical energy. At age 3 he climbed a tree to get onto the roof of his house, and his parents were calling him a "hellion." A year later he began playing soccer. Welker didn't start with football until sixth grade; then he played both sports through high school.

"There was no walk-through for me," he says of his days on what others call the "practice" field. "I would tell the coaches: 'I only have one speed.'" What he did do in those years, a lot, was throw up. That's how hard he pushed himself on the field.

Welker has another quality that no one knows how to measure: intense competitiveness. "Even in practices, I didn't want anybody else to beat me on any sprint, ever," he recalls. Today, he says, that translates into a desire to prove himself in every play of every game. To dominate, to use one of his favorite words. To be "uncoverable." "That's my mindset," he says. "That's what I think about when I'm training and getting ready." And when all this training is a summer memory? When he's out on the field this fall? "My thought process is, I'm gonna kill this guy.'"

Spoken like a young firebrand. Except he's not. Welker may be only 33, but as he enters his 11th season, only one other Bronco on the roster—Peyton Manning—has more experience. Yet I cannot get Welker to talk about life after football. He is not about to get distracted by "new ideas and projects," as the Grit Scale quiz puts it. His career in football is a long-term goal that gets longer every year. If you want to measure him by his career receptions, he's right up there in the mid-800s with Larry Fitzgerald Jr.

In another two years, Welker has a chance to break 1,000 catches. He thinks it will happen, "as long as these ankles and knees stay together and I can keep on playing and enjoying it." That would be amazing. Hall of Fame amazing.

Meanwhile, all those young players on the Broncos roster are seeking his advice. And the one bit of wisdom he frequently imparts has to do with shaking off a bad play. "Young players, they're on such a big stage, and bad things happen," he says. "I tell them, 'The last play doesn't matter anymore. It's the next one. So don't let a bad play become another bad play.' You've just got to get rid of it and say 'It happens.' Move on to the next one. And do better next time."

I think they got an important lesson in grit. And so did we.