This story is part of Peak, The Athletic’s desk covering the mental side of sports. Sign up for Peak’s newsletter here.One day last fall, Erling Haaland was doing something he did not want to do.Sitting on an exercise bike inside Manchester City’s training facility, he was pumping his arms and legs inside a hypoxic chamber, a controlled environment that simulated high altitude and intense heat, reducing the amount of oxygen in the room.To meet his goal, Haaland had to raise his heart rate to 150 beats per minute, which required a momentary sprint.“Did I want to be doing this? No,” he said, looking into a camera lens. “Will I do it? Yeah … because why not? It’s good for my body. It’s good for my mind.”Haaland, a 25-year-old Norwegian striker, is one of the breakout stars of this World Cup, a 6-foot-5 Viking meme with angelic blond hair, otherworldly athleticism and a gift for viral content.He’s scored seven goals, leading Norway to the quarterfinals for the first time, and six days after leading the “Viking Row” chant after a victory against Brazil, he’ll be guiding the boat Saturday against England.And yet one of the most interesting things about him might be his mentality — a simple, almost unintentionally profound method for pushing his body to the limit.“If you tell yourself you’re tired, you’re going to be tired,” Haaland said. “If you tell yourself, ‘I’m not that tired, it’s OK,’ you’re not that tired. Simple as that.“So it’s also a really psychological thing because our bodies can handle so much more than we think. A lot is in the mind.”Haaland, of course, is not alone in this philosophy. Distance runners are known for using positive self-talk to survive marathons. LeBron James, the NBA superstar, declared that “tired is only in the mind.” And former NFL quarterback Tom Brady once put it more bluntly: “F— your tired body.”