As opposing hockey teams hop off the bench to chase the puck at the Colorado Avalanche’s home venue, it’s not just the skill of Avalanche players such as Nathan MacKinnon that leaves them gasping. Denver’s Ball Arena is located a whopping 5,280 feet above sea level—a geography that gives the powerhouse Avalanche a real physical advantage.

The Avalanche, which had the best record of any National Hockey League (NHL) team during the regular 2025–2026 season, are widely considered to be this year’s favorite to win the league’s ultimate prize—the Stanley Cup. Part of the reason may have to do with the physics behind how the human body converts oxygen into energy. Although air is made up of 20.9 percent oxygen at all elevations, the effective percentage of oxygen—that is, the amount of oxygen in each breath someone takes—changes with altitude. At sea level, the barometric pressure, or the pressure exerted by the weight of the atmosphere at a given point, compresses air, and thus oxygen molecules, closer together. As a result, it feels like we’re getting the full 20.9 percent of oxygen in the air we breathe at this elevation. The higher you go, the less pressure there is, which results in more space between the molecules and less oxygen in every breath you take. In Denver, the effective ratio of oxygen in the air drops down to around 17 percent.

While that might sound like a disadvantage, humans are adaptable, says Martin MacInnis, an associate professor of kinesiology at the University of Calgary. Tissues that don’t receive enough oxygen don’t perform nominally. They enter a state called hypoxia, which prompts the body to compensate by producing more hemoglobin, the protein in red blood cells that is responsible for carrying oxygen around the body’s tissues.

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“When your body is exposed to [high] altitude, it provides a signal to increase the amount of red blood cells you’re going to keep in circulation,” MacInnis says. That allows the body to optimize how much blood oxygen in the bloodstream it can utilize, a metric known as VO₂ max.

“Oxygen is our measure of energy transfer, so it’s really how much energy your body can utilize in a period of time,” MacInnis says. “The ability to use more means you can do more work, which, in hockey terms, would [mean] you could sustain a higher intensity than someone with a lower VO₂ max.”

The Avalanche play 41 regular NHL season home games and do most of their practices and gym workouts at high elevation—and the players reap the benefits, says Randy Wilber, a senior sports physiologist at the U.S. Olympic & Paralympic Training Center. The center’s location in Colorado Springs is not a coincidence: Olympians across a number of sports have sought to improve their own athletic performance there via high-altitude training.

“In my opinion, it allows for an additional ‘training stress,’ which, if balanced by adequate recovery, can effectively result in the Avalanche players feeling like they have ‘five gears’ instead of ‘four gears’ and having the confidence in knowing that they have that five-gear advantage over their sea-level opponents,” Wilber says.

The advantage is double-pronged: the Avalanche benefit both from the team’s players receiving better cardiorespiratory conditioning and from their opponents suddenly being thrust into a challenging oxygen environment. Data collected by NHL strength and conditioning departments indicate that visiting players experience a 5 to 10 percent decline in performance during the first 10 minutes of a game against the Avalanche in Denver, says Mario Leone, an adjunct professor of physiology at Quebec’s University of Sherbrooke.

“The data indicates that visiting players’ oxygen saturation (SpO₂) in Denver frequently drops below 90 percent during high-intensity efforts,” Leone says. “The direct consequence of this environmental hypoxia is a marked increase in oxygen deficit from the very onset of exertion. Due to the reduced partial pressure of oxygen, the visiting team’s aerobic metabolism struggles to activate, displaying a significantly greater delay than under normal conditions.”

There is a complicating factor. Although study after study has found that training at altitude leads to increased levels of hemoglobin and improved VO₂ max, the improvement is most useful for athletes in aerobic sports, such as distance running, where the focus is on sustaining performance over long periods of time. In that sort of exercise, energy is generated through how the body uses oxygen. By contrast, sports like sprinting and powerlifting are done in short spurts, and the body responds by burning through energy sources that are already stored in the muscles. Hockey is somewhere between the two: Activity bursts are generally short, but they can become stretched, such as when a team gets hemmed into its defensive zone. That means that while the benefits of a higher VO₂ max are still present in the players, they may not be as pronounced as they might be in some other sports.

“There are periods of long, intense skating where you’re not moving a lot, but you’re definitely working hard, and then probably where it has the biggest impact is on the recovery side,” MacInnis says. “So with a higher VO₂ max, you’re going to recover better between shifts, and then you’re more refreshed for your next shift.”

The ability to recover quicker could spell trouble for the Las Vegas Golden Knights, who begin their third-round series against the Avalanche on Wednesday. The Knights play in a city that’s roughly 2,000 feet above sea level, where the effective oxygen is around a hearty 19.4 percent.

Both teams have had their share of recent success: the Knights were champions in 2023, and the Avalanche were the year before. The Colorado players’ improved VO₂ max and potential higher hemoglobin count can’t explain that success: the team posted the worst record in the NHL just under 10 years ago. Ultimately, altitude training is beneficial, but having players such as James Norris Memorial Trophy–winning defenseman Cale Makar is perhaps even better.

“I don’t know that the Avalanche needs the advantage,” says MacInnis, adding that they swept their first-round opponents and lost just one game in their second-round matchup. “It’s not like every year Colorado is just running away with it. I think it’s just the players they have right now.”