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Altitude’s effects on eracing: Pro riders and pro coaches weigh in

Science-based rules of thumb for power decreases, plus some nuances explained.

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High elevation can benefit bike racers in a few ways, from boosted hematocrit from sleeping at elevation to increased speed due to reduced air density. But high elevation also means a reduction in threshold power due to reduced oxygen.

In on-the-road bike racing, competitors are all on the same page by being in the same place, be it high or low. With the UCI esports world championship, or eracing in general, competitors are literally all over the globe. And with the Zwift algorithm providing each rider with the same effective air density model, there is only a penalty for being at altitude.

An accepted rule of thumb for professional coaches and physiologists is a 1 percent decrease in threshold power per thousand feet of elevation above sea level up to about 5,000ft. Above that, and the drop-off gets steeper.

“It’s nuts how big an impact it has,” said Greg Abbott, the co-founder of NeXT eSports, which was nine racers from six countries at Zwift worlds. “They could probably use geo-location to give a performance bump in the game and it would be totally fair.”

These two studies on the effect of altitude on an athlete’s sustainable power output are used as points of reference by professional coaches as well as staff at USA Cycling. The Bassett study on cycling world hour records was published in Medicine and Science in Sports and Exercise in 1999, and the Peronnet study was published in 1991 in the Journal of Applied Physiology, based on runners at altitude.

A quick scan of the Zwift worlds start list reveals that most riders are at or near sea level. Zwift declined to provide the exact locations of riders, citing privacy concerns. For NeXT, however, their highest rider is Niki Hug in Olten, Switzerland, which is 1,300 feet above sea level.

“The effects to your body when racing at altitude are higher heart rate and lower power output,” coach Jake Rytlewski wrote in a piece on the subject. “Since you are getting less oxygen to your muscles, your body increases its heart rate to help bring in more oxygen, which means you reach your max output quicker. This leads to a lower Functional Threshold Power (FTP) and also makes it harder to recover from maximal efforts.”

It isn’t just human engines that are affected by reduced oxygen; gas-powered vehicles are affected, too. A motorhome resource group says one popular diesel engine loses 3.5 percent of power for every 1,000ft of elevation over 3,000ft. “A good rule of thumb is that you will lose 10 percent of the rated output every time you gain 3,000ft in altitude,” Mark Quasius wrote on the RV Tech Library.

Ashton Lambie is the world champion and world record holder in the individual pursuit, which is a 4km time trial. Lambie has raced and trained around the world at various elevations, and he chose the high altitude Aguascalientes, Mexico velodrome for his world-record effort. He said the Bassett chart above tracks fairly well with his own power output changes.

Lambie is also part of the NeXT team, and last year while training at the Olympic Training Center in Colorado Springs, Colorado, he was able to pull off a unique trick, using the OTC’s altitude chamber to simulate sea-level oxygen levels for racing a Zwift team time trial.

“It was for our first playoff round around February of 2021, I think, and I did have it cleared with the OTC, since USAC sports physiologist Lindsay Golich is a huge fan of Zwift racing as a workout,” Lambie said. “I was training at the OTC for the Berlin world champs, if I remember correctly, but still wanted to get some solid aerobic work in.”

“I’d say it was a noticeable benefit, but it also had some weird parts, like an insanely high heart rate going from altitude to sea level right away. We’re talking like 200bpm for 10 minutes during the TTT,” Lambie said. “So if you live and train at altitude, it can definitely be an advantage [to go race low], but not possibly as huge as people think! You have to have the muscles ready to use the extra oxygen, especially for high-intensity, anaerobic efforts.”

Beyond the reduction in steady-state power, eracers at altitude might be at a disadvantage in other ways, says Jeff Winkler, a professional coach who specializes in Zwift racing and training.

“The [Bassett study] table captures the differences in output, but not how acute recovery rates might be affected,” Winkler said. “For example, if you did a two-minute maximal effort, would you recover faster at sea level or at 5,000ft? Or if you did an elevation-adjusted effort at both elevations by going 5 percent harder at sea level, would the recover rate be the same or still quicker at sea level? I would assume recovery would be faster at sea level in both cases. I am not aware of any studies, but I have not searched extensively.”

Acute recovery aside, Winkler said the shorter the effort, the less impact elevation makes on the power output.

“Given that the table reflects the impact on aerobic capacity, it follows that the impact should be scaled with the degree of aerobic contribution to the effort,” he said. “Sprint power is generally thought to not be impacted by elevation, so does it follow that anaerobic efforts are also less or not affected? I haven’t seen any specific studies on this nuance, either, but it does seem to be a reasonable application of the science.

Repeated alactic and anaerobic efforts are affected because the PCr regeneration system is aerobic and requires oxygen. I just read a study of soccer players that supports these ideas, where even though peak speed and acceleration frequency is largely unaffected, the average speed of these efforts is decreased due to the recovery effect, and total distance covered decreases in matches at elevation.”

Winkler pointed out that another virtual cycling software BigRingVR allows for altitude correction for riders in game.

“I can have it increase power in-game for the speed calculation but it still reports/records my actual power in the .fit file,” he said “They simply add an asterisk to the recorded time on the leaderboard.”

Zwift does not have any altitude correction currently.

“It would be fairly straightforward for Zwift to assign a rider a home elevation for a ride, like using GPS data from the Zwift Companion App, and simply apply a multiple to in-game power,” Winkler said. “While this would be an improvement, it probably wouldn’t completely account for the disadvantages for the Zwifters at elevation.”

An American in France

What’s it like to be an American cyclist living in France? Watch to get professional road cyclist Joe Dombrowski’s view.