Triathlete Jenny Fletcher Tests VO2 Max at the Mammoth Performance Lab

Oct 20, 2020

After 15 minutes of easy running, Jenny Fletcher does some gentle stretches then straps an oxygen mask to her face and gives a thumb’s up to confirm that she’s ready.

It’s not a normal workout for Fletcher, a 41-year-old professional triathlete and Los Angeles resident who is training in Mammoth Lakes for the Ironman Australia this June.  She’s at the Mammoth Hospital Performance Lab inside the SPORT Center performing a VO2 Max test with sports physiologist Tim Tollefson.

“Last year because I turned 40 I thought I would do a full Ironman—one and done, check it off the list,” Fletcher says. “It went really well, so then I was like, wait, I can do better.”

“You can train all you want, but a race like the Ironman is so mental,” she says.

“You go through all these phases while you are racing. The body is always capable, but you have to keep your head strong.”

Fletcher, who previously focused on the half Ironman distance, thought she was ready to retire, but in February after taking a long break she got it into her head that she wanted to qualify for the 2017 Ironman World Championships in Kona. But with the points scoring season well under way Fletcher was off to a late start.

“It’s almost an impossibility, but I figure why not try,” she said.

Having trained in Mammoth Lakes before, Fletcher understands the benefits of high altitude for endurance sports athletes, so she came back for an extended training stint. This time she enlisted the help of Mammoth Track Club head coach, Andrew Kastor with the goal of improving at the marathon. Running, she says, is her weakest of the three disciplines.

She’s also recruited the assistance of Tollefson, who is performing the VO2 max test and will analyze the results to help Fletcher and her coach maximize training while in Mammoth Lakes.

“Running by effort or feel can be challenging for someone when they are at high altitude because their heart rate is through the roof until they adapt,” Tollefson explained.  “We can use (the VO2 max) parameters to more precisely set up a program for the athlete, so they know what is appropriate for their daily runs or workouts.”

VO2 max is the maximum amount of oxygen, measured in milliliters, that the body can use in one minute per kilogram of bodyweight. More simply, it measures the rate the body utilizes oxygen during exercise. The higher the number, the better the oxygen uptake.  

At her peak, Mammoth Lakes’ local and Olympic bronze medalist Deena Kastor measured a VO2 max of 82.5, one of the highest numbers ever recorded for a female athlete. It’s no wonder she is the former World Marathon majors champion, an Olympic bronze medalist and the American record holder in the marathon and half marathon.

Norwegian cyclist Oskar Svendsen recorded the highest known VO2 max score of 97.5 in 2012. Svendsen, however, was the first to point out that the numbers don’t tell all.

“The figures are not important,” he said in a statement. “I’ve been beaten by many with lower oxygen uptake than me over the years.” 

While the VO2 max does not guarantee competitive success, knowing the number can be beneficial for an athlete. The importance of performing workouts at the right pace, or effort, is important to reap the benefits of altitude training.

When Fletcher receives her scores, she will send the information to Coach Kastor, who will then use a formula handed down to him by former Mammoth Track Club head coach Terrence Mahon. Paces will be formulated for Fletcher, which she can implement in her training. What is also beneficial is the comparison of VO2 max scores over time, Kastor says.

“You can graph and chart the numbers to see progress,” Kastor explained. “One can argue: well is the person racing faster? The VO2 max is just another reassurance or validation to know if the training is working. Is that number getting higher?”

For the test, Fletcher starts running on the treadmill at a one percent grade, which feels relatively easy to start.

“With this test we’ll be taking Jenny on a progressively steeper incline on the treadmill until failure,” Tollefson explains.

Each minute Tollefson increases the grade one percent and asks Fletcher to rate her perceived exertion. The test typically lasts 10-12 minutes.

“The further you get in the test, the better our values are going to be,” Tollefson says. “I will be encouraging you, cheering you, giving you positive feedback. A lot of times people wonder if they could have done more, so before you stop, ask yourself if you can go just a little bit longer”

After more than 10 minutes with the treadmill peaking at a 10 percent grade, Fletcher can’t take anymore. She grabs railing with both hands, and hops off.

“That’s so hard,” she says.

Tollefson pricks her finger and draws blood to measure blood lactate level as a variable to see if Fletcher achieved the threshold.

She grimaces. “We are so tough, until we are not tough,” she says.

The Mammoth Performance Lab is open to athletes of any fitness level and also offers sub-maximal exercise testing and wellness programming for the general community. Learn more at

Monica Prelle

Monica Prelle is an outdoors, wine, and travel writer who would rather be running, climbing, or mountain biking. See more of Monica's posts here, read more of her work at and connect on twitter @monicaprelle

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