Little-Used Fitness Measure Could Be Key to Exercise Results

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Jan. 6, 2022 – There’s been a push in recent years encouraging doctors to prescribe exercise as medicine, telling their patients how often, how long, and how hard to work out to improve health.

A new Brigham Young University study suggests doctors could take that initiative to the next level, prescribing exercise plans that result in a  specific health outcome; say, lowering your blood pressure or losing weight. 

“The findings of this study, and others, suggest that we should be able to more consistently and accurately prescribe exercise like medicine,” says senior study author Jayson Gifford, PhD, an exercise sciences professor at BYU. 

These exercise prescriptions would be tailored to patients based on a largely ignored fitness measure called critical power, or maximum steady state – the fastest you can go while maintaining a pace you can sustain for a long time. 

By crafting workouts around critical power instead of the more frequently used VO2 max (maximum effort), we could more accurately predict health outcomes, just as we can with medicine, the researchers say in the Journal of Applied Physiology.

“We’ve known for centuries that exercise is part of the way to develop a healthy and long life,” says Jordan Metzl, MD, a sports medicine doctor at the Hospital for Special Surgery in New York and author of The Exercise Cure. “But it’s only in the past 70 years that we’ve recognized the medicinal value of exercise.”

Metzl, who was not involved in the study, helped develop an annual seminar at Cornell Medical School teaching medical students ways to prescribe exercise that go beyond the “30 minutes per day” cookie-cutter advice. Still, doctors and other health care professionals often struggle to prescribe exercise to prevent or treat disease. And a recent study from Oxford found that when doctors do give weight loss advice, it’s often vague and hard for patients to use. 

“The drug of movement is one of the safest and most effective forms of preventive health,” says Metzl. “We need to get the medical community fully engaged in prescribing exercise for their patients.” 

This study suggests that a focus on critical power could be key in making that happen. 

What the Research Found

In the study, 22 adults completed 8 weeks of either moderate-intensity training or high-intensity interval training, or HIIT.  The intensity levels specified in both plans were based on VO2 max. So, the people in the study trained at given percentages of their VO2 max.

Both groups saw improvements in endurance, but results varied greatly from person to person. Those mixed results could be explained by individual differences in critical power. 

“Improvement was much more correlated with the percentage of critical powers the individuals worked at rather than the percentage of their VO2max, like exercise physiologists have thought for years,” says lead study author Jessica Collins, a researcher at Brigham Young University. 

Not only that, but several subjects who did not improve their VO2 max did see an increase in critical power and endurance. 

“People tend to only focus on VO2 max,” Gifford says. “Many might see the lack of increase in VO2 max for some people and conclude that the training was ineffective. I personally believe that a lot of potentially useful therapies have been ruled out because of an almost exclusive focus on VO2 max.”

Turns out, critical power varies a lot from person to person, even among those with similar VO2 maxes.

“Let’s say you and Jessica had the same VO2 max,” explains Gifford. “If we had you both going at 70% of [your VO2 max], it could be above your maximum steady state, which would make it really hard for you. And it could be below her maximum steady state, which would make it easy for her.” 

This means you are each stressing your body differently, and that stress is what triggers improvements in fitness and endurance. 

“Below critical power, the metabolic stressors are well-managed and maintained at elevated-but-steady levels,” Gifford says. “Above critical power, the metabolic stressors are produced so fast that they cannot be controlled, and consistently accrue until reaching very high levels that cause failure.” 

Knowing your critical power means you can predict how those stressors will build up, and you can tailor an exercise program that provides just the right stressor “dose” for you, Gifford says. 

Such programs could be used for rehab patients recovering from a heart attack or from lung disease, Gifford suggests. Or they could help older adults improve endurance and physical function, Collins notes. 

But first, researchers must confirm these results by programming workouts based on people’s critical power and seeing how much different measures improve.

How to Find Your Critical Power

Critical power is not new, but exercise physiologists and medical professionals have largely ignored it because it’s not easy to measure. 

“People generally train off VO2 max or maximum heart rate, which is even less precise,” Gifford says. 

Finding people’s critical power in the study involved multiple timed trials and calculating the relationship between speed/power and time, Gifford explains. 

But for a rough measure of your critical power, you could use an app that measures functional threshold power (FTP), something Gifford refers to as the “Walmart version” of critical power. “It’s not exactly the same, but it’s close,” he says. (The app Strava features FTP as well as a pretty sophisticated power analysis.) 

Or skip the tech and go by feel. If you’re below your critical power, “it’s going to be challenging, but you’ll feel under control,” Gifford says. Above your critical power, “your breathing and heart rate will continuously climb until you fail in about 2 to 15 minutes, depending on how far above you are.” 

Still, you don’t need to know your critical power to start exercising, Collins notes. 

“The beauty of exercise is that it is such a powerful drug that you can see benefits without fine-tuning the workout this way,” he says. “I would hate for this to become a barrier to exercising. The important thing is to do something.”


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