“Critical Power” Exercise Prescriptions – Bigger Improvements and Longer Lasting Benefits

Using what’s called “critical power,” the team hopes to develop exercise prescriptions.

Anyone who trains frequently knows the phenomenon that two people with comparable fitness levels can do the same exercise and have very different results. It’s extremely frustrating for people who can’t make progress despite their best efforts.

Exercise science researchers at Brigham Young University have tried to address the issue because they are familiar with this feeling. Good news: they think they have understood the code.

The team’s recently published study reveals a more practical method for determining the level of intensity each individual should exercise for best results. A study published in the Journal of Applied Physiology describes a new system for creating “prescribed” workouts that deliver results regardless of an individual’s current state of health.

“One day, we may be able to prescribe exercise-like drugs,” said Jayson Gifford, professor of exercise science at BYU and lead author of the study. “To prescribe a drug, you need to have predictable results for each dose of drug. We found that the exact same thing applies to exercise.

According to the study, exercise that is personally prescribed based on what is called “critical power” produces greater improvements in endurance and longer lasting benefits for the individual. The authors define critical power as the highest level of our comfort zone. “That’s the level at which we can perform for a long time before things start to get uncomfortable,” said study lead author Jessica Collins, a former BYU graduate student.

It works something like this: Suppose two friends have the same maximum heart rate. Prior knowledge of exercise would imply that if they ran together at the same pace, they should have very similar experiences. However, it turns out that when these two friends run at 10 km/h, the exercise is simple for one but difficult for the other. These disparities in experiences at the same speed and percentage of maximum heart rate are due to the fact that 6 mph is less than one friend’s critical power but greater than the other friend’s critical power.

When exercise is below a person’s critical power, their body can compensate for the energy challenge and achieve a comfortable and controlled homeostasis. However, when exercise is above its critical power, the body cannot fully compensate for the energy demand, resulting in exhaustion.

Traditionally, individualized exercise has been recommended based on a fixed percentage of one’s maximum rate of oxygen consumption (VO2 Max) or maximum heart rate. Collins and Gifford said that using “critical power” is a better way to prescribe exercise because it not only accurately serves athletes and those who are in top shape, but it also serves those who are more elderly or have a more sedentary lifestyle.

“This type of research helps all types of people, regardless of their current activity level,” Collins said.

For the study, Collins, Gifford and their coauthors recruited 22 participants between the ages of 18 and 35 who were in good health but had low physical fitness. Participants underwent eight weeks of supervised physical training during which they were randomly assigned to high-intensity cycling training or moderate-intensity continuous cycling training. Exercises were traditionally prescribed based on an individual’s maximum heart rate or VO2 Max.

Researchers have found that prescribing exercises based on VO2 Max as a benchmark leads to alarming variability in results. There were participants who benefited a lot from the training period and others who did not, even though the training was personalized to them. They compared this to each individual’s critical power and found that it explained 60% of the variability in their results. If the exercises had been prescribed using critical power as a benchmark against their heart rate, the results would have varied less, meaning the workouts would have been more effective and beneficial for each participant.

“One of the main reasons people don’t exercise as much as they should is that they’ve tried something in the past and it didn’t work the way they hoped,” he said. Collins said. “The advantage of basing exercises on critical power is that we can almost always guarantee the outcome, which allows us to help people achieve their fitness goals.”

To calculate a person’s critical power, the researchers asked participants to cover several exercise distances (i.e. running, cycling) as fast as they could. They then took the average speed and fed that data into a proprietary formula that determines the relationship between exercise distance and exercise time to produce a critical power number. They found that a person’s critical power can increase dramatically with physical training, making things that were previously difficult less difficult, less uncomfortable, and less tiring.

“Exercise is so good for you that you’ll see some kind of benefit no matter what you’re doing,” Gifford said. “This research is simply informing people that they can further optimize their exercise, so that they get the most out of it. We are excited to know when it will become more accessible for people to know their personal critical power in the near future.

Reference: “Critical power and primordial work explain the variability of endurance training adaptations not captured by V̇oh_2max” by Jessica Collins, Olivia Leach, Abigail Dorff, Jessica Linde, Jason Kofoed, Megan Sherman, Meagan Proffit and Jayson R. Gifford, October 6, 2022, Journal of Applied Physiology.
DOI: 10.1152/japplphysiol.00344.2022