Nobody’s against fitness for children. But health secretary Robert F. Kennedy Jr.’s proposal to make physical activity a “vital sign” akin to blood pressure or growth is raising questions among physiology experts.

Physical fitness for all and children’s health are two tentpoles of Kennedy’s Make America Healthy Again mission, articulated in the MAHA Commission’s initial meeting in May, outlined in a draft Make Our Children Healthy Again report in August, and solidified in the full report Tuesday. The report promotes “physical activity as a vital sign,” a concept that usually defines a healthy level as 150 minutes of movement per week.  

The commission, though, urges states to establish specific metrics of fitness, for example, a “predicted VO₂ Max” as a baseline for evaluating Medicaid managed care organizations on how much they were improving health. Other factors would include sleep, nutrition, and potentially “select high-quality supplements.” 

And decades after President John F. Kennedy introduced it, the Presidential Fitness Test will return to gauge the speed and strength of America’s schoolchildren, the report says.

When the test was introduced in the 1960s, people worried that kids were spending too much time indoors, sitting around and watching television. To jump-start their fitness, physical activity at school was encouraged with routines from running to rope climbing, culminating in an annual test.

STAT has requested comment from the Department of Health and Human Services, which directed inquiries to the White House, asking about the basis for these ideas and what might come next. 

What is VO₂ max, anyway? 

Experts told STAT they were puzzled by the mention of VO₂ max as a metric. VO₂ max stands for the maximum volume of oxygen the body can take in and use in a single minute during intense physical activity. It’s determined by a standard exercise stress test that measures breathing with a mask to calculate oxygen consumption to define fitness.

As a measure of cardiorespiratory fitness, higher is better. It’s less clear whether it makes sense to apply an adult athlete’s numbers, aka VO₂ max, to children at play.

“It’s a really great test. It’s not really something you can do in someone under 9, 10 years old in a really good way,” Jared Hershenson, a pediatric cardiologist who directs cardiac exercise and rehabilitation at Children’s National Hospital, told STAT. “If you’re talking about trying to measure someone’s fitness who’s younger than that, there really is not any objective test that can do that, or quantitative test that can be done.” 

VO₂ max is difficult to measure, even for adults, I-Min Lee, a professor of epidemiology at the Harvard T.H. Chan School of Public Health who studies the role of physical activity in preventing chronic diseases, also told STAT. You need lab equipment and you need participants to exercise to the point of exhaustion. Picture people on an exercise bike or a treadmill, going full out while wearing a mask to track their oxygen levels. Many people won’t or can’t do that, she said.

There are predictive equations that do not rely on lab tests, but instead use age, sex, body mass index, or other numbers. With varying degrees of accuracy, they are more feasible in large numbers of people to advance health. 

Lee underscored that physical fitness and physical activity are related, but not identical constructs. Activity is something we do; fitness is a physiological construct. 

There’s this catch: Fitness can be improved by regular activity, but it’s also partially genetically determined, she said. “I could exercise to death, but I will never be as fit as Lance Armstrong, since I don’t have his genes.”

Outside the lab, VO₂ max is familiar to weekend warriors who upload their runs, swims, rides, and hikes to sports/social media sites like Strava to track training and add up kudos from their friends. That less-than-lab-quality number is derived from heart rate and other data collected by the watches on their wrists, made by Garmin or Apple or Coros, among others in the burgeoning market for wearables.

In June, Kennedy predicted wearables for all in the next four years, but later pulled back, saying in a statement to Axios that “they are not for everyone because of concerns like cost and personal privacy.”

Eric Topol, a cardiologist and geneticist at Scripps Research Institute, scoffed at measuring VO₂ max in kids when for adults, studies have shown there are more practical ways to measure fitness that don’t involve lab testing or expensive wearables.

“To do that in children? Are you kidding me?” he said in an interview. “That is just absurd. But that’s just the anti-science movement that keeps spewing out things that are not substantiated or possible or likely.”

Topol himself gets his VO₂ max data on his iPhone, but acknowledges its limits compared to a physiology lab. For children, it would make more sense to give them a wristband with a pedometer to measure activity. “Let’s go with something simple and cheap like that, that everybody could have,” he said.

We’re not there yet for children’s wearables, Hershenson said, while acknowledging its potential if government and private sector entities work together. There are no reliable fitness data from wearables in pediatric patients, for the children Hershenson’s center sees who have significant challenges or for healthier kids. Companies on their own might have variations in how they measure fitness. 

“In any of the technology, as far as I know, nothing’s been correlated with exercise tests. You’d have to have some sort of standard testing to compare it to,” he said. “I think it’s going to have to be some sort of surrogate,” maybe heart rate recovery, which tracks how fast the heart rate returns to its normal resting state after exercise.

Presidential Fitness Test, redux

Then there’s the metric President Trump wants to bring back to life: The Presidential Fitness Test. Started in the 1960s by President Kennedy and modified decades later by President Obama, it sent schoolchildren racing the mile and doing situps, pushups, pullups, and rope climbs.  The new report says HHS and the Department of Education will partner with the President’s Council on Sports, Fitness, and Nutrition to help states and schools reintroduce it.

Hershenson isn’t sure measuring how many pullups a child can do is going to help gauge fitness, strength, or flexibility as they pertain to future health. 

“I think it’s useful to have some sort of baseline. But you need to know what to do with that,” he said. “We’re not just gonna throw a ball around. Let’s find ways that we can improve this person’s strength and this person’s fitness by doing stuff that’s fun.”

That also means thinking about barriers to exercise. 

“To me the best approach is always treating each person individually to figure out how I can help them be the best version of themselves,” Hershenson said. “That’s extraordinarily difficult when you’re making massive public policy for however many millions of people, but I think it’s going to be different for everybody.”

“Is it a questionnaire? Is it measuring heart rate recovery? Is it measuring how many steps you take?” he asked. “It’s probably not going to be perfect.” 

STAT’s coverage of chronic health issues is supported by a grant from Bloomberg Philanthropies. Our financial supporters are not involved in any decisions about our journalism.