Headlines recently touted that 3 minutes of intense exercise could be “better” physiologically than 90 minutes of moderate activity. They’re based on a recent study published in Cell Reports Medicine — and you can see why on its face the idea would be attractive to patients. Or anybody.

While it’s more complicated than many consumer publications make it sound, it does appear exercising at a certain intensity brings unique benefits — so much so that study co-author Kate Baird, CSCS, an exercise physiologist at Hospital for Special Surgery in New York City, said doctors should be talking to people about exercise intensity, perhaps even encouraging them to experiment with higher intensities.

That said, many physicians have a hard time getting their patients to do any exercise, let alone intense exercise, and experts agree that it’s better to get enough moderate exercise than to get hung up on intense exercise and end up doing…nothing. “For the vast majority of the population, it’s just: Exercise as much as you can and sometimes push a little bit,” said Brendon Gurd, PhD, professor in the School of Kinesiology and Health Studies at Queen’s University in Kingston, Ontario, Canada.

Still, what happens inside the body during intense exercise is undeniably intriguing from a standpoint of health benefits and vitality. Here’s what to know about what’s happening physiologically during moderate vs intense exercise and how to guide your patients (and yourself!) accordingly.

Intensity, Defined for Doctors

Defining intensity gets to the heart of why exercise science research is so challenging, Baird said. Intensity has historically been quantified by metabolic equivalents (METs) that associate an oxygen consumption cost with various activities, such as walking 4 mph. But it turns out this is overly generalized (even though it’s what the American Heart Association guidelines are based on).

“Vigorous intensity in the guidelines would be approximately six times our resting metabolic rate,” Gurd said, but a fit athlete’s 6 MET thresholds will be very different from those of somebody with low fitness.

“So instead of using absolute terms, we try to use relative intensity,” Baird said, which assumes that a given workload can be challenging to different people in different ways depending on their conditioning status. A few common ways to look at relative intensity include:

• Percentage of heart rate max
• Percentage of VO₂ max
• Rate of perceived exertion (RPE)

But perhaps the clearest way to understand if someone is getting the physiologic benefits of “intense” exercise is whether or not they’re exercising above or below their lactate threshold. There are two thresholds: L1, or the aerobic threshold, and L2, or the anaerobic threshold.

Some researchers believe the benefits come after L1, whereas others, including Baird, believe it’s at L2, which may correspond to an RPE of 17-18 out of 20 or 77%-95% of someone’s heart rate max, per a 2023 opinion piece in Sports Medicine. “Beyond that second lactate threshold or second ventilatory threshold, that’s really where the high-intensity domain lies in my opinion,” Baird said.

In the Cell Reports Medicine study, one group did six 30-second bursts of bike sprints, which represented going above the second lactate threshold and the other group did 90 minutes of continuous exercise at an intensity lower than that threshold. The group that exercised for less time above their second lactate threshold saw greater positive effects in the plasma proteome and metabolome, as well as greater “cross talk” between the muscle, liver, and adipose tissue. All effects were associated with health improvements.

Gurd draws the line after the first lactate threshold, and he co-authored a 2025 paper published in Sports Medicine concluding that prioritizing higher exercise intensities above this threshold is critical to maximize cardiometabolic health benefits compared to zone 2 exercise (anything done below the first lactate threshold). However, he said, there may be even more benefit at even higher intensities, such as above L2.

For patients, Brad Schoenfeld, PhD, CSCS, a professor in exercise science at Lehman College in Bronx, New York, suggests an older-school measurement for lactate threshold: the talk test. “If you’re able to talk without gasping for air, if you’re able to carry on a conversation,” he said. “Once you’re not able to carry on a conversation, at that point, you’d be in a higher-intensity format or above your lactate threshold.”

What Happens to the Body With Intense Exercise?

It all comes down to how the body solves for the demand of exercise, Baird said. “Exercise in itself is an ATP [adenosine triphosphate] problem,” she said, referring to ATP. “When your brain tells your body to start exercising, the faster and harder it is, the faster we need to replete ATP in the muscle.” The body solves this “problem” through various conditioning adaptations.

In other words, when you do something like an all-out sprint, your body perceives that as an emergency and that sets off a much faster molecular response to try to hedge for the fact that there may not be enough ATP to fulfill the demands of the activity. “You’re going to accumulate a high level of blood flow change and blood vessel shear,” Baird said. When you stop, your body begins preparing for it to happen again.

Gurd said that when ATP is broken down, adenosine diphosphate is produced within skeletal muscle, which is basically an “error signal” that activates signaling molecules to induce mitochondrial biogenesis. “That leads to more mitochondria within muscle and then that can impact metabolic health,” he said. “The higher the intensity of exercise, the greater the error signal.”

Additionally, the effects on the heart are different during higher-intensity efforts vs lower ones. “My heart rate is going to blast out of this world during a 30-second sprint,” Baird said. “It is not during a 90-minute steady state [workout].” That means during the latter, the beats are not as big and strong, there’s not as much ventricular refilling, and the heart won’t stretch as much. “When you get to a very fast, very big heartbeat, you are teaching your heart to stretch and contract rapidly and to get better at eccentric stretch and strength in the heart,” she said.

The outcome: “Cardiac output can grow from that central adaptation and so your VO₂ max goes up,” Baird said. “That’s one of the more hallmark adaptations from high-intensity exercise.”

And that, really, is the endgame for higher-intensity exercise: “VO₂ max is the strongest predictor of mortality and morbidity risk, so if you have a high VO₂ max, you’re at low risk of developing cardiovascular and metabolic disease,” Gurd said.

All of this said, there are absolutely benefits to less intense exercise, too. “It teaches your body to be more a more mobile, capable human,” Baird said.

“For VO₂ max, high-intensity exercise is the better option for sure, but for life, for glycemic control, for being mobile, for walking around, for going on runs, for mental health, for muscle fiber conditioning, musculoskeletal health, moderate-intensity exercise is the better bet,” said Schoenfeld. And while higher-intensity exercise can be more efficient, moderate exercise is easier and often more enjoyable for the average person.

So what begins for some as an either-or debate finishes as more of a both-and situation, as least as it applies to patients with low- or moderate-level fitness.

How to Advise Your Patients on Exercise Intensity

The experts agree that there are unique benefits to intense exercise, but whether and how much someone should do isn’t one-size-fits-all. “Part of the trouble with high-intensity exercise is that you can really burn the stick at two ends and get yourself in trouble with things like overtraining and decreased recovery,” Baird said. “There are always risks associated with any amount of exercise, and certainly there are with high-intensity exercise.”

But beyond any risks, intensity may not even be necessary for less fit populations: Zone 2 exercise can have similar adaptations to those seen in more fit people with higher-intensity exercise, Gurd pointed out. “I’m definitely not anti-zone 2; I am pro exercise of any intensity, but if you’re trying to maximize that error signal and maximize adaptation, you probably want to be targeting a higher intensity than zone 2.”

So if your patients are already doing roughly 150 minutes of moderate-intensity exercise (per federal guidelines), and are otherwise healthy, it could make sense to suggest adding in some degree of intensity to the mix. “If you add in some of that vigorous, you start to move up the curve and start to get more adaptation and better benefits,” Gurd said.

Baird puts it this way: “The best recipe is to get a variety of different types of intensity in your exercise routine.” That could be a few longer sessions below the lactate threshold and then one or two sessions where you get your heart rate close to its max. “We know from the study that it is such a potent stimulus and there’s a lot of benefits,” Baird said. “It doesn’t mean you do it every day.”

The key, per Schoenfeld: Urge them to progress gradually and slowly to avoid injury and overtraining. It also may make sense to refer them to a fitness professional: “Encouraging people to seek some guidance in how to build up toward being capable of doing high-intensity exercise, I think is important,” said Baird.

While any type of aerobic activity can “count,” Baird said that cycling is accessible, particularly stationary bikes. “If you have balance issues, musculoskeletal issues, then you can get on a bike and largely be stable and be able to work hard,” he said. But at the end of the day, Baird said, “Do the modality that you like so that you’ll do it. That’s the thing about exercise — none of this matters if you can’t find a way to make it a part of your daily hygiene.”

The experts cited in this article reported no relevant disclosures. Disclosure information for study authors is available in the original study publication.