The Slow and Low Exercise Elite Athletes Swear By

You should perform most of your cardiovascular exercise at a level that lets you carry on a slightly strained conversation. It broadly means exercising at a relatively low effort for a long time.

This type of training helps us produce energy more efficiently by stimulating our cells’ powerhouses, the mitochondria. Over time, this translates to better health and athletic performance.

Elite athletes spend much of their time training at this intensity. It is the secret to helping them go faster for longer, says Dr. Benjamin Levine, a sports cardiologist at UT Southwestern Medical Center in Dallas.

In fitness speak, that mellow intensity level is called Zone 2. Its benefits have recently been touted in books like the bestseller “Outlive: The Science & Art of Longevity," by Peter Attia and on the eponymousperformance-focused podcast hosted by Stanford University neuroscientist Andrew Huberman.

One of the easiest ways to determine the zone you are exercising in is to use what is known as a talk test:

Zone 1: A brisk walk where you can effortlessly carry a conversation.Zone 2: A relaxed jog or easy bike ride where you can still talk, but every few words are interrupted by an audible breath.Zone 3: A run where you can’t maintain more than three sentences.Zone 4: An effort where you can respond with only a yes or no answer.Zone 5: An all-out sprint where you can’t speak.

“The idea that the human body was designed to maintain a not-easy-but-not-hard intensity for a long duration goes back to the hunter-gatherer days," says Mathias Sorensen, an exercise physiologist at the University of California, San Francisco.

He believes the Zone 2 hype has recently caught on because people want an alternative to the idea that every workout needs to be no pain, no gain. And people who have tried it have found the workouts can provide significant health benefits, assuming they still meet a weekly average of about three to five hours, he says.

Zone training defined

When our body is under strain, it can obtain energy from a variety of sources, like fat or carbohydrates, and convert it to power, Sorensen says.

The intensity of that strain determines what the body uses for fuel. Zones are a loose attempt to define those physiological adaptations.

Pro cyclists generally use a six- and sometimes seven-zone model determined by power output. The American College of Sports Medicine adopted a five-zone model for the general population based on a percentage range of maximum heart rate.

Many cardio machines and fitness trackers, like the Apple Watch and Fitbit, monitor heart rate with good enough accuracy for the average exerciser.

If you don’t have access to a gadget, you can estimate the zone you are training in with the talk test or rate of perceived exertion scale. This technique asks exercisers to rate how hard they are working on a scale from 0 to 10, with 10 equaling max effort. Sorensen notes that the drawback of this method is that it is subjective.

The magic of Zone 2

Because Zone 2 exercise burns fat, cardio machines, such as ellipticals, often have a “fat-burning" mode. But don’t view this type of training as the key to weight loss, Levine says.

Instead, the real benefit of Zone 2 training is that it optimizes the performance of our mitochondria, the battery packs in our cells that convert fats and carbohydrates into energy.

Dr. Iñigo San Millán, a former competitive cyclist and assistant professor at the University of Colorado School of Medicine, started exploring the idea that mitochondrial function could be key to peak performance almost three decades ago. He has applied his findings to the training routines of professional athletes, including two-time Tour de France winner Tadej Pogačar.

“Mitochondria are stimulated by exercise," he says. “I wanted to see if that stimulation varied depending on intensity."

By measuring the levels of lactate in athletes’ blood, he discovered that Zone 2 exercise improved mitochondrial function far more than other intensities. Athletes with optimal mitochondrial health burn fuels and recycle lactate, a byproduct produced in the body during vigorous exercise, more efficiently.

“If your body can’t clear lactate, it binds up the muscles and slows you down," he says. He concluded that mitochondrial function is a better indicator of endurance performance than even VO2 max, a measure of your maximal aerobic capacity.

San Millán has turned his research to nonathletes.

As we age, we lose mitochondrial function. And research has linked mitochondrial dysfunction to Type 2 diabetes, cardiovascular disease and cancer. He believes Zone 2 training could have profound effects not just on fitness, but also longevity and health.

Not a cure-all

Zone 2 is just one part of the larger fitness puzzle, Levine says. It builds our endurance base, but our workouts should be diverse and include strength training and high-intensity training (Zone 4). San Millán recommends 80% of training fall into Zone 2 and 20% in Zone 4.

A mellow effort might sound easy, but the trade-off is that it requires time: around one hour, three to four times a week. People who find cardio a snooze or athletes used to always pushing the pace sometimes struggle with long, slow training days, Levine says.

If you plan to use your Zone 2 exercise to catch up with a friend, find someone with a similar fitness level, Sorensen says. An elite cyclist’s Zone 2 is likely the average person’s Zone 4.

Over time, your body will adapt. With regular exercise, the 3-mile hike that once left you breathless will be a breeze to complete while holding a conversation.

Write to Jen Murphy at workout@wsj.com