Why Easy “Zone 2” Workouts Became the Biggest Thing in Fitness 

If you're a casual-to-obsessed observer of fitness trends, you may have come across “Zone 2” training—heard about it on Huberman Lab, or The Drive with Peter Attia, perhaps. It has become a buzzword with both professional endurance coaches and optimization-focused tech dudes. (The CEO of surveillance software giant Palantir, Alex Karp, told Axios he adhered to a Norwegian-influenced regimen centered around Zone 2, to take just one example.)

The jargon-y term might make this type of training sound extremely complicated, and its loudest proponents tend to be the type of guy who enjoys tracking (and Tweeting about) the time spent in this sometimes-elusive state, with the most advanced watches, chest straps, and tracker-embedded underwear. But it’s actually just exercising at a relatively low intensity for a long period of time. Experts who spoke to GQ broke down the mechanisms behind this type of training and why pretty much any guy stands to benefit from adding it to their fitness regimen. 

Sports cardiologist Dr. Benjamin Levine at UT Southwestern Medical Center, who has worked with the US Olympic Committee and USA Track and Field, explained that exercise is often broken up into five approximate “zones” of intensity marked by a percentage range of maximum heart rate. While trainers often have their own system, Zone 5 is typically 90-100 percent of your max heart rate (achieved with intense exercise like all-out sprinting), while Zone 1 is 50-60 percent of your max (a brisk walk). Zone 2 is just a bit harder than that: an easy jog or a relaxed bike ride, though this varies athlete-by-athlete. (Eliud Kipchoge's Zone 2 pace would be off the charts for most people.) 

At lower intensities, from rest to Zone 2, the body relies mostly on fat for energy. Fat is an efficient fuel source, but requires plenty of oxygen to be metabolized. As intensity increases, carbohydrates, in the form of stored glycogen (basically a .zip file of glucose), become the body’s primary source of fuel. Zone 2 is the space where you’re doing the maximum level of effort while not going beyond the “crossover point” from aerobic (“with oxygen") to anaerobic (“without oxygen”) where carbohydrates begin to become the preferred fuel source instead of fat, and lactate (a byproduct of glucose metabolism) begins to accumulate. 

The zones aren’t clearly delineated, as Jeff Christle, a clinical exercise physiologist at Stanford explained—they are percentage ranges of maximum heart rate, with “soft borders” between zones. There isn’t some kind of magic that happens if you’re a few beats above the general 70 percent of maximum of heart rate range. 

But long efforts in Zone 2 do seem to have some unique characteristics that help improve overall fitness. Almost three decades ago, exercise physiologist and professor at the University of Colorado School of Medicine Iñigo San Millán was interested in mitochondrial function as “the epicenter of performance.” (You may remember these cellular structures from high school as the “powerhouse” of the cell.) A former competitive cyclist, he wanted to find an exercise intensity that improved mitochondrial function. By measuring athlete’s lactate via blood samples, he found training in Zone 2 to be the best way to stimulate mitochondrial function and cultivate a cardio base that both professional cyclists and weekend 5k athletes can benefit from. (As the head of performance for UAE Team Emirates cycling, and personal coach of 2020-2021 Tour De France winner, Tadej Pogačar, he mostly works with the former.) He said that Zone 2 training, for athletes of all types, is a way to ensure athletes can go faster, for longer. 

The broader adoption of Zone 2 training is welcomed by experts. Christle pointed out that it is generally safer and more sustainable than the high intensity interval training fad that preceded it. The best exercise is the type that you can keep doing, after all.

For those interested in Zone 2 endurance work, Christle said, it requires a relatively high volume: 1 hour of exercise, 3-4 times a week, for 12 weeks—the length of time it takes to change exercise behavior. It also takes patience—many athletes find low-intensity work boring, and it can be a blow to the ego to spend your entire Saturday jog getting passed. 

And while there's an enormous range of heart-rate tracking fitness tech out there, the data-obsessed athlete may struggle to dial their personal Zone 2 into their fitness tracker. San Millán, without naming any specific brands, said that some technology may be able to capture data and assemble sophisticated graphs, but that does not mean they’re representing reality. 

Dr. Levine went even further. “You should throw everything out the window if you've not had a maximal exercise test to measure your heart rate and ventilation for yourself,” he said. These tests typically involve getting on a treadmill or bike while breathing into a tube, while your oxygen uptake, carbon dioxide output, heart rate, and other stats are monitored. If you haven’t taken this test, he explained, your heart rate zones are generated algorithmically. Given that an individual’s actual maximum heart rate can vary by up to 20 beats per minute from the most common calculation methods, that makes such measurements “complete, utter garbage.”

Dr. San Millán is similarly doubtful about fitness trackers and their Zone 2 calculations. “After 30 years, and doing research, I still would not be able to tell you where your Zone 2 is without testing you.”

The gadget-focused athlete can sometimes find themselves contorting their behaviors to satisfy their trackers: taking another spin around the block to finish their Strava run with a nice round number, or adding a rower session to a weightlifting workout to max out the Whoop band “Strain” metric. Forum posts from Garmin, Polar, Apple Watch show users having difficulty finding and staying in Zone 2. And while all-out effort is rewarded (like with Strava’s "suffer score" or WHOOP’s “strain score”) extended low-intensity exercise is not always highly regarded by the machines. Garmin users, for example, have found their low-and-slow training marked as “unproductive.” 

But even if you aren’t drawing blood during workouts to measure lactate, there is a non-invasive way to approximate if you’re training in Zone 2: the “talk test.”

“You should be able to talk, but it should be a little bit strained,” said Christle. “But you should be able to talk in full sentences at that rate.” This zone, which is around 70 percent of maximum heart rate, should be attainable for almost all people, he said. (Of course, this has been conventional wisdom for endurance athletes for decades—in many ways the Zone 2 fad is simply putting a theoretical framework on what many athletes already know.) 

For all the attention that Zone 2 training has received, it’s important to remember it’s just one slice of a larger fitness pie. More specifically, it should be around 80 percent of that pie, with the remaining 20 percent done at high intensity. Elite athletes do lots of low-intensity training, San Millán said, but athletes like Pogačar are also subjecting themselves to training that would be “unbearable for most humans.”

For higher-intensity work, Levine likes the old Norwegian ski team workout: a 4x4 aerobic interval workout consisting of 4 minutes at 95 percent of max heart rate, followed by three minutes of rest, repeated four times. “That's the best workout to build aerobic power in a short period of time.”

Because fat burning occurs in Zone 2, it is sometimes thought to be a magical tool for weight loss. (Elliptical machines and treadmills that indicate when you’re in “fat burning” mode likely haven’t helped.) But people looking at Zone 2 training for weight loss are “barking up the wrong tree,” Christle said. “It's not a weight loss strategy. And just exercise never has been, never will be. It's pretty simple math.”

Instead of thinking about Zone 2 as a weight loss tool, it’s more accurate to consider it a way you make your internal engine more efficient. Instead of picturing a six pack, picture a beautiful mitochondria within the cell, operating at peak efficiency. 

All three experts recommend simply listening to your body. There is no “magic” about a particular number, he said, and that should be encouraging. Spend most of the time slow-cooking your exercise, with shorter bouts of pan-searing high-intensity, and you can rest knowing you’re training like a high-performing endurance athlete, no matter what your fancy watch says.