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.)