Spending Hours on Zone 2 Cardio? Build VO2 Max More Efficiently With This 8-Minute Plan
Zone 2 cardio has been sold as the most important cardio protocol for building your aerobic base. Most think that doing enough of it will help you burn more fat and protect your metabolic health for the long haul.
But Kristi Storoschuk, a Ph.D. candidate in the Muscle Physiology Lab at Queen’s University, has spent much of her doctorate1 combing through the research on this topic, and she's not convinced. In fact, she questions what the term "aerobic base" even means.
On the latest episode of the mindbodygreen podcast, I sat down with Storoschuk and she explained zone 2 isn't the holy grail we've made it out to be and why, if your cardio time is limited, you're likely spending in the wrong place.
What zone 2 actually means
Let's start with a definition. "Zone 2 is where we have a very low steady state lactate level in our blood, where we're basically clearing it at the same rate that we're producing it," Storoschuk explained.
Serious zone 2 devotees use a blood lactate meter to confirm their blood lactate stays below 2, the standard cutoff."Everyone else relies on proxies, like a percentage of heart rate or the talk test. In the lab, Storoschuk said researchers ask people to count backward from 30, then ask whether they were able to do it comfortably. "If we can keep a comfortable conversation while exercising, then we're probably in zone 2," she said. "But as soon as we can't get out sentences without huffing and puffing, then we're above zone 2."
However, Storoschuk isn't sold on these proxies. Most people do Zone 2 without lab equipment, meaning they are basically throwing darts blindly at a target. A recent study comparing common descriptions of zone 2 found definitions ranging from 30 to 80% of VO2 max, which are hugely different intensities.
Where the zone 2 evidence falls short
The pitch for zone 2 rests on mitochondria and fat burning. Train in zone 2 and you improve mitochondrial content and function along with your ability to use fat for fuel, which supports insulin sensitivity and lowers your risk of metabolic disease. When Storoschuk and her team went looking, they found that zone 2 can trigger these adaptations, but not all the time.
"When we look at higher intensities, we see that higher intensities always do these things," she said. This means the literature does not support zone 2 being the optimal intensity for achieving these adaptations.
Storoschuk considers this simplified messaging irresponsible. "Someone could be dedicating three hours a week to zone 2 because they're told that it's doing some magical thing, when if they just did 40 minutes in zone 3 or 4, or did a 20-minute HIIT session in zones 5, they would be getting better results and better health benefits." Choosing efficiency over volume is often the smarter trade, especially when your calendar is full.
Where the zone 2 obsession actually came from
Storoschuk traced the hype back to observational data on elite athletes who train 20 to 40 hours a week. They spend roughly 80% of their training time in zone 2, with the other 20% at high intensity. But they are training 20 to 40 hours a week, meaning 20% of 20 hours is a full four hours at high intensity. That's more than most recreational exercisers do in total.
"We're conflating the training habits of an elite athlete and their high mitochondrial content, their high fat oxidative capacity, their great insulin sensitivity. We're attributing it to that 80% they're doing in zone two, but it could very well be that 20% that they're spending at high intensities," Storoschuk explains.
Plus, the fitter you are, the more compressed your zones become. An elite athlete's zone 2 sits much closer to their VO2 max, because there's a ceiling on how high a heart rate can go. This means their zone two is so much more of an effort than a recreational individual.
Why your aerobic base gets built above zone 2
One of the stickiest zone 2 claims is that you need it to build an aerobic base before adding higher intensity on top. Storoschuk disagrees on two counts. "One, there's no definition of aerobic base, and so that's kind of hand-wavy in itself." Plus, the evidence actually supports a smarter cardio structure that includes work above your lactate threshold.
"The evidence would support the use of higher intensity, beyond your lactate threshold, to pull your lactate threshold up, instead of trying to exercise below it to push that ceiling up," she said. The adaptations come from pulling that ceiling up by doing the intense cardio, then becoming more comfortable with it over time.
She pointed to a study comparing zone 2, zone 3, zone 4, HIIT, and sprint interval training groups against a control. "Every group that was above zone 2 improved VO2 max and lactate threshold, but the zone 2 group didn't improve compared to the control," she said. And that includes lactate threshold, which is considered the ceiling of the very base zone 2 is supposed to build.
Prioritizing intensity when your cardio time is limited
"As your time commitment goes down, the importance of intensity goes up," Storoschuk said. And the good news is, she isn't hunting for one optimal protocol. She believes there are lots of ways to improve your cardio, and she shopes the variety of options feels liberating for people.
However, if like a prescription, here's are Stroschuk's suggestions how to spend the time you have:
- If you have 30 minutes and love a continuous run: Go for the run. "But don't try and purposely stay at a low heart rate when you feel good enough to go harder," she said.
- If you have 20 minutes: One minute all out, one minute off. "The goal is making each one-minute bout genuinely vigorous, even if you dwindle off at the end.
- If you have 15 minutes a week: Three Tabata sessions of 20 seconds all out and 10 seconds of rest, eight rounds, on an assault bike, rower, or ski erg. Each session is four minutes. "You shouldn't be able to go much longer than 20 seconds at the speed that you're going," she said.
Any of these slot easily into a time-efficient interval plan for the week.
Frequency matters as well. Storoschuk pointed to a meta-analysis finding that six sessions a week produced better VO2 max improvements than four, which beat two, independent of intensity and how long the intervention ran.
Kristi's own routine is deliberately unstructured
When I asked how Storoschuk applies any of this to her own training, her answer was surprisingly flexible. She exercises most days without a rigid program, checking crossfit.com for the workout of the day and subbing in something else if it doesn't appeal to her.
She was 30 weeks pregnant at the time of our conversation, so her routine looked different than it used to, but her philosophy hadn't changed. "Maybe it'll be an hour run, maybe it'll be a 15-minute run, maybe it'll be a CrossFit workout, maybe I will lift," she said. "I don't think what I do is optimal, but I've been doing it for probably over a decade, which means it's sustainable and probably the best thing for me."
The takeaway
Storoschuk's goal is not to convince people that zone 2 is bad. She just wants people to know it is not the end-all-be-all when it comes to cardio. "I think zone two is better than nothing," she says, "but I don't think you need zone two,"
What she objects to is swapping out higher intensities for zone 2 because someone online said it's essential. If your cardio time is limited, spend it above zone 2, in a dose you can recover from and repeat week after week.
And remember, the training habits of athletes with decades of conditioning don't automatically translate to someone squeezing in a workout between meetings.