A MINUTE AGO, high-intensity interval training (HIIT) was king. Now that gut-it-out-and-get-the-glory work might need to share the crown with slow and steady efforts, thanks to what they do for your body, your longevity, and maybe even your mind.
These are the famous zone 2 workouts – keeping your heart rate between about 70 and 80 per cent of its maximum – that runners, smart gym-goers, and everyone with a podcast is talking about these days. Think of zone 2 as the range between easy and moderate cardio, where you can carry on a conversation but someone on the other end of a phone would know you’re not sitting down. (Find out more on how to know you’re in the zone by checking out the chart below.)
What happens in zone 2 powers up your endurance, your lifting routine, and your general performance as a human being, proponents say. It may even help fend off cancer and diabetes. Here’s what to know about the zone.
How an easy effort brings you gains
Endurance athletes, such as marathoners, cyclists, and Ironmen, have long understood that zone 2 training is a key to performing well on race day. Look at Eliud Kipchoge, the world’s fastest marathoner, who spends four days a week running in zones so low that any decent runner could keep up with him. Lower-zone training yields high results in endurance sports, says Mike T. Nelson, Ph.D., C.S.C.S., owner of Extreme Human Performance, possibly “because the athletes aren’t out there just trying to fry themselves crazy every single day.” A long Z2 effort today leaves room for more time on your feet or in the saddle the next day. But another huge perk of Z2 training, no matter your sport, is the adaptations your body is making deep within your cells.
It all goes back to your mitochondria, the parts of your cells that generate ATP – that’s the fuel that drives muscle contractions. With age, your mitochondria get kind of like old dog fur: sparse, damaged, and inefficient. “Zone 2 cardio basically helps build your mitochondria,” says Kenneth Jay, Ph.D., a sports scientist who’s done research with the National Research Centre for the Working Environment in Copenhagen.
All this Zone 2/mitochondria research started in an effort to improve athletic performance. Scientists found that two key drivers of that performance – the ability to clear lactate more efficiently and the ability to oxidise fat better – both depended on those cellular powerhouses. “These things can only happen through the mitochondria,” says Iñigo San Millán, who’s been researching mitochondria for decades and is affiliated with the University of Colorado School of Medicine as well as with the UAE Team Emirates cycling team. Understanding that, researchers drilled down into what exercise intensity could improve mitochondria the most. “And that was zone 2,” San Millán says.
Lactate is a byproduct of using glucose for energy. It ultimately decreases the pH in muscle. “That decreases the velocity and force of the muscle contraction,” he says. Getting better at clearing it allows you to be stronger and more powerful.
Another big benefit of robust mitochondria, fat oxidation, means your body gets better at using fat as an energy source during endurance efforts. This can help stave off fatigue. In the future, maybe there will be a pill to help build mitochondria. “But right now, the only way we know of to improve mitochondrial function is to move,” he says.
These cellular improvements can help you make gains in lifting, too, by aiding your recovery between sets. People without healthy and plentiful mitochondria “just don’t have the capacity to regenerate ATP fast enough to repeat something. And what they are repeating is half the output of what they were doing at the beginning, so they’re not getting the stimulus that they could,” Nelson says.
Building mitochondria might not get you ripped, but it deserves a lot of cred given that “mitochondrial dysfunction” has been linked to cardiovascular disease, cancer, diabetes, and obesity. Shoring them up with Z2 essentially gives you a more efficient engine to get through life on. “It’s like a hybrid car,” explains Stephen Seiler, Ph.D., a professor of sports science at the University of Agder in Kristiansand, Norway, who’s studied the effects of high- and low-intensity zone training in athletes. Redlining your workouts saps battery power. “You can run off the battery for a little while, but to pay it back, you’ve got to have your main engine.” Consistent Z2 work not only helps you recover better between sets or workouts, but it also helps you get better at everyday challenges: a family hike, an extra-long-haul rush to your flight gate, or a grueling workweek.
Does Zone 2 work have to be cardio?
In general, yes. That’s because a big benefit of zone 2 cardio is that it helps keep your heart strong and, basically, stretchy. When you lift heavy, your heart’s left ventricle – the one that pumps oxygenated blood to the rest of your body – gets tough, too. Heavy loads make your heart contract in a way that causes it to lay down more muscle fibres. But it lays them down inside the chamber, so there’s less room for blood. (Don’t get smug, endurance folks; a huge amount of cardio makes yours too thin and can leave you vulnerable to issues like atrial fibrillation.) Zone 2 cardio moves blood through the heart in a way that keeps walls at a healthy thickness and stretchiness, so it functions better, says Jay.
How do you know if you’re in zone 2?
Zones were developed according to the different energy systems that are utilised at different exercise intensities, explains San Millán. It’s important to train all of the zones (outlined in the chart below) he explains. But when you’re aiming to shore up your mitochondria, 2 is the one that you want.
It’s hard to give an exact percentage of max heart rate (MHR) for zone 2, as it’s different for everyone. Someone who’s not trained might hit zone 2 at 60 per cent of MHR, while someone who exercises regularly might hit it at 70 to 80 per cent of that.
San Millán uses plenty of fancy equipment for research. But when it comes to finding your zone 2, he says you can do it just fine without all that. Use the talk test: if you can speak just like you’d do at your desk or in a quiet place, you’re in zone 1. If you are breathing a little harder and the person on the other end of a phone call would know that you are working out, that’s zone 2. If you are forcing it and having a hard time keeping conversation going, that’s 3. (Squeaking out only yes or no answers tends to be zone 4 or 5.)
How much time do you need to spend in zone 2?
There’s debate on exactly how much time you need in Z2 to get the benefits. If you’re mostly sedentary now, any amount of low-intensity movement will help. If you’re somewhat or very fit, experts typically promote a minimum of 30 to 40 minutes of zone 2 cardio twice a week. (For endurance athletes, 80 per cent of total training volume in Z2 is a good guideline). San Millán thinks you need even more to really get the mitochondrial benefits that can help performance, stave off chronic illness, and improve lifespan. “In my opinion, you need at least 300 minutes a week,” he says.
The trick is not turning that cardio workout into a sprint endorphin rush. Once you start pushing the intensity, different training adaptations are taking place. And that’s not what you’re looking for in these workouts.
Zone 2 is chat-paced work, so bring friends. Fit people might need more than a casual walk; hiking and rucking uphill can get you there. Keep it interesting in the gym by giving ten minutes each to the rower, bike, and treadmill. The key is control. “There is a warrior aspect to this as well,” Seiler says. When you have the discipline to stay in the zone, “there can be a Zen there in going out and finding your rhythm and not be influenced by the person who runs past you that day,” he says.
When it comes to creating a more efficient engine, easy really does do it. It’s not like you can’t have go-hard-or-go-home workouts; you need those, too. But being smart about adding a little low can feed your high.
This article originally appeared on Men’s Health US.
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