What is zone 2?
Zone 2 cardio is low intensity, steady state cardio performed at 60-70% of one’s maximum heart rate (MHR). Zone 2 creates a number of specific adaptations in the body that are not achieved as efficiently in higher or lower heart rate zones. That means going for a walk (which typically results in heart rates lower than zone 2), or doing a fast run (which typically results in heart rates higher than zone 2), does not result in all the fitness adaptations you need to be the best athlete you can be.
Why do it?
Zone 2 can help an athlete’s recovery, output at higher intensities, ability to use fat stores for fuel, use energy more efficiently, and do harder work for longer durations. As an athlete, a strong cardio base at lower intensities is a prerequisite for improving their cardiac output at higher intensities. If you’re not good at the easy stuff, you’re limiting your capacity to be good at the hard stuff. If your goal is to be a high performing athlete in any sport, your aerobic engine is one of the most important things to look after, so neglecting zone 2, especially if it’s a limiter for your output, will only set you back.
The adaptations in the body to zone 2 include increased size of the heart muscle, expansion of the network of blood vessels in the periphery, more mitochondria in the cells to produce energy, more enzymes in the cells to convert energy stores to the energy that we use to produce force, increases storage space in the muscles for carbohydrates, and can stimulate the release of chemicals that make you feel happier (and are protective against dementia).
Zone 2 is also not very taxing on the body since it’s done at a low intensity, making it very easy to recover from. Unless your current training is really at the absolute peak of the volume you can tolerate, most people can add in a session or two of zone 2 each week and see benefits without additional strain on the body.
Because it’s done at low intensity, you can enjoy yourself: netflix, podcasts, audiobooks and conversations with friends are all on the cards in zone 2 in a way that might not be possible when working in higher heart rate zones. That means zone 2 can be very fun and easily incorporated into your routine.
How to train in zone 2
The best types of training for zone 2 are cyclical, monostructural, steady-state activities. Typically in CrossFit we do these on ergs: rowers, ski erg, concept2 bikes, or assault bikes. Their outdoor counterparts are equally appropriate choices. Some people do slow and steady sled pushes - that will work too, although I've never hated myself enough to give that a crack. All you need to do is grab one (or more, if you choose to rotate through different machines) of the above and start moving until your heart rate is sitting in the appropriate range. Then stay in that range for at least 30 minutes at a time.
Not many people can run in zone 2. Most people’s heart rates will be too high to effectively run in zone 2. People on the fitter end of the spectrum might be able to do a slow jog on flat terrain in zone 2, but I rarely recommend running to clients when aiming to train this way.
Simply having your heart rate in the recommended range is not enough - lifting sessions where your heart rate recovers down to zone 2 in between lifts is not training zone 2 cardio. In zone 2 training, the muscles need to be constantly contracting and relaxing, requiring the heart rate to overcome the tension in the muscles (this is part of what makes the heart stronger). During resistance training, the heart cannot overcome the muscular tension, which is why we see a heart rate spike from big lifts and the ‘pump’ that occurs during weight training. This means these forms of training are not zone 2 training.
Calculating MHR and zone two heart rate ranges
Some people will have a good feel for what their MHR is from wearing heart rate monitors during their training and seeing numbers at or very close to their maximum heart rate. You can use a prediction from a watch for your MHR, or you can get a rough idea by subtracting your age from 220. People who have remained fit for their whole life will often have a higher MHR than this calculation predicts, so use real data for your heart rate where possible rather than calculated or projected data.
For example: I’m 28 years old at the time of writing, so my theoretical MHR is 192 beats per minute (bpm). Because I have data from the last 3 years where my HR went higher than this, I know my MHR sits between 198 and 202, so I use 200bpm as my MHR instead.
With this MHR, multiply by 0.6 to get the bottom end of your zone 2 heart rate range, and 0.7 to get the top end of your zone 2 range. For me, this is 200(0.6) = 120 and 200(0.7) = 140 so my range is 120-140 bpm.
How to do zone 2 if you don’t have a heart rate monitor
My first advice here is get a heart rate monitor. They really are quite helpful for staying in training zones you want to be in, and can make a big difference to your training outcomes.
If you don’t have, can’t get, or really don’t want to get a monitor, that’s fine. You can feel when your body is in zone 2 by checking in on a few tests:
Could you hold a conversation at the pace you’re currently working at? In zone 2, you should be able to speak full, uninterrupted sentences without stopping to catch your breath or panting excessively.
Is your breath deepening? In zone 2, we expect to see a deepening of breathing, rather than a large increase in breathing rate. You should feel a moment when you start to inhale more air in one go, without needing to pant.
Can you continue to comfortably breathe through your nose? Most people will be able to breathe nasally during zone 2 for the full duration of the activity.
Is your core temperature warming? We should see an increase in blood flow and a rise in temperature - not everyone will get sweaty doing zone 2, but you may not be working hard enough if you feel no different to a leisurely stroll.
How to incorporate zone 2 into your training
You don’t need to wait to start zone 2 training. If you’re exercising for general health, start with 30-60 minutes each week, split into one or two sessions. If you’re an athlete trying to build an aerobic engine because you know that’s what’s holding you back, try accumulating 2 hours each week in 2-4 sessions and then dial back when you feel you’ve improved significantly. Zone 2 adaptations can be maintained much more easily than they are built.
Not all cardio training should be in zone 2
Let’s be clear - higher intensity cardio is a requirement for those wanting to get truly fit. Zone 2 is only part of the equation. That said, it’s part of the equation that is frequently overlooked for the vast majority of people (particularly in CrossFit). You must continue to do higher intensity work if you want to progress your fitness goals.
Balance your training time wisely between lifting, skill work, and structured cardio sessions to get the most out of your body. Group fitness classes like CrossFit can cover your bases for some, but not all of these components, so consider how you can best structure your training to touch on all the things you want to do as well as what you need to do for your desired outcomes.