HIIT vs Zone 2 training. What’s the best choice?
It’s common to think that Zone 2 training and high-intensity interval training (HIIT) sit at opposite ends of the conditioning continuum. But it’s not always as clear-cut.
For the heart to respond to the oxygen demands of the body, it can either a) increase the volume of blood it can pump with each beat. Or b) beat faster. Both options get the job done. This is where the HIIT vs Zone 2 confusion stems from.
For the sake of keeping this blog from spiraling out of control, we’ll only focus on the heart differences and benefits of Zone 2 and HIIT.
The Science of Zone 2 Training
With Zone 2 training, the goal is to keep your heart rate at around 60-70% of your maximum beats per minute (bpm). This moderate pace allows the left ventricle of the heart to stretch with blood between beats. Therefore increasing the volume of blood the heart can pump with each beat.
Zone 2 training develops an eccentric heart.
This means the heart builds muscle on the outside of the left ventricle. Which then further contributes to a more robust aerobic fitness and a slow, long, and strong heartbeat.
The Science of HIIT
Things can get overly complicated because the term “HIIT” itself is as about vague as it gets. Broadly speaking, HIIT training falls into two categories: max effort and sub-max effort.
They might sound similar, which is why people often confuse the two. But the heart responds to them differently.
Max Effort HIIT
This HIIT requires an absolute heart rate max effort, followed by a full recovery to a resting heart rate before going again. The work/rest ratio for a highly trained individual is usually somewhere around 1/3-5.
Depending on their aerobic conditioning. A more efficient aerobic heart will circulate the oxygen-filled blood faster. Leading to a faster recovery.
How the heart reacts to max effort HIIT
As the max effort demands a huge amount of oxygen in a very short time, the only way for the heart to deliver this oxygen is by beating fast. With this rapid rate, unlike Zone 2 training, there’s not enough time for the blood to pool in the left ventricle.
Max effort HIIT develops a concentric heart.
This builds muscle on the inside of the left ventricle. Allowing the heart to beat even faster. This quality is obviously important for most sports that require a brief explosive effort. But concentric training is important for day-to-day living too as it teaches the heart to deal with maximum heart rates.
The risks of too much concentric heart training
When the concentric heart develops too much it can significantly reduce the size of the left ventricle. Which can then lead to an increased risk of a cardiac event. Luckily, it’s really difficult to develop an overly concentric heart. As you’d have to ignore almost all other cardiovascular conditioning. Planned or indirect.
Most people who think they’re doing maximal effort HIIT, especially in gym classes, are somewhere closer to 90% effort, at best. This brings us to the most approachable type of HIIT training.
Sub-max effort HIIT
Go as hard as you can until you reach close to 90% of your maximum heart rate. Rest until the heart rate returns to 60%. Go again.
Not everyone has the access to a heart rate monitor. If your effort is somewhere around 9/10 in the perceived rate of exertion (RPE) and you go again at around 5.5/10 RPE, you’re pretty close to those numbers.
You can also base your recovery on the talk test. Once you can talk comfortably, it’s time to go for another rep.
Here’s where the difference between Zone 2 training and HIIT gets murky
Both Zone 2 and sub-maximal effort HIIT develop the eccentric heart. Provided that in HIIT, the heart can recover to 60%.
The physiological changes in the heart are based on the average heart rate throughout the HIIT session. And when you tap 90% and return to 60%, the average heart rate during the session will probably sit somewhere between 60-70%. The same as zone 2 training.
To get even murkier, sub-max HIIT can also develop concentric heart
If you do a 90% effort and go again before reaching 60%, there’s a good chance you’re developing a concentric heart. The heart has to stay at a rapid rate throughout the training session. Not leaving enough time for the blood to pool in the left ventricle.
Like we said in the beginning, the heart benefits of HIIT vs Zone 2 are not as clear-cut as we’d like to think.
Should you ignore Zone 2 and just do sub-max HIIT?
They both have similar heart benefits, after all.
HIIT is more time efficient, but it has its downsides
Compared to Zone 2 training, HIIT is a bigger stressor for the body and takes longer to recover from. Skipping the process of building an aerobic base and compounding HIIT on top of a body that is already stressed can lead to overtraining symptoms. And if you’re already feeling average (lack of sleep, stress, kids, etc.) HIIT is likely to be another stressor you don’t need.
HIIT is limited to specific modalities
Because of the intensity, you want to stick with movements and exercises that don’t take a lot of skill. A stationary bike or airdyne is the best option. Maybe hill sprints if your running mechanics are in check. Kettlebell swings are good for those who’ve mastered the form and can maintain it when tired.
We have worked with a bunch of people who come to us after doing months, or even years of four or five HIIT-style group fitness classes a week. Without seeing any noticeable results.
That’s partly because HIIT training is often marketed as the “catch-all” solution to training that helps you lose fat, improve fitness, and build strength. But because of the light weights and sub-optimal rest periods, people end up training nothing substantially. Feeling exhausted at the end of the workout doesn’t mean the workout was successful.
HIIT requires training at high heart rates
Which is not always ideal for someone with an existing heart condition. Or for those who are new to training.
Besides, HIIT training is tough. Both mentally and physically. Some people just don’t like the feeling of the intensity required to make HIIT effective.
Zone 2 training takes longer, but it’s more approachable
Zone 2 training is relatively easy to recover from. And it’s doable even if you don’t feel like bringing your A-game.
You can do Zone 2 training with a wide range of activities
Almost anything goes as long as your heart rate averages around 60-70% and you don’t use heavy weights.
It’s not HIIT vs Zone 2, but HIIT and Zone 2
The one you choose to do on any given day depends on your current level of conditioning, preferences, and life circumstances.
Our Guidelines
If you’re new to cardiovascular conditioning, start by building your aerobic base with Zone 2 training. Keep at it until your resting heart rate (measured over a month, first thing in the morning) consistently sits under at least 65 beats per minute.
It’s this aerobic base that then allows you to recover faster during HIIT.
As your conditioning improves, add one or two weekly sub-maximal HIIT sessions. Take your time to build the intensity to 90%. Provided you don’t have an underlying health condition where a higher heart rate could trigger a cardiovascular event.
If you already have a strong aerobic base, When you’re feeling fresh, do a 90% HIIT session twice a week. Other times, and when you’re not feeling so fresh, stick with Zone 2 training.
If you loathe the idea of sub-max HIIT, stick with Zone 2. And if you loathe the idea of Zone 2 training, a few sessions of sub-max HIIT per week are probably enough for a healthy heart. Provided you’re still doing other forms of physical activity.
And, if you already have a decent history of sub-max HIIT training, attempt a max-effort HIIT session every once in a while.
Just remember to take those rest periods seriously.
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