What is Zone 2 Cardio?

The guy who won the marathon that you just raced is running his easy runs slower than you are.

I said this to one of the runners that I coach recently.

I had been having a debate with him about “zone 2 cardio.” He had a pretty good race in the Philadelphia marathon, and on the heels of that race he started training for Tokyo this spring. After reviewing some of his training data, I told him that he looked like he was working too hard on his easy runs based on his heart rate. 

This athlete is a very intuitive runner, meaning that he likes to run based on how he feels on the day. Some days he pushes harder, some days he takes it easier, and to be fair, this strategy has served him decently well so far. And in many cases, intuitive running can be a great skill when done correctly. So what was his initial reaction to being encouraged to slow down? 

Skeptical at best. 

Why should he slow down when he feels good training at his current intensity? 

But when I looked at this athlete’s situation, I saw something different:

• A runner with a history of stress fractures

• A runner who initially came to see me during a marathon training cycle for achilles tendinopathy that took a while to get under control

• A runner who has had a difficult time achieving his goal time in a marathon over the last few training cycles

• A runner who is currently running a shortened training cycle for his first international marathon this spring

So after sharing some research about zone 2 cardio that didn’t seem to strike a cord, I shared with him that fact that my mentor, who happens to know the gentleman who won the Philly marathon, shared with me.

Clearly this “zone 2 cardio” stuff is working for people. But why? How can we make sense of running slow to run fast?

In this blog post, we'll dive deep into the lauded training zone that promises to:

• Dramatically improve your aerobic capacity

• Reduce injury risk

• Build a rock-solid endurance foundation

Whether you're a weekend warrior or an aspiring marathon runner, get ready to discover the training method that could completely revolutionize your running journey. Buckle up—your fitness transformation starts here.

What is Zone 2 Cardio?

To understand what zone 2 cardio is, we have to understand what heart rate zones are in the first place.

A heart rate zone describes a range of how fast your heart is beating relative to your maximum heart rate. Generally, the higher the zone number, the closer you are to your heart rate max (HRM). Zone 2 cardio typically refers to zone 2 of a 5 zone model, where zone 1 is basically resting, and zone 5 is maximum effort. (1) 

Another way to answer this question is to look at a 3 zone model. This model is based on lactate turn points 1 and 2 (LT1, LT2). You might have heard of lactate before, but let's talk about how it fits into this puzzle.

Lactate is a molecule that helps your cells convert nutrients into energy (cellular metabolism). (2) When you exercise at a certain intensity, typically zone 3-4 on the 5 zone model, your body starts to produce lactate and hydrogen ions at a higher rate. The hydrogen ions make your blood more acidic, and your body starts buffering (make less acidic) the more acidic environment created by the hydrogen ions. 

Therefore, measuring how much lactate is in your blood is another way for us to measure how hard your body is working.

But here’s the thing! Your body can’t buffer forever. This is where the lactate turnpoints come in. Lactate turnpoint 1 (LT1) is the point at which your body starts accumulating lactate as it goes from an easier effort (aerobic) to a harder effort (anaerobic). 

When looking at the three zone model, zone 1 is below LT1. Zone 2 is between LT1 and LT2 where lactate is gradually accumulating. Zone 3 is referring to efforts above LT2, where lactate is quickly accumulating.

But why does zone 2 cardio work?

Exercising in zone 2 trains your body to be more efficient at using oxygen in its energy metabolism, which allows you to run faster with less effort. This improvement can be described as an improvement in aerobic capacity.

Let’s look at an example of zone 2 cardio training improving aerobic capacity in practice.

Let’s say you do a baseline 40 minute run at an average HR of 150 beats per minute (bpm), and you make it 4.44 miles. 

After performing some zone 2 training for 4 weeks, you do the same 40 minute run keeping an average HR of 150 bpm, but this time, you make it 4.9 miles. This indicates that you were able to run faster while keeping the same HR as before. This happens because your body gets more efficient at using oxygen in its energy metabolism, along with other reasons. This is one way to get faster. 

Not only can zone 2 cardio help you get faster, it can also help you train more consistently. Zone 2 cardio puts little stress on your body comparatively to harder efforts in zones 3-5. Too much training stress => injuries => less consistency of training, and consistency is incredibly important. So the question becomes, “what type of training builds fitness while causing the least amount of training stress?” I think you see where this is going.

There is definitely room to include some higher intensity training in a program, but many in the distance sport community recommend a pyramidal distribution of training, with the highest volume of training below LT1, a medium amount between LT1 and LT2, and a small amount above LT2. 

Need more proof that zone 2 cardio is worth doing? Just look at this chart showing some insight into different athletes training distributions for the Boston Marathon:

Credit: Joel Sattgast PT, DPT @trexopt

How to implement zone 2 cardio into your training

There are a couple different ways to identify what zone you are exercising in.

1. Use a HR max calculator and a HR monitor on your runs

   1. There are equations to estimate your max HR, one is 220 - (your age), another supposedly more accurate one is 208 - (0.7 x your age)

   2. Once you know your max HR, you can calculate what HR you should be working at for zone 2. For example, a 30 year old would have a max HR of 187. Zone 2 is 60-70% of their HR max, which would be between 112 and 131 bpm.

2. Use Rating of Perceived Exertion (RPE)

   1. RPE is a subjective measure of effort that correlates very closely with your HR zones. LT1 typically correlates with RPE 4-5, so zone 2 cardio would be ~ RPE 2-4/10.

Final Thoughts

Zone 2 cardio or zone 2 training is considered an essential component of any aerobic sport training including distance running, cross country skiing, cycling, and many others. It can help you get faster and stay healthy. At Alterra, we specialize in helping runners get and stay healthy, which always includes reviewing their training intensity distribution. If you're local to Boston's Back Bay area and are looking to dial in your training, we'd love to help! Book a free discovery call to chat with a therapist today.

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Next on your reading list:

1. Is Running Gait Analysis Worth It?

2. How Strength Training Can Make You a Better Runner

3. Physical Therapy Treatment for Runners

Sources:

1. Exercise Heart Rate Zones Explained. (2023, December 12). Retrieved from: https://health.clevelandclinic.org/exercise-heart-rate-zones-explained/ 

2. Robergs RA, McNulty CR, Minett GM, Holland J, Trajano G. Lactate, not Lactic Acid, is Produced by Cellular Cytosolic Energy Catabolism. Physiology. 2018;33(1):10-12. doi:https://doi.org/10.1152/physiol.00033.2017

3. Seiler S, Tønnessen E. Intervals, Thresholds, and Long Slow Distance: the Role of Intensity and Duration in Endurance Training. SPORTSCIENCE. 2009;13: 32-53.