Zone 2 Training

I spent a week in Barcelona at the end of August so decided to use the free time out there to put in a good amount of running. I ran 20km on each of the first four days of the holiday, giving me 100km for the 7 day period. It was fun enough exploring a new place, running along the coast and across the city. Albeit, the sun made it a bit challenging on the last couple of days.

The reason I did this was to give my fitness a sharp super charge, all the runs were relatively slow, keeping my heart rate in what is called zone 2. It worked brilliantly as I could feel myself getting fitter almost day to day from my heart rate data.

Zone 2 training is now the rage of every fitness podcast it seems. However, most people are a little confused by it. There is a difference between “easy” exercise vs “zone 2,” and there are reasons why sometimes it is better to do an easier Zone 2 session than a harder intervals workout. We touched on some of these subjects a few weeks ago when I wrote about mitochondria, but let’s look at zone 2 training specifically and what you can do to reap its benefits.

How Energy Works

The body has three energy systems. The biggest is the aerobic system, which takes oxygen from the air to produce energy. At lower intensities the body can supply all energy using just the aerobic system. The cells can choose to use carbohydrates or fat to fuel it. Which source it picks depends on how the body has been trained and the exercise intensity.

As intensity increases the body begins to use the anaerobic energy system. This provides quicker energy through breaking down carbohydrates, producing lactate. Some lactate can be recycled, but once effort rises too high the body cannot contain it, blood lactate shoots up and you get fatigue, burning legs and rapid breathing.

Heart Rate Zones

Your heart rate varies from your resting pulse (asleep or sitting down) up to your maximum (the fastest it will ever go). Between these two points sit the heart rate zones:

Zone 1 – Low heart rate at rest or very light activities
Zone 2 – Comfortable, steady, ‘easy’ aerobic exercise
Zone 3 – Comfortable exercise with higher reliance on carbohydrates
Zone 4 – Hard effort where lactate rises sharply, muscles burning
Zone 5 – Maximum all-out effort, very painful & hard work

Most of life’s activities lie in zone 1. Zone 2 represents early aerobic exercise, where you feel comfortable and can maintain conversation. Zone 3 feels moderate. Zone 4 is tough with a localised burn. Zone 5 is all-out effort.

Zone 2

The general formula for Zone 2 is 180 – your age. It should feel comfortable and conversational. But here’s where confusion kicks in. Many people can also talk at Zone 3, especially newer exercisers. The difference is subtle.

Genuine Zone 2 = The Maximum Fat Burning Output by Your Body.

The reason Zone 2 gets attention is that it trains your body to use fat for energy at the highest possible rate. It also helps increase both the efficiency and number of your mitochondria, making your aerobic system stronger and more efficient at sparing carbohydrate stores.

Respiratory Quotient (RQ)

RQ is the ratio of fat vs carbohydrate as a fuel source. Elite athletes have a better RQ, meaning they can exercise aerobically while using more fat than a lesser trained person. This spares carbs for when intensity rises.

The problem is many people think they’re doing Zone 2 but are actually using a lot of carbs at that pace. Your maximum fat burning point may be at a lower heart rate than you expect. The only way to know this is lab testing, so erring on the lower side of Zone 2 is often wiser.

Zone 2 vs Easy Exercise

Easy exercise protects your joints and reduces fatigue, but it doesn’t necessarily mean you’re at max fat oxidation. True Zone 2 specifically develops mitochondria and RQ. Both eas and Zone 2 have value, but they are not the same thing.

Zone 2 Problems

For many people, Zone 2 feels too easy to be “worth it.” Many people can’t jog and stay in Zone 2, which feels demoralising. This is why it is not the primary focus for new exercisers. It doesn’t feel challenging and doesn’t deliver the immediate “sweaty workout” reward. On the flip side, if you hate exercise you could get benefits by doing this easy feeling type of activity.

The other issue with zone 2 is volume and injury, especially for runners. Most elite runners are pushing 150-200km+ a week and this takes a toll on the joints, even at zone 2 speeds. Cyclists have it easier here, they can rack up huge zone 2 training volumes without injury risk. For running, the body takes years to build tolerance to the pounding, so chasing big Zone 2 mileage too quickly will usually end in injury.

What This Means For You

If you only have limited time each week (say 20–30 minutes once or maybe twice), then Zone 2 is not your priority. If you build your exercise habits up a little more, then you can consider adding in a session of Zone 2.

Where it becomes essential is once you get to a good level of ability and want to push it upwards significantly. There is a reason why every good runner or cyclist puts in loads of km/week. At this point you need to lean heavily into Zone 2, as it is one of the best ways to build your aerobic base, fat-burning capacity, and long-term endurance.

I’ve referenced this previously around Peter Attia’s debates on how to dedicate 3–4 hours of aerobic exercise per week to the different zones when trying to optimise health. This of course is a topic that doesn’t apply to most people who struggle to find 4 hours a month. But if your goal is ultimate health, the path will eventually need to include regular aerobic exercise and some dedicated Zone 2.

Photo – My running in Barcelona and the effects of it this weekend showing up as I took a minute off my Nuclear Games time in Essex

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