Why Zone 2 is the Essential Training Zone for Triathletes — Triathlete

You’ve heard about it, you’ve talked about it, and you’ve used it in workouts: Zone 2 is the bedrock of endurance training. But do you actually know what Zone 2 is, beyond the “endurance training zone” description? You’ve heard that you should spend a ton of time training in Zone 2, and that doing so will make you faster, but do you know how that actually works?

Whether you’re a person who likes to know the “why” behind your training or you’re someone who needs to be convinced that they really have to run that slow, this deep dive into Zone 2 will answer all the questions you have and possibly a few that you wouldn’t have thought to ask. You’ll come out the other side not just a smarter athlete, but one who executes their training with intent and therefore gets fitter and faster. And really, isn’t that the whole point?

What is Zone 2?

On the surface, Zone 2 is simply the second zone in a tiered system of training intensities. When we talk about Zone 2, we are typically referring to its place within a 5-, 6-, or 7-zone system. In some cases, though, training can be prescribed using a 3-zone system, in which case what we’re talking about would actually be Zone 1.

Here’s the thing about these multi-zone systems, though: No matter how many zones they define, the systems are all based on two physiological boundaries – the first and second lactate thresholds, aka LT1 and LT2. A 3-zone system uses only those physiological boundaries to define training zones, where 5-, 6-, and 7-zone systems subdivide the spaces below, between, or above these boundaries.

You’re probably already familiar with the LT2, the boundary that helps define high-intensity training zones, because that’s the one we generally call “lactate threshold.” The first lactate threshold, or LT1, is an equally meaningful physiological boundary but one that’s less well known. We need to understand LT1 in order to understand Zone 2, and to understand LT1, we need to talk about energy systems.

Our bodies use two main energy systems to fuel our muscles for any activity longer than a few seconds: our aerobic and anaerobic energy systems. (We use a third system, the phosphagen system, for very quick bursts of energy.) All of our energy systems produce adenosine triphosphate (ATP), which is our muscles’ primary fuel source.

Our aerobic system uses oxygen as one component of its relatively slow ATP-producing mechanism (hence the name), and can produce fuel for hours and hours in a well-trained athlete. Our anaerobic system is not oxygen-dependent and produces meaningful levels of lactate as a by-product of its relatively quick but short-lived (just minutes at maximum output) ATP-producing mechanism. Both our aerobic and anaerobic systems are always on, producing ATP across all intensity levels and zones, even if you’re just sitting on the couch reading this article.

And this brings us back to LT1. Below LT1, activity is almost entirely powered by our aerobic system, with only a very small, steady anaerobic contribution as that system idles in the background. Those energy contribution proportions result in low and stable lactate levels in our blood.

Once we move an iota above LT1, our anaerobic system starts to rev up, and our blood lactate levels begin to rise. So LT1 represents the tipping point between stable and increasing blood lactate levels, and the moment that our anaerobic system’s contribution begins to climb.

Which finally lands us back at our original question: Zone 2 (or Zone 1 in a 3-zone system) represents the heart rate, effort level, power, and pace at which we are working below LT1, and therefore where we are fueling activity almost exclusively with our aerobic system.

Why do we care?

Defining Zone 2 doesn’t necessarily explain why everyone places so much importance on it in training. We’ll dive into the details, but let’s start with the TL;DR version: Everyone wants to get faster, and a big aerobic engine contributes heavily to speed – at low intensity where the aerobic system dominates, but also at higher intensities that are more commonly associated with the anaerobic system. You build that big aerobic engine by training in Zone 2.

A big aerobic engine is an aerobic energy system that produces large amounts of ATP at any given rate of oxygen delivery, or heart rate. This engine doesn’t live in one place within the body, and it’s not even a singular engine.

Each muscle has its own aerobic engine, which is actually the muscle’s collection of mitochondria – organelles within each muscle cell that produce ATP during aerobic metabolism. A bigger aerobic engine means muscles with bigger collections of mitochondria that are highly efficient at producing ATP.

Below LT1, where your aerobic energy system supplies 90+% of your ATP, or fuel, more and more efficient mitochondria mean more fuel production, which means your muscles can work harder and you can go faster.

But above LT1, even after your anaerobic system increases its ATP production, your aerobic system keeps working. In fact, your aerobic system works harder as your effort level increases, right up until you hit VO2 max – your maximum rate of oxygen use and therefore the upper limit of your aerobic system.

And as the aerobic energy system works harder, more fuel production means your muscles can work harder and you can go faster, just like below LT1. So a stronger aerobic engine provides more fuel to your muscles across all effort levels, which makes you faster at all training intensities.

What’s the benefit of training in Zone 2?

You build this big aerobic engine by using your aerobic engine: The more you ask of it, the better it becomes at doing its job. Specifically, using your aerobic energy system prompts mitochondrial adaptations that both create new mitochondria, increasing mitochondrial density within the muscle, and improve mitochondrial ATP-production efficiency.

And yes, you do use your aerobic engine across all zones and effort levels, and it does work harder at higher intensities, but work done above LT1 comes at a cost. Above LT1, both the aerobic and anaerobic systems work harder, and the anaerobic system creates fatigue at a disproportionately higher rate than the aerobic system.

So the best way to spend a lot of time using your aerobic engine, to be able to train today and again tomorrow and the next day, is to find the optimal trade-off between aerobic training stress and the resulting fatigue burden – which is, of course, Zone 2.

Zone 2 in triathlon training

To effectively use Zone 2 in training in order to build that big aerobic engine and improve your pace or power across all effort levels, first, you need to be able to find your Zone 2 for each sport with reasonable accuracy. Field threshold tests can help estimate your power/pace and heart rate at lactate threshold (LT2), which can then be fed into standard zone curves to define your Zone 2 pace, power, or heart rate.

Lab-based threshold testing is the gold standard for defining not just your power/pace and heart rate at LT2 but also at LT1, which then defines the top of your Zone 2. But lab testing isn’t generally feasible for the average athlete, so we do have ways of finding Zone 2 outside of that.

Typical Zone 2 ranges by sport and metric

• Swim: 60-70% of max heart rate; 10-15 seconds per 100 slower than 1000m/yd pace
• Bike: 81-89% of lactate threshold heart rate; 65-75% of threshold power (FTP)
• Run: 85-89% of lactate threshold heart rate; 78-86% of threshold pace (typically 2+ minutes per mile slower than 5K pace); 80-88% of threshold power

But the thing is, all of these tests have some drawbacks. Thresholds found through field tests are estimates to begin with, and the zone curves aren’t individualized to your physiological profile. Lab testing can be very accurate – but it only provides a snapshot of your physiology on the day you are tested, and your pace, power, and heart rate at LT1 and LT2 are going to evolve along with your fitness.

Plus, testing in one sport doesn’t necessarily lead to zone definition across all three sports. So it’s helpful to know that the conversational threshold – the effort level at which you have broken a sweat but can still hold a conversation, speaking in complete sentences without need to take a breath in the middle – is a surprisingly good boundary for the top of Zone 2.

We often think of Zone 2, and work done below the conversational threshold, as having a rate of perceived exertion (RPE) of 4 out of 10; once you find your 4/10 RPE for one sport, you can generally translate it to the other two.

After defining Zone 2 across all sports using quantitative metrics, RPE, or both, you want to spend copious amounts of time there, for all three sports, since each has its own muscle-specific aerobic engine.

How much time should triathletes spend in Zone 2?

Typically, you’ll allocate roughly 70-85% of your weekly training minutes – across swim, bike, and run, and including warm-ups, cool-downs, and recoveries between intervals – to training in Zones 1 and 2. Zone 1, your very easy effort level, will generally only show up in warm-ups, cool-downs, and recovery minutes, so the vast majority of those minutes will be spent in Zone 2.

And yes, especially on the run, you might be cranky about spending so much time at what feels like a slow (but surprisingly comfortable) effort level. Stick with it for 6-8 weeks, though, and the transformation of your pace or power at that effort level will be all the proof you need to keep Zone 2 as the bedrock of your training.