Why Zone 3 Cardio Is Just as Effective as Zone 2

There are benefits to training in heart rate zone 2, and you’ve likely heard plenty about them. But what happens when your heart rate climbs into zone 3 during a run or a gym workout? The surprising truth: You don’t lose the advantages of zone 2 training. Zone 3 might even be just as effective, or perhaps even more beneficial.

The appeal of zone 2 training lies in its ability to build your aerobic foundation and burn calories with minimal fatigue. But guess what else zone 3 training does? It also strengthens your aerobic base, burns even more calories, and usually results in just a bit more fatigue than zone 2. So, why aren’t we all incorporating more zone 3 cardio into our routines?

Zone 2 gets more credit than it deserves

There are different reasons for opting for either lower or higher intensity cardio workouts. Currently, easy cardio is gaining a lot of attention, especially the idea of doing more zone 2. Before heart-rate monitors were commonplace, determining what ‘easy’ felt like was something you gauged on your own, or by comparing your pace to what you could sustain during a race. For non-athletes, the ‘talk test’ was used: If you could still chat while jogging, you knew you were at an easy, steady pace.

Now that most people own heart rate monitors, we're fixating on specific numbers. These devices even color-code your heart rate zones, telling you when you’re in zone 2 or 3. A slight increase in your heart rate? You’ve slipped out of zone 2 and into ‘workout jail!’

In reality, your workout isn’t drastically altered if your heart rate hits 153 beats per minute compared to 152. The difference between 145 and 155 beats per minute probably isn't significant either, as long as both numbers are within the effort level where you can still carry on a conversation.

Workout zones aren’t exactly a perfect science

Most heart-rate zone systems are merely designed for convenience and don't truly reflect what's happening in your body. While there are actual thresholds in exercise intensity, like the moment when talking becomes difficult or when lactate builds up faster than your body can remove it, these do not perfectly align with the common five-zone system.

To refresh your memory, the five-zone system is based on where your heart rate sits as a percentage of your maximum heart rate. Each zone is defined by specific percentage boundaries, and the zones are typically described as follows:

• Zone 1: Rest or minimal effort

• Zone 2: Easy, conversational pace

• Zone 3: ???

• Zone 4: Pretty challenging

• Zone 5: Maximum effort

It's a nice concept, and many people find it useful, but the zones aren’t grounded in any scientific evidence showing that you gain particular benefits at 60-70% of your max heart rate, and different benefits at 71-80%. If you're skeptical, take a look at how various gadgets and apps define these zones differently: Your ‘zone 2’ might fall between 60-70% on an Apple Watch, but be 65%-75% on a Peloton. At 73%, the Apple Watch might put you in zone 3, while the Peloton would call it zone 2. So, who’s right? In truth, neither.

Exercise research doesn’t rely on heart rate zones of this type—at least not in the way apps use them. Instead, studies often assess intensity by looking at factors such as whether you're above or below your ventilatory threshold (essentially, if you can still talk while exercising) or lactate threshold (measured through blood chemistry, but essentially the maximum effort you can maintain for extended periods). Sometimes, they use METs, which measure energy expenditure during physical activity, or they may express everything in terms of oxygen consumption (the basis of the term VO2max). Occasionally, studies provide heart rate-based guidelines, but these are typically based on personalized measurements, not the generic zones found in apps or videos on YouTube.

A conversational pace actually spans both zone 2 and much of zone 3.

Let’s dive deeper into the concept of the “talk test” or “conversational pace.” This idea of maintaining a pace where you can still talk comfortably is rooted in science, specifically the ventilatory threshold. This guideline for easy cardio pace is based on it.

Imagine you begin with a walk and gradually speed up each minute. Eventually, you’ll reach a point where your breathing becomes more labored, and your speech starts to shorten. If you were talking with a friend, you’d be able to utter just a few words at a time, rather than speaking freely. That’s your ventilatory threshold (VT), also known as VT1.

When athletes or coaches refer to an easy pace or effort, they typically aim for a pace below the ventilatory threshold. Despite common belief that zone 2 corresponds with the VT, the actual conversational pace is closer to 80%, which falls at the upper end of zone 3. For instance, a study on recreational runners found their VT1 to be around 78% of their max heart rate. Importantly, the runners’ heart rates were directly tested, rather than relying on age-based formulas, which are often inaccurate.

If your goal is to train at an easy pace, or you follow the 80/20 rule for running (where 80% of your sessions are easy), you should be doing your easy runs or cardio in zones 2 and 3, not just zone 3.

Zone 3 is still considered an aerobic zone, so it’s still classified as an easy pace.

Rather than separating zones 2 and 3, it’s more logical to view them (along with zone 1) as a spectrum. At the lower end, your pace is slower, calorie burn is minimal, and it may feel like you’re hardly working at all. (Enter, cozy cardio!)

On the higher end of zone 3, you’re still getting significant aerobic benefits, improving your mitochondria, capillaries, and more—but in a shorter amount of time. For those aiming for calorie burn per hour, zone 3 is the most efficient.

Cyclists often refer to this training range as the ‘sweet spot.’ It offers some of the benefits of harder training without pushing you to exhaustion. For runners, zone 3 might encompass tempo runs, race-pace runs, or even some of your faster ‘easy’ runs.

So, why bother with zone 2 if you can reap its benefits in zone 3? It depends on your overall plan: if you’re doing frequent training, you may want some sessions in zone 2 to conserve energy while still covering more distance. But if you're only running, say, three times weekly, those zone 3 sessions probably won’t overtax you.

You shouldn’t place too much emphasis on your heart rate anyway.

This takes me back to my frustration with heart-rate monitors. (A frustration rooted in affection; I track my heart rate during runs and find it beneficial in various ways.)

Your heart rate doesn’t solely align with your exertion; it’s also influenced by many other variables. For instance, it reacts to hot weather by showing higher numbers. It may also rise if you’re more fatigued, near the end of your run, or a bit dehydrated. During a race, you might notice your heart rate higher than usual right from the start, simply due to nervousness. Certain medications, like beta blockers, can also lower your heart rate significantly.

Then, there's the matter of whether your fitness tracker’s zones are calibrated accurately (even knowing that, yes, these boundaries are arbitrary). If you’ve never pushed yourself in an all-out race or series of hill sprints, your watch might not have recorded your true maximum heart rate. So, when it tells you your max is 184 because you’re 36, it’s just applying a formula. This is as logical as buying shoes based on the average shoe size for a 5’6” woman instead of actually measuring your feet (or trying on the shoes). If you go for an easy run and your heart rate stays in “zone 5” the entire time, trust me, that’s not your zone 5.

If your heart rate drifts into zone 3 during a “zone 2” training run, that might be accurate, or it might not be. But even if it is, as long as you can still talk and breathe relatively easily, you’re still gaining plenty of benefits from that zone 3 cardio.

Is zone 2 or zone 3 more effective for fat loss?

Both options are great! If you can only fit in cardio a few days a week and don’t mind pushing yourself, zone 3 is a fantastic choice. It’s less taxing than HIIT but offers more intensity than zone 2.

However, if you have extra time, aiming for 50 to 60 minutes of exercise per day, as researchers have found, is ideal for weight loss and long-term maintenance. (Here’s one interesting study showing that this level of exercise works even without dietary changes.) It’s a lot of work! To reach this target, most people wouldn’t want to do it all in zone 3, but zone 2 becomes much more manageable. The more you exercise, the more you’ll need to incorporate lighter workouts like zone 2 to balance out the tougher days.

If you're getting in a lot of exercise, it's important to include at least some time in zone 2, while also adding some zone 3 or higher if that’s your preference. If you’re only working out a few times a week, zone 3 might be a better option.

What is the optimal heart rate zone for cardio?

Every zone provides unique benefits, so if your goal is to enhance your cardio fitness, make sure to spend time in all of them.

• Zone 1 is ideal for warming up, cooling down, and providing recovery during breaks between intervals.

• Zone 2 is suited for longer, sustained efforts. It’s typically fine to substitute zone 2 for a rest day.

• Zone 3 helps you adjust to more intense work than zone 2. It burns more fat but comes with slightly more fatigue compared to zone 2, as we've mentioned. This is also the zone where you'll practice your race pace if you're preparing for events like a half marathon.

• Zone 4 brings you close to your lactate threshold, improving endurance when pushing hard. This zone is critical for athletes but is generally used just once or a few times per week, not for every session.

• Zone 5 is extremely intense and perfect for HIIT training (with recovery in zone 1, like walking, between tough intervals).

In general, it's better to spend more time in the lower zones and add higher zones for variety. Using the 80/20 approach to running, 80% of your workout time should be in zone 2 and low zone 3, while the remaining 20% should be in zones 3 and higher. While this isn't the only way to structure your training, it’s a widely-used method that ensures a good balance across all zones for runners.

Is zone 3 a "gray zone" with no benefits?

Definitely not! This label originated from coaches and writers encouraging people not to make medium-hard training the focus of all their sessions. Instead, they recommended including some easier sessions (zone 2) and harder ones (zone 4 for threshold training and zone 5 for intense intervals). This training style, emphasizing the highest and lowest zones, is known as "polarized training." However, this advice mistakenly led to the myth that people should avoid zone 3, which was never the case.