There are benefits to training in heart rate zone 2, and you’ve probably heard all about them. So what happens when your heart rate spikes into zone 3 on a run or when you’re doing cardio at the gym? Surprise—you don’t lose the benefits of zone 2 training. In fact, zone 3 is arguably just as good for you, or maybe even better.
Remember, the reason people are excited about zone 2 training is that it builds your aerobic base and burns calories without incurring much fatigue. Guess what zone 3 training also does? Yep, it builds your aerobic base, burns even more calories, and usually only incurs the tiniest bit more fatigue than zone 2. So why aren’t we all doing more zone 3?
Influencers and heart rate monitors have ruined “easy” cardio
There are reasons to run (or do any cardio) at lower intensities, and reasons to use higher intensities. Before heart rate monitors were widespread, you had to judge “easy” by yourself, or by comparing your speed of running to what you knew you could do in a race. For non-athletes, we had the “talk test.” If you could hold a conversation while jogging, you knew you were at an easy, steady pace.
But when everybody has a watch that tells them their heart rate, suddenly we’re looking at specific numbers, and our watches color code the numbers so you know when you’re in zone 2 versus zone 3. Your heart ticks up a beat? You’re out of your zone. Straight to jail.
But the reality is, your body isn’t getting a drastically different workout at 153 beats per minute than it was at 152. There probably isn’t even much difference between, say, 145 and 155, as long as they’re both within that conversational-ish effort level.
Zones aren’t real
Heart rate zones are made up. They’re a cute idea, I suppose, but they’re not based on any scientific findings that say we get such-and-such benefits at 60-70% of max heart rate, and such-and-such different benefits at 71-80%. If you aren’t convinced, just look at how different gadgets and apps define the zones differently. Your “zone 2” might be 60-70% on Apple Watch, but 65%-75% on a Peloton. The zones are made up, okay?
Research on the benefits of exercise doesn’t use heart rate zones, at least not of this type. They may measure intensity in a few different ways, including whether you are above or below your ventilatory threshold (basically, whether or not you can talk while exercising) or your lactate threshold (measured through blood chemistry, but basically the highest effort you can sustain for a long time). Sometimes they’ll measure METs, which relate to how much energy you use to do work. Occasionally they’ll send participants home with heart rate-based guidelines, but those tend to be based on the measurements above, rather than cookie-cutter heart rate zones.
Conversational pace includes zone 2 and most of zone 3
So let’s take a closer look at that idea of the “talk test” or “conversational pace.” The guideline to keep your easy cardio at a chatty pace does come from a scientific concept: the ventilatory threshold.
Imagine you start out at a walk, and every minute or so you increase your speed a bit. As you work harder, you’ll hit a point where you can’t keep breathing normally. Your breath becomes a little ragged, your sentences choppy. That point is your ventilatory threshold, or VT (sometimes called VT1).
When athletes or coaches talk about easy pace or easy efforts, they usually want you below your VT. The way people talk about zone 2, you’d think that the VT occurs at the top of zone 2. But nope—conversational pace is closer to 80%, which is the top of zone 3. For example, here’s a study on recreational runners that found VT1 to be, on average, at 78% of the runners’ max heart rate. And they tested the runners’ max heart rate, rather than using a formula based on age. (Never trust the default formulas.)
So if you’re trying to train at an easy pace, or if you’re using the 80/20 rule to keep 80% of your runs easy, you can do those easy runs or cardio sessions in zones 2 and 3, not just zone 3.
Zone 3 is still aerobic and still easy
Now that we understand that the zone 2/zone 3 distinction is arbitrary, it will make more sense to look at zones 2 and 3 (or even zones 1 through 3) as a continuum. At the lower end, you’ll be running or pedaling slower, burning fewer calories, and feeling like you’re barely doing any work. (Hello, cozy cardio!)
At the higher end, so the top of zone 3, you’re still getting a lot of aerobic work done, still benefiting your mitochondria and your capillaries and everything else, but you’re doing it in less time. If you’re interested in calorie burn per hour, zone 3 is more efficient.
Cyclists sometimes call training in this range the “sweet spot.” It gives you some of the advantages of harder training without making you too fatigued. For runners, zone 3 may include some of your tempo runs, some of your race-pace runs, and some of your faster “easy” runs.
So what’s the point of zone 2, if you can get all of its benefits in zone 3? That depends on the big picture of your training. If you’re doing a lot of training, you’ll probably want some of it to be in zone 2 just to save some energy while you’re getting more miles on your feet. But if you only run, say, three times a week, it’s unlikely that those couple of runs will wear you down very much even if you do them all in zone 3.
You shouldn’t read too much into heart rate, anyway
Let’s bring this back to my little grudge against heart rate monitors. (It’s a grudge borne of love; I track my own heart rate when I run and find it useful in many ways.)
Your heart rate doesn’t only track your training effort; it also responds to summer heat, showing you higher numbers for the same effort. It can also show higher numbers if you’re more fatigued, it shows higher numbers at the end of a run compared to the beginning, and it may show higher numbers if you’re a bit dehydrated. When you run a race, you may find that your heart rate is higher than expected at the start, just because you’re a bit nervous.
And then there’s the question of whether your zones are set correctly (even knowing that, yes, their boundaries are made up). If you’ve never run an all-out race or series of hill sprints, your watch may have never seen your maximum heart rate. So if it says that your max must be 184 because you are 36 years old, it’s just grabbing numbers from a formula. That makes as much sense as buying shoes based on the average shoe size for a 5’6” woman, rather than actually measuring your feet (or trying on the shoes). If you go out for an easy run and find that your heart rate was in “zone 5” the whole time, I guarantee you that isn’t your zone 5.
So if your heart rate creeps into zone 3 on a “zone 2” training run, that may or may not be accurate. But even if it is, if you can still breathe and speak more-or-less normally, you’re getting plenty of benefits from your zone 3 cardio.