There are benefits to training in heart rate zone 2, and youve probably heard all about them. But what happens when your heart rate spikes into zone 3, whether when you're on a run or doing cardio at the gym? Surpriseyou dont lose the benefits of zone 2 training. 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 helps you build your aerobic base and burn calories without incurring much fatigue. Guess what zone 3 training also does? Yep, it helps you build your aerobic base, burn even more calories, and usually only incur a tiny bit more fatigue than zone 2. So why arent we all doing more zone 3 cardio?Zone 2 is overratedThere 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 what was easy by yourself, or by comparing your speed of running to what you knew you could do in a race. Non-athletes 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 were looking at specific numbers, and our watches color code the numbers so you know when youre in zone 2 versus zone 3. Your heart ticks up a beat? Youre out of your zone. Straight to workout jail!But the reality is, your body isnt getting a drastically different workout at 153 beats per minute than it was at 152. There probably isnt even much difference between, say, 145 and 155, as long as theyre both within that conversational-ish effort level. Workout zones arent realThe most popular heart rate zone systems are made up, with no precise relationship to what's going on in your body. Your body does have some dividing lines when it comes to exercise intensity (like the point at which you can't speak comfortably anymore, or the point at which lactate accumulates faster than you can clear it), but these don't correspond exactly to the typical five-zone system. The five-zone system, as a refresher, is based on where your heart rate falls as a percentage of your maximum heart rate. There will be specific percentages defined as the boundaries of each zone, and the five zones are usually described something like this: zone 1: rest or minimal effortzone 2: easy breezy conversational pacezone 3: ???zone 4: pretty hardzone 5: maximal effortIt's a cute idea, and many people find this system helpful, but these zones are not based on any scientific findings that prove we get such-and-such benefits at 60-70% of max heart rate, and such-and-such different benefits at 71-80%. If you arent 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. Research on the benefits of exercise doesnt use heart rate zones, or 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 theyll measure METs, which relate to how much energy you use to do work, or they'll put everything in terms of oxygen consumption (this is where the term VO2max comes from). Occasionally these studies will send participants home with heart rate-based guidelines, but those tend to be drawn from their personal scientific measurements, rather than the cookie-cutter zones you get from an app or from watching a video on youtube. Conversational pace includes zone 2 and most of zone 3So lets 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, youll hit a point where your breath becomes a little ragged, and your sentences choppy. If you were conversing with a friend, you'd be grunting out a few words at a time, rather than casually telling a story. 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, youd think that the VT occurs at the top of zone 2. But nopeconversational pace is closer to 80%, which is the top of zone 3. For example, heres 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 youre trying to train at an easy pace, or if youre 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 easyNow that I've explained why the zone 2/zone 3 distinction is arbitrary, you see why it makes more sense to look at zones 2 and 3 (or even zones 1 through 3) as a continuum. At the lower end, youll be running or pedaling slower, burning fewer calories, and feeling like youre barely doing any work. (Hello, cozy cardio!)At the higher end (or the top of zone 3), youre still getting a lot of aerobic work done, still benefiting your mitochondria and your capillaries and everything else, but youre doing it in less time. If youre 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 whats the point of zone 2, if you can get all of its benefits in zone 3? That depends on your big picture: If youre doing a lot of training, youll probably want some of it to be in zone 2, if only to save some energy while youre getting more miles on your feet. But if you only run, say, three times a week, its unlikely that those couple of runs will wear you down much even if you do them all in zone 3.You shouldnt read too much into your heart rate anywayThis brings me back to my grudge against heart rate monitors. (Its a grudge borne of love; I track my own heart rate when I run and find it useful in many ways.)Your heart rate doesnt only track with your training effort; it also responds to a lot of other factors. For example, it responds to summer heat, showing you higher numbers in hot weather. It can also show higher numbers if youre more fatigued, or at the end of a run compared to the beginning, and it may show higher numbers if youre a bit dehydrated. When you run a race, you may find that your heart rate is higher than expected at the start, just because youre a bit nervous.Some medications can alter your heart rate as wellbeta-blockers, for example, notoriously lower your heart rate. And then theres the question of whether your fitness tracker's zones are set correctly (even knowing that, yes, their boundaries are made up). If youve 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, its just grabbing numbers from a formula. That makes as much sense as buying shoes based on the average shoe size for a 56 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 isnt 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, youre getting plenty of benefits from your zone 3 cardio.