Heart rate zones are big in the fitness world right now, and every smartwatch and fitness tracker will happily tell you what “zone” you’re in when you do a workout—any workout. Even strength training. Even yoga. So you might be tempted to interpret this data in terms of the benefits of zone 2 training. If my heart rate is in zone 2 when I’m lifting weights, that means I just did an hour of zone 2 training, right? Unfortunately, no.
Heart rate zones are for cardio
The idea behind dividing up your heart rate into “zones” is an idea that comes from cardio training. In fact, the whole idea of monitoring your heart rate for athletic (rather than medical) purposes originated with a Finnish ski team, and then spread to other athletes. Your heart rate can tell you how hard you are working, relative to your past performances, and you can use that information to gauge whether you should go harder or ease up. Or, as Finnish triathlete Pauli Kiuri is quoted saying in this article at Polar, “At some point during your run, you get the feeling that you can’t push any further. But if your rate is still only 160 bpm, you just have to believe that yes, you can!” (Polar was the maker of those early heart rate monitors.)
Heart rate training makes sense for endurance sports, because the harder you work, the faster your heart beats. So you can use that relationship in reverse: the bigger the number on your heart rate monitor, the harder you must be working. Your heart rate is a good gauge of the work you're doing.
But that relationship with heart rate doesn’t hold for other types of exercise. Your heart rate during a yoga session doesn’t tell you how deep into a stretch you are getting. Your heart rate during a strength training session doesn’t tell you whether you are lifting light or heavy. That’s why your heart rate during a strength training session does not matter.
Heart rate zones tell you what your whole body is doing, not what your heart is doing
Heart rate, properly used, is what’s called a proxy metric. We don’t track heart rate because heart rate matters; we track heart rate because it tells us about something else that matters—in this case, the intensity of cardio exercise.
We get certain benefits from zone 2 cardio: low-fatigue calorie burn, increased mitochondrial and capillary density, and improved VO2max, to name just a few. These benefits come from the fact that our entire body is doing exercise.
Our muscles are contracting repeatedly, requiring our mitochondria to work harder to power them. Those mitochondria need more fuel, so our cells become more responsive to insulin so they can pull in blood sugar more effectively. To do all of this, we need more nutrients and oxygen to reach our muscle cells, so we grow more capillaries to supply them, and to take away metabolic byproducts.
That big picture is what “zone 2” training is all about. Just increasing our heart rate, without doing all of that other stuff, isn’t going to get all those good zone 2 adaptations.
That’s why sitting in a stressful meeting at work for 30 minutes is not the same thing as going for a jog. Nor is watching a scary movie the same thing as doing a HIIT workout. And, sorry to sauna enjoyers, but a sauna is not a replacement for exercise. (Sauna sessions do seem to improve the health of your blood vessels somewhat, but they don’t do all the other stuff.) When you think about it, zone 2 cardio has a lot more in common with zone 3 (or even zone 5) cardio than it does with “zone 2” lifting.
What zone should strength training be in?
Trick question! Zones don’t matter for strength training. There are a bunch of ways you can gauge how hard you’re working when you lift weights, but none of them require a heart rate monitor. Here are some common ways to judge your effort:
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How the weight on the bar compares to the most you’ve ever lifted: for example, you might do squats at 80% of your max.
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How many reps you are doing at a given weight: five reps at 80% is harder than one rep at 80%.
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How fast you move as you complete the rep: the harder it is, the slower it will move; there are even gadgets to measure this. Sometimes this property is called “bar speed,” as in, how fast the barbell is moving.
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How many more reps you think you could have done: if you had three reps "in the tank," that's an easier set than if the last rep you did was the last rep you could possibly have done.
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How sore, fatigued, or “pumped” a muscle feels: within some styles of training, this can help you to figure out how much work a muscle group has gotten during the day’s training.
Some things that do not correlate with how much strength you are building include how out-of-breath you feel after a set of exercises, or how fast your heart beats during or after the exercises.
The truth about heart rate when lifting is that your heart rate may be higher, your breathing harder, and your rest times longer, if your cardio fitness kinda sucks. This is a sign that you may want to improve your fitness by doing actual cardio (zone 2 or otherwise), so that you gain those adaptations and don’t have to sit around as long sucking wind between sets. But your cardio fitness is only playing a supporting role here; it’s not the point of the strength session, and it’s not a useful metric for gauging your effort.
Or to put it another way: when you do five sets of five squats at 80% of your max, you get the same strength benefit whether you do it with three-minute rests and a smile on your face, or eight-minute rests and a sky-high heart rate.
OK, but what zones are normal for strength training?
Realistically, you’ll see your heart rate jump up and down throughout your strength training session. During rests, your heart rate may be in zone 1 or 2; during short sets of an exercise, you may see your heart rate spike into zone 3 or higher.
You’ll likely see a higher heart rate during sets of many reps (sets of 10 reps may result in a higher heart rate than sets of three reps). It doesn’t really matter what heart rate zone you find yourself in.
The only thing that really matters, if you’re checking your heart rate chart afterward, is that your strength session has big spikes and dips. If it looks relatively even, you’re probably not resting enough. For comparison, here are my charts for a recent jog (at the upper end of zone 2/lower zone 3) and for a strength training workout that included snatches, deadlifts, and squats. Note the longer rests and higher spikes toward the end of the workout, when I squatted heavy for six reps at a time.
If zones don’t matter, why does my app tell me what zone my strength training was in?
The short answer is: because they can, not because they should.
When endurance athletes first adopted heart rate training, it was a replacement for running by timed paces or just by feel. They understood that it was a measure of how hard their whole body was working during a run or a ski session.
It’s different in the modern era, when every watch measures heart rate. The truth is that heart rate zones are on all your result screens because heart rate is easy for your watch to measure, zones are easy for your app to calculate, and because the company that makes the app and watch wants to please you with a bunch of cool looking charts after every workout.
Heart rate zone charts make your workout feel more important, and getting to see them is like a little reward sticker to keep you in the habit (and, from the company’s point of view, using its product). Enjoy the charts if you think they’re pretty, but keep your eyes on the metrics that matter—which, for strength workouts, includes pretty much everything except your heart rate.
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