Buzz
While your heart rate increases when you're pushing yourself, it’s often seen as a sign of sufficient effort... but it's not always accurate. Heart rate can reflect effort only in specific exercises and might even mislead you if the tracking isn't set up correctly.
Avoid depending on heart rate tracking to estimate calorie burn
When your fitness tracker reports the calories you've burned, it's largely estimating based on your heart rate (and a few personal details, like your size).
It's mostly accurate that the higher your heart rate during a cardio workout, like running, the more calories you're burning. However, your watch can't always distinguish when your heart rate spikes due to other factors like stress or overheating, not just your workout intensity.
This metric is also well-known for being inaccurate in certain activities. Running and cycling are straightforward because of the repetitive motions, and the movement of your legs on the pedals is likely close to the data used by app developers to calibrate their algorithms. But if you're doing something like barre, Crossfit, or skiing, the watch doesn't really know what you're doing or how intense it is. Spoiler: It doesn't.
Compounding the issue is the fact that the calories we burn during exercise don't reflect our total calorie expenditure, which is what really matters. Our bodies are good at conserving energy when we're burning a lot during exercise, so you might burn 500 calories on a run but then burn 400 fewer calories later in the day due to fatigue.
The bottom line is, if you're concerned about how many calories you're burning, you shouldn't rely too much on a heart rate monitor. A more effective approach is to adjust your calorie intake based on whether your weight is going up or down.
Don’t focus on heart rate while you're strength training.
If you're a regular cardio enthusiast, you might be used to viewing your heart rate as a solid indicator of intensity. The higher the heart rate, the more effort you're putting in. But this doesn't apply to strength training.
Consider this: If you're lifting a light weight repeatedly, your heart rate will rise and stay elevated. For instance, here’s the heart rate graph from a ten-minute set of kettlebell clean and jerks I did:
That’s a challenging workout and it will build some strength, but it’s nothing compared to one of my typical weightlifting sessions. For example, I did barbell clean and jerks, clean pulls (a type of explosive deadlift), jerk recoveries, and various core exercises for over an hour. These lifts were mostly quick but heavy enough to leave me out of breath afterward. Those spikes on the graph represent those moments. Afterward, I would rest for a few minutes, and that’s when you see my heart rate drop back down.
Looking at the heart rate graphs, it's clear that the ten-minute set was a consistent, fairly intense cardio workout. I couldn’t have pushed myself much harder, as my cardiovascular system wouldn’t have been able to supply enough blood and oxygen to my muscles to keep going. From a lung perspective, it’s similar to a tempo run.
In contrast, the barbell workout wasn’t very demanding on my heart or lungs, and it didn’t contribute much to improving my cardiovascular endurance. If we were to judge these workouts based only on their heart rate graphs, the barbell workout would appear almost meaningless.
However, cardiovascular benefits are not the goal of strength training. The purpose is to challenge our muscles to become stronger. Heart rate doesn’t reflect the true difficulty or weight of the lift. Just by glancing at the graph, I can't tell which set was 42 kilos and which was 49 kilos; I need to check my training journal for that info. If I compared this workout to one I did a few years ago, when I was weaker, it would probably look the same, even though the weights would have been much lighter.
It’s crucial to recognize this difference, because if you evaluate your strength workouts solely by heart rate, you'll be tempted to raise your average heart rate by doing more reps with lighter weights—essentially turning them into cardio sessions. (That ten-minute kettlebell workout I used as an example? It's really a mix between cardio and strength, offering a tough challenge for my heart and lungs and a moderate one for my muscles.) If you want to build strength, you must lift heavy, which means resting between sets and allowing your heart rate to drop.
Use heart rate to measure cardio intensity—with some caveats
Now let’s talk about where heart rate really excels, which is assessing the intensity of cardio workouts like running and cycling.
So how do you translate your heartbeats into an intensity measure? The easiest way is to use an app that converts your heart rate into heart rate zones. In a five-zone system, if you're out for a LISS (low intensity steady state) jog, your heart rate will likely stay in Zone 2 most of the time.
These zones are useful for training because each one has a slightly different impact on your body. Zone 2 is perfect for enhancing aerobic capacity without overexerting yourself. Zones 3 and 4 push your body to work harder, though it’s harder to sustain work in these zones. So, if you're heading for a long run, wearing a heart rate monitor will help ensure you're staying in Zone 2 for the benefits you seek; on another day, you might do a shorter workout that keeps you hovering between Zone 4 and Zone 5 to condition your body for tough efforts.
You don't have to track your heart rate in order to train effectively at these effort levels. Before the advent of smartwatches and heart rate monitors, coaches simply advised runners to go for an "easy run," and recreational joggers would try to maintain a "conversational" pace, meaning they could breathe easily enough to chat. When doing high-intensity intervals, you don’t need a heart rate monitor to know when you're sprinting full out – your body will make it obvious.
How to train with heart rate, if that's still something you're interested in.
If you're just starting out with cycling, running, or another steady-state cardio activity, you can focus on effort level rather than using your heart rate as a guide. If you'd like to keep track of your heart rate, I suggest observing what number you see at each effort level you experience.
What number do you notice when you're walking or warming up, with little to no effort involved?
What number do you see when you're moving at a pace where you can easily carry on a conversation?
What number do you observe when you're working at a level that's challenging but sustainable for about half an hour or even longer if needed?
What number do you see when you're going all-out? (It’s probably hard to check because you’re exhausted, but you can always review it afterward.)
These heart rate numbers generally align with Zones 1, 2, 4, and 5, with Zone 3 representing the zone between an easy pace and the challenging but manageable pace.
The reason I'm asking you to notice rather than calculate is that if you're new to exercise, your calculations are likely to be inaccurate. Heart rate zones are usually described as percentages of your maximum heart rate, but if you've never experienced an all-out, race-day effort, you won't truly know what your max heart rate is.
There are several formulas available to estimate your max heart rate, and the simplest one is to subtract your age from 220. For instance, a 30-year-old would have a max heart rate of 190. The problem with this method is that it’s a one-size-fits-all formula that fits almost nobody.
Here's an example: I'm 41, and my actual max heart rate is over 205. If I relied on the formula, a heart rate of 152 would put me in Zone 4, but in reality, that’s just a nice easy Zone 2 pace for me. During that ten-minute kettlebell set, I spent a good portion of it with my heart rate exceeding what the formula would consider my max. Some people, on the other hand, have max heart rates lower than the formula predicts, and they experience the opposite issue. They'll be gasping for air, with burning legs, yet their watch might say they're in Zone 3. That’s not correct, either. Using miscalibrated zones can leave you either overexerted or undertrained.
Your heart rate can also fluctuate depending on your body position. For instance, I’ve noticed I can’t push my heart rate as high when cycling as I can when running. (Swimming, where your body is horizontal, typically results in even lower numbers.)
So, don't stress over your exact heart rate in the beginning. Higher numbers indicate that your cardiovascular system is working harder, while lower ones mean you’re exerting less effort. Pay attention to how the effort feels at a given heart rate, and adjust your “max” in your app’s settings to whatever makes the zones feel right. Keep in mind that heart rate is just a tool to refine your training. Your body will know how hard it’s working, whether you have a smartwatch or not.
