If you care enough about your health and/or fitness to be thinking of buying a heart rate monitor, you may also care enough to want to know which you can trust to give a correct reading.
There are, for example, perhaps a dozen different ways of determining body fat percentage. None can be said to be 100% accurate, but some gadgets on the market have been known to give readings that differ by fifty freaking percent from more reliable methods (percent, not percentage points).
So the question is: is buying a wrist-worn heart rate monitor even worth it?
The Fundamentals of Heart Rate Measurement
There are basically only two kinds of sensors used to measure pulse rates: electrical and optical. The differences between them are key to understanding heart rate monitor accuracy.
Direct electrical sensors generally come in the form of chest straps and, instead of measuring the actual pumping action of your ticker, record the nerve signal telling its muscles to contract. Although accurate, they’re not all that comfortable to wear. They can chafe, slide down, feel constricting, and, unless you choose a slimline model that you can hide under a shirt, are far from being a fashion accessory.
Optical sensors measure pulses of blood flow directly… for the most part. You see, it would be expensive and impractical to install the same kind of optical sensor your camera has in a wrist strap or earbud. Instead of actually photographing your blood vessels as they expand and contract…
…they are looking only at the total reflectivity of a small area of your skin. (This explanation is a little oversimplified; if you like, you can read more about photoplethysmography here)
How the Type of Sensor Affects Heart Rate Monitor Accuracy
Blood absorbs more light than most other tissues. Your skin and flesh are slightly translucent, and the surface darkens by approximately one 1/50th when a pulse of blood goes through the area.
This change is not exactly huge, but that wouldn’t be a problem except for one thing: numerous factors actually influence how dark that little patch of your skin appears to be from moment to moment. Optical heart rate monitor accuracy would be superb if this wasn’t the case.
A chest strap monitor’s input signal looks something like the following, assuming that you’re heart is beating normally:
Unlike with an optical reading, there are few interfering signals and the waveform is easily recognizable: as long as the electrodes are making contact, the device should give a pretty accurate indication of how fast the wearer’s heart is beating.
How Important, Really, Is Accuracy in a Heart Rate Monitor?
As mentioned in a previous article, the ideal heart rates of individual people will be different, even when resting. The same goes for what they’ll require from a device meant to measure and record it. You shouldn’t bring a knife to a gunfight, but neither would you normally chop celery by shooting at it. It’s all about different aims and intentions.
If you’re a bodybuilder, athlete, or someone of that ilk, you might be more interested in the consistency of the measurements you’re getting. At different times, environmental factors like temperature, aging, and battery level can affect the measurement accuracy of electronic circuits, including those found in heart rate monitors.
Consequently, a poorly-designed device might give a reading that’s at least in the general ballpark, but it could mislead you totally if what you’re really interested in is the trend over days or weeks. This is one way, along with the better support that comes from having a large user base, in which higher-priced heart rate monitors are usually better than their counterparts.
How Heart Rate Tracking Can Really Help to Improve Fitness
Whether you’re someone who trains at a very high level of intensity, or you just want to push your workout as far as it will go, you will generally be interested in identifying changes over time rather than your heart rate monitor’s accuracy as such.
In other words, you’ll want to know how hard you’re pushing yourself and whether you’re making progress, but knowing that your pulse rate is exactly x when you’re doing y is useful only in a clinical context, or, of course, for bragging rights. Long-term tendencies are the ones to watch.
Noticing a decrease in heart rate variance, for instance, can help diagnose a case of overtraining before it results in a month or more of enforced downtime.
You might care a little more about heart rate monitor accuracy when it comes to measuring your recovery heart rate – how much your pulse slows directly after exercising and an excellent indicator of aerobic fitness. Perfect precision is not necessary to see whether you’re improving or not, but it does help.
Someone who is aiming to maintain a “yellow” or “orange” level of intensity in their workout, though, probably won’t care too much whether their heart is actually beating at 126 or 131 bpm. Even more basic devices are perfectly adequate for this, although you may want to spend extra if your target zone is narrower than 20 bpm.
Some Wristbands are More Equal Than Others
Statistically speaking, the best way to strive for accuracy in most realistic situations is to (A) try to take your measurements under similar conditions every time, and (B) take a lot of them. In this way, the errors quickly start to cancel out each other.
Most devices with any kind of connectivity have (b) covered. It should be no trouble at all to view your heart rate over the course of a workout, or several months, and even correlate this to things like dietary changes. Where cheaper manufacturers tend to fall down, unfortunately, is point (a).
“Similar conditions” for the owner of a fitness tracking wristband is obviously a non-starter, exactly because of how much they struggle with movement. If you jog, lift weights, and play tennis, for example, your heart rate monitor might work well for one of these activities but give confusing readings for the others. This, again, is where the big brands in the heart rate monitor field, and their research budgets, come out on top.
Unfortunately, wristband heart rate monitors will always be at a disadvantage. Depending on the type of exercise and which device you choose, the eventual error may be as small as 2% or over 30%. The most expensive devices, unsurprisingly, usually give the best results.
Can I Get the Same Results Using an Ordinary Watch and My Finger?
Doctors, of course, will use professional equipment when trying to diagnose a patient. Usually, though, they’ll still take a patient’s pulse by hand almost as soon as they arrive, but this is more of a quick check and also something of a tradition. Doing this accurately, especially on yourself, is a little more difficult than most people think. If you want decent results without spending anything, though, you can use an app for the same purpose, perhaps directly after taking a run or waking up.
To sum up all of the above: when it comes to heart rate monitor accuracy, you really do get better and worse devices, with chest straps almost always falling into the “better” category. Of course, this doesn’t mean that precision is the only important factor: saving money is certainly dear to all of our hearts. The comfort and everyday wearability a wristband model offers is also fantastic compared to chest straps, most of which make you look a little like a cyborg, anyway.
Others will serve as an alarm clock, monitor the quality of your sleep, help maintain an ergonomically correct working place, and even prompt you to stand up while working and move around when you’ve been sitting around too long. A chest strap, by contrast, usually does only a single thing, although it does it very well.
TL, DR – Which Should I Buy?
Optical heart rate monitoring is inherently a more complex and less reliable technology than electrocardiography. It comes with some interesting quirks that affect heart rate monitor accuracy, like when Apple belatedly discovered that its smart watch heart monitor didn’t work on tattooed skin. Similarly, in a few cases, a person’s pigmentation or general cardiovascular health might mean that they don’t get quite the results they expect, although improved design seems to have taken care of many of these problems.
For this reason, it’s not recommended that wrist-worn heart rate monitors intended for fitness use be used for clinical research or medical purposes.
They are, indeed, less accurate at measuring heart rates, especially at the low end of the price range. In general, though, products from reliable brands tend to work well 90% of the time for 90% of users. This is obviously unacceptable when monitoring a patient likely to go into cardiac arrest, but if you’re only trying to improve your 10 mile performance, who cares about outlying data points?
If you’re thinking of getting a wearable heart rate monitor for medical reasons, get one recommended by your doctor, and only once he actually recommends getting one. Without medical training or access to other resources, you’re likely to drive yourself crazy over every tiny variation. If you must resort to home care, instead try to see if a dependable home test kit exists for whatever condition you suspect you have.