Chest Strap or Wrist-Based Heart Rate Monitors: Which Is Better?

This spring, Polar announced a brand new heart rate sensor that fits on a redesigned chest strap. We looked down at our wrist-based HR monitors and wondered: Why? 

For most athletes, traditional heart-rate monitors are a pain—literally. Despite sleek designs and copious amounts of lubrication, many athletes who use chest-strap monitors sport a telltale scar just below the breastbone: chafing, and lots of it.

Though these chest-mounted electrodes provide valuable training information—keeping easy runs easy, for example, or making sure hard rides are at the correct level of exertion—the discomfort of wearing a chest strap can be a deal breaker for many athletes. Heart-rate data is simply not worth the annoyance.

Some fitness trackers have tried to sidestep this issue with new technology—specifically, wrist-worn devices that forgo the chest-strap electrodes. Garmin was one of the first to offer a heart-rate monitor sans strap—the Forerunner 225, launched in 2015, used LED sensors on the back of the watch face to measure heart rate. Several other brands, including Polar, Apple and Fitbit, followed suit, advertising freedom from chest-strap chafing.

It sounded too good to be true. And according to the latest data, it probably is. A study out of the University of Wisconsin says wrist-worn heart-rate monitors aren’t as reliable as most people think they are.

After hooking study subjects up to electrocardiograph (ECG) technology and wrist-worn heart-rate monitors, Dr. Lisa Cadmus-Bertram and her team recorded heart-rate data at rest and during exercise. What they found was that the heart-rate monitors were within 5 beats of the ECG reading while at rest. But during exercise, the wrist-worn heart-rate monitors were found to fluctuate by a range of 20 to 40 beats per minute.

“At any given instant, the reading might be anywhere from perfect to quite a bit low or quite a bit high,” says Cadmus-Bertram. “For triathletes and other endurance athletes, that’s important because when you’re racing, you’re only going to glance down every now and then. So if at that moment you look down, the reading isn’t accurate, it can really mess with your head game.”

Dr. Michael Emery, medical director of The Center for Cardiovascular Care in Athletics, says these inaccuracies could be inconvenient at best. “As an athlete who is really trying to ‘dial in’ their training using heart rate zones derived from objective testing, if the monitor isn’t helping you get in those proper zones, then it is not helping you train as precisely as what is required to be that level of an athlete,” Emery says.

At worst, the inaccurate HR info could be deadly for some athletes with heart conditions.

“From a medical standpoint, if we instruct a patient to keep their heart rate in a specific range, either empirically derived or based on those measured during a stress test,” Emery says, “then they could potentially be exercising in heart rate ranges not appropriate for their medical condition, simply because the monitor was inaccurate.”

So what’s an athlete to do? It depends. For daily wear, a Fitbit or Apple Watch may be just fine. The heart rate data provided at rest can still reveal signs of overtraining or under-recovery by showing overall weekly trends. But when it comes time to work out, both Emery and Cadmus-Bertram say you best stick with the electrodes.

“For the data geeks who really care about precision and accuracy,” says Cadmus-Bertram, “it’s likely worth staying with the old-school chest strap.”

Skewed Data

Even with the most accurate reading, heart rate doesn’t tell the whole story. Several factors can affect the reading on the monitor.

Elevation
Heart rate increases at higher elevations, especially if you haven’t trained in high-altitude conditions.

Core temperature
As core temperature increases, heart rate will follow suit.

Fatigue
A low heart rate could be a sign of fitness, or it could be a sign of fatigue. When you’re feeling sluggish, your heart rate is slow to climb during workouts.

Caffeine
That morning cup of Joe wakes up more than just your brain—your cardiovascular system also gets a jolt.

Emotions
Nerves, excitement, frustration, anger—when you’re feeling all the feels, your heart rate kicks up a notch.