Issue #10: Heart Rate Monitor Accuracy

✍️ Author’s Note

Welcome to this edition of the Threshold Lab newsletter! I’m Stephen Pelkofer, an experienced HYROX athlete with a men’s pro personal best of 59:41 and men’s pro doubles best of 51:39. If you want to learn about my story and how I got here, check out this Instagram post here.

The goal of this newsletter each week is to pick a training topic related to running or HYROX, do deep research into it, and provide actionable protocols that the reader can take away and apply to their training immediately. Let me read the research, listen to the experts, and then give you the tools to make it work for you.

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👨‍💻 Introduction

Topic: Heart Rate Monitor Accuracy

If you’re tracking your training with a Garmin, Coros, Amazfit, Apple Watch (or other smart/fitness watches), you’ve probably wondered: How accurate is my heart rate reading? Can you trust your wrist sensor during a threshold run or a HYROX circuit? There are many studies that have looked into this – today, we’ll focus on the data from one of those studies: Variable Accuracy of Wearable Heart Rate Monitors during Aerobic Exercise (2017).

⚡ Fast Finds for the busy reader

• Your devices can and will misreport your heart rate (sometimes by a lot).

• If you want to take heart rate training seriously, you should look into buying a chest strap monitor.

• No device is 100% accurate all the time (another reason to dial in your rate of perceived effort).

🔬 Deep Dive

The experiment: Gillinov’s team at Cleveland Clinic tested five popular devices during exercise: four wrist-worn optical monitors (Apple Watch, Fitbit Blaze, Garmin Forerunner 235, TomTom Spark Cardio) and one forearm optical band (Scosche Rhythm+). Each of 50 fit volunteers wore two different wrist devices (one on each arm), a forearm monitor, and a Polar H7 chest strap, all while also hooked up to a clinical ECG as the standard for true heart rate. The subjects did 18 minutes of cardio split across a treadmill, stationary bike, and elliptical at light, moderate, and vigorous intensities. This setup put each gadget through its paces, from steady running to full-body effort, and allowed the researchers to compare every reading against the ECG (the gold standard for HR accuracy).

Chest strap vs. wrist sensors: Not surprisingly, the chest strap (Polar H7) was almost spot-on with the ECG – essentially perfect agreement (correlation ~0.996). These electrode-based monitors directly detect the heart’s electrical signals, and being strapped close to your heart gives them a clean signal. In contrast, all the other devices use optical sensors to shine light into your skin and measure blood flow pulses. It’s a clever technology (no chest strap needed), but it’s inherently more vulnerable to noise. Movements, muscle contractions, or even how snugly the device is worn can all interfere with the optical signal. Essentially, the wrist and arm monitors have to guess your heart rate from subtle blood flow changes, whereas a chest strap listens to the actual electrical beat.

So, how did the wearables stack up? Overall, the Apple Watch came out on top among the optical HR monitors, showing the highest agreement with the true heart rate readings. In the study’s stats, Apple’s wrist sensor was the only one that consistently met the “acceptable” accuracy threshold (a correlation >0.80 with the ECG) in almost every test. The Garmin Forerunner 235 and TomTom Spark performed moderately well (decent in many conditions, but with some drop-off in tougher scenarios). The Scosche Rhythm+ (a forearm band) and the Fitbit Blaze were less reliable. The Apple Watch was usually within a few beats of the chest strap, while some others could be off by 10-20+ beats at times.

Exercise type matters: On the treadmill (steady running), all the monitors did fairly well, with most wrist devices staying in a reasonable range and the Apple Watch nearly as accurate as the chest strap. On the stationary bike, only the Apple Watch, Garmin, and Scosche forearm band maintained acceptable accuracy while cycling. The biggest challenges came with the elliptical trainer. With arms stationary (holding handles on an elliptical without arm levers), the Apple Watch was the only device that stayed very accurate (it still tracked well, possibly because of the steady rhythm). But once arm movements were added (elliptical with arm levers moving back and forth), none of the wrist or arm devices met the accuracy threshold in that scenario. Every optical tracker struggled when the exercise involved vigorous upper-body movement. The more motion and muscle involvement, especially from the arms, the more an optical wrist/arm sensor can get thrown off. In contrast, the chest strap barely flinched in any of these tests – it delivered consistent readings across the board, regardless of exercise modality.

Why do wrist monitors go wrong? The researchers mentioned several reasons why our watch might misread your heart rate. Motion is a big one: when you’re swinging your arms, doing burpees, or rocking an elliptical, the jostling and muscle flex can confuse the optical sensor. Fit and positioning matter too: a loose watch or one that slides on sweat can lose the lock on your pulse. Physiological factors play a role: variations in skin tone or tattoos (which affect how light penetrates), and poor tissue perfusion (like cold hands on a winter run reducing blood flow) can all introduce error. Optical sensors basically need a clear “view” of blood pulsing, and a lot of factors can blur that view. Chest straps, on the other hand, pick up electrical signals directly from the heart’s activity – they’re less affected by motion or external conditions, as long as the electrodes maintain contact. The downside, of course, is comfort and convenience. Not everyone loves tightening a band around their chest for every workout, and it can be an extra hassle in training. Wrist trackers are simple and comfortable, but the convenience comes with a trade-off in data quality.

Time for the lab notes ⬇️

🧪 Lab Notes

In every issue of this newsletter, the “Lab Notes” are going to be the protocols that you can apply to your training and routine right away. The goal of this section is to translate the science into actionable steps for the reader, whether you’re a recreational runner/HYROX competitor, or someone pushing the limits of their peak potential.

• Use a chest strap for high-intensity or critical workouts: When you’re doing threshold intervals, VO₂ max repeats, or any session where hitting the right zone is key, strap on a chest monitor. Also throw on the chest strap for activities with lots of arm motion or full-body effort (think rowing, ski erg, wallballs).
  

• Wrist devices can be good enough for steady aerobic work: For most easy runs, long steady-state cardio, and daily activity tracking, a quality wrist wearable is usually accurate enough. When intensity is low to moderate and the movement is rhythmic (a simple run or ride), the optical monitor tends to stay in a reasonable range. If you’re in Zone 2 or just building base miles, a few beats of error won’t derail your training.
  

• No device is 100% accurate all the time: I have found this to be especially true with HYROX training. Burpee variations and ski erg tend to mess up readings from my Polar heart rate strap more often than not. As I talk about a lot, this is another great reason to really get in touch with Rate of Perceived Effort (RPE) and your own subjective difficulty scale for judging intensity.

That’s it for this edition of the Threshold Lab. If you enjoyed this and found it useful, please share it with a friend!

What I Read and Researched for this Issue

• [1] Gillinov, Stephen & Etiwy, Muhammad & Wang, Robert & Blackburn, Gordon & Phelan, Dermot & Gillinov, Alan & Houghtaling, Penny & Javadikasgari, Hoda & Desai, Milind. (2017). Variable Accuracy of Wearable Heart Rate Monitors during Aerobic Exercise. Medicine & Science in Sports & Exercise. 49. 1. 10.1249/MSS.0000000000001284.