Heart rate monitors have become indispensable tools for runners and fitness enthusiasts. Most smartwatches and fitness bands use wrist-based optical sensors, known as photoplethysmography (PPG). In one study, an evaluation was conducted on how accurate these wrist-worn devices are—both at rest and during exercise—for people in normal heart rhythm (sinus rhythm) and those with atrial fibrillation (AF). Here is a summary of the study.
Right: Chest strap electrodes picking up the heart’s electrical signals (ECG).
What They Did
- Participants: 81 individuals were tested, including people with normal heart rhythm and those in atrial fibrillation.
- Procedure: Participants wore commercially available wrist-based PPG heart rate monitors while resting and performing treadmill exercises.
- Their heart rates were compared against the gold-standard electrocardiogram (ECG) readings worn at the same time for comparison.
What They Found
- At rest, wrist monitors were relatively close:
- For people in sinus (normal) rhythm: average error was around 4.6 bpm.
- For those in AF: average error increased to 7.0 bpm.
- During intense exercise, accuracy dropped significantly:
- In sinus rhythm: average error soared to 13.8 bpm.
- In AF: error ballooned to 28.7 bpm, nearly half a minute difference in heartbeats.
These discrepancies weren’t random—some devices tended to overestimate, while others underestimated heart rates. The variations were both statistically significant and large enough to matter if you’re training by heart rate zones. American College of Cardiology
What This Means for Runners
At Rest or Easy Runs
- Wrist-based monitors generally offer acceptable accuracy for daily tracking and recovery. They’re comfortable and convenient—great for monitoring resting heart rate or light activity.
During High-Intensity Efforts or Irregular Heart Rhythm
- Accuracy drops sharply in these conditions.
- For runners using heart rate zones during workouts (e.g., tempo runs, intervals), wrist sensors might misguide your effort—especially if you or someone has AF.
The Bottom Line for the Running Community
Wrist-based heart rate monitors are convenient and adequate for general use and easy workouts. But their readings can become unreliable during intense efforts or in the presence of arrhythmias like atrial fibrillation. If precision matters—especially during structured training or for those with heart rhythm concerns—consider pairing your wearable with a chest strap or clinical-grade device for greater accuracy.
Reference
Read the full study “Accuracy of Wearable Heart Rate Monitors During Exercise in Sinus Rhythm and Atrial Fibrillation” for a deeper dive.
How Wrist-Based LED Heart Rate Monitors Work vs. Chest Strap Monitors
Wrist-Based Monitors: Photoplethysmography (PPG)
Wrist monitors work using green LED lights that shine into your skin. Blood absorbs light differently depending on whether your heart is pumping or in between beats.
- When your heart contracts, more blood flows through your vessels, absorbing more light.
- When your heart relaxes, less blood is present, and more light reflects back.
A tiny optical sensor in the back of your watch measures these changes in reflected light. By analysing the rhythm of these changes, the watch estimates your heart rate.
Srengths
- Comfortable and convenient (always on your wrist).
- Works well at rest and during steady, low-intensity exercise.
Limitations
- Motion, sweat, or skin tone can interfere with accuracy.
- Often struggles during high-intensity workouts, intervals, or rapid changes in effort.
Chest Strap Monitors: Electrocardiography (ECG)
Chest straps measure heart rate using electrical signals, the same principle as an ECG in a hospital.
- Small electrodes on the strap detect the tiny electrical impulses your heart produces each time it beats.
- These signals are transmitted to your watch or phone and recorded as a heart rate reading.
Because chest straps read your heart’s actual electrical activity, they are highly accurate, even when you’re sprinting, climbing hills, or racing.
Strengths
- Gold standard for accuracy.
- Reliable across all intensities and conditions.
Limitations
- Less comfortable than wrist monitors.
- Requires wearing and adjusting a strap before each run.
A Recap of Which One Should You Use
- Everyday runners: A wrist-based monitor is usually “good enough” for general training and lifestyle tracking.
- Performance-focused runners: A chest strap ensures reliable data during intervals, races, and training sessions where precision matters.
- Best of both worlds: Many athletes wear a wrist monitor for convenience, but switch to a chest strap for key workouts.
For more information and troubleshooting tips check out Garmin’s page.