Ever notice that your heart rate data seems off on your watch during a run? Watches use optical heart rate sensors which can suffer from an issue known as “cadence lock”. This occurs when your watch reports your cadence as your heart rate, instead of your actual heart rate.
Wondering how to prevent this issue? Let’s dive in…
How do watches measure heart rate?
Watches use a technology called photoplethysmography (PPG) to measure heart rate. With this technology (often colloquially called “optical sensors”), LEDs project light onto the skin on your wrist, and a photodector on the watch measures how much light is absorbed.
This is why you see the bright lights on the underside of your watch face, like this:
Because hemoglobin in your blood absorbs light, changes in blood flow (which occur with every heart beat) affect the measurement of light absorption. The watches use algorithms to convert all this data into the heart rate information that’s displayed on your watch.
Pretty cool, right?
The problem is that sometimes, this technology is subject to inaccuracies, and cadence lock is one such issue.
Why does cadence lock happen?
Cadence lock occurs when your watch’s optical sensor “locks on” to your steps per minute instead of your actual heart rate.
This can happen because the movement in running creates a lot of “noise” that makes it difficult for the watch to accurately measure heart rate. This is particularly true if your watch is bouncing around a lot on your wrist and not strapped on tightly.
As you run and your wrist moves, each stride may let a little light into the sensor under your watch, creating different data points and making it look like your heart rate is your cadence.
You can see how this would happen with my watch strapped on lightly like the photo below; the light and sensor are visible because of the excess space.
It does appear that different watches and fitness trackers may vary in their overall accuracy and their risk of cadence lock. This may be due to the design of light and sensors across different models (or, in the case of inaccuracies at rest, possible algorithmic differences too).
For example, a 2019 study found that an Apple Watch 3 was fairly accurate during running with a mean error of only 1.77 bpm, whereas a Fitbit Charge 2 had a mean error of −14.73 bpm (i.e. it underestimated heart rate by that amount).
The study also noted “as movement became more erratic during certain conditions and as heart rate increased, the devices became less accurate.” This was found to be the case in another study as well, where speeds over 8mph on a treadmill resulted in inaccurate heart rate data on all watches studied.
What does cadence lock look like?
Cadence lock can occur in two ways:
1. Runners with a fast cadence may see their heart rate appear abnormally high, or suddenly jump.
For example, perhaps they’re running at an easy pace with a 145 bpm heart rate, then suddenly heart rate jumps to 170-175 bpm for the rest of the run, despite no change to pace or effort. That’s likely cadence lock, not their actual heart rate changing.
2. Slow runners with a low cadence may notice their heart rate looks abnormally low. This may occur less frequently, but does still happen.
In these cases, heart rate data at the start of a run may lock with cadence, and your true heart rate may not kick in until the watch finally recognizes it mid-run.
Below is a personal example from the second type. I am a slower runner and incorporate walking breaks. In the example below, I was on a rooty trail, and I had my dog with me so there was lots of abrupt slow downs to sniff things, haha, which is why the cadence is so low on average (110-140 steps per minute). You can see that the watch was locked on cadence up until 2.5 miles, when my actual heart rate of around 160 bpm kicked in.
How to prevent cadence lock and get more accurate heart rate data
If you’re finding that your watch may be experiencing cadence lock, here are a few ideas for getting more accurate data:
- Make sure your watch is strapped on tightly. If your watch is bouncing around too much, this can make it difficult for the optical sensor to accurately measure heart rate data as the ambient light will be let in under the watch and affect the readings.
- Try wearing your watch further up your arm. Despite strapping it tightly to my wrist, I never seem to get the most accurate measurements there. Instead, I strap it on one or two notches looser, then slide it up my arm until it’s snug (like the photo below). This has worked much better for accuracy!
- Clean the optical sensor regularly; logic would seem that if the sensor is dirty it may have a harder time detecting the light absorption correctly.
- If you need very accurate heart rate data (for example, if you are doing heart rate training), your best bet is switching to a chest strap instead of using a watch. Chest straps use a different technology (electrical detection) which isn’t subject to the issues with cadence lock. (Chest straps may also eliminate the possible issues with skin color affecting heart rate data, as some research suggests optical sensors may have lower accuracy in those with dark skin tones.)
The Bottom Line
Cadence lock can be a pain with heart rate data on watches, but there are a few ways to reduce the risk of this happening. If you truly need accurate data though, a chest heart rate strap is a better option.
Feel free to pin this post to share with others!
- Cranberry Pomegranate Smoothie - November 17, 2025
- 25 Minute Treadmill HIIT Workout - November 17, 2025
- Banana Berry Granola Smoothie - November 17, 2025
Leave a Reply