Your heart rate reads 150 warming up. Here's what actually fixes it.

The symptom

You know this one if you have it:

• Heart rate reads 150+ during an easy warm-up jog, then settles to 130 after ten minutes
• Intervals show heart rate climbing thirty seconds after the effort started, then overshooting on recovery
• The number locks onto your cadence and stays there — a classic optical artefact
• Cold mornings are much worse than warm afternoons

Every one of those is a signature of optical sensing at the wrist. None of them is a fault in your particular watch, and none is fixed by buying a better one.

The sensor-level reason

An optical sensor shines light into the skin and measures how much scatters back off moving blood. Three things degrade that, and the wrist is bad at all three.

It moves.The wrist is a joint in constant independent motion. Every movement shifts the sensor against the tissue underneath, and the algorithm has to separate a pulse signal from that motion. When cadence and heart rate are numerically close, it sometimes locks onto the wrong one — which is exactly the "heart rate equals my step rate" artefact.

Perfusion is low. A wrist is tendon and bone with comparatively little muscle. Less blood flow near the surface means a weaker signal, and cold weather constricts peripheral vessels further — which is why cold mornings are worse.

Contact is poor. A wrist is not round, and a strap tight enough for a good optical seal is uncomfortable, so nearly everyone wears one looser than the sensor needs.

The evidence, and why it is unusually clean

The 2025 Sensors study (n = 28) did the experiment that settles this. It put three identical Whoop 4.0 units on each participant at once — wrist, forearm, upper arm — plus a Polar Verity Sense on the forearm and a Garmin Forerunner 55 on the wrist, all against a Polar H10.

Identical sensors at three placements removes brand from the equation entirely. Any difference is caused by position alone.

• At rest: everything agreed within 1 bpm. Your watch is fine for resting heart rate.
• Cycling warm-up: wrist devices showed limits of agreement of roughly ±10-19 bpm. The upper-arm unit had the lowest error.
• Graded treadmill: the forearm Verity Sense recorded a bias under 0.1 bpm with concordance of 0.997. Wrist devices sat at 0.85 or below.
• Burpees: every optical device fell below 0.50 concordance, and the wrist Garmin showed a bias of about 33 bpm.

The authors' conclusion is that proximal placements consistently outperform the wrist across all activities. Not for one brand. Consistently.

What actually fixes it

If you will wear a chest strap: the Polar H10

It measures electrical activity directly rather than inferring it optically, which removes the entire problem rather than mitigating it. It is what researchers use as their reference standard. It pairs to Bluetooth and ANT+ simultaneously, so it works with the watch you already own regardless of brand, and Polar states 400 hours on a replaceable coin cell.

If you will not: the Polar Verity Sense

Be honest with yourself here, because a strap in a drawer measures nothing. The Verity Sense is the armband that actually appeared in the study and recorded a bias under 0.1 bpm on the treadmill protocol. Worn on the forearm or upper arm it is genuinely strap-equivalent for steady work.

If you want the cheapest version of the same idea

The Wahoo TICKR FIT gets the sensor onto your forearm for less. It has no published validation of its own — you are buying the principle rather than the measured result.

This won't fix it if…

Your training is explosive or stop-start. During burpees every optical device tested fell below 0.50 concordance, armbands included. If you do CrossFit-style work, only a chest strap will serve you. There is no optical product at any price that solves this.

Your watch is genuinely broken. If the reading is erratic even at rest, sitting still, that is not the placement problem described here. Check the sensor window for dirt and contact support.

You only care about resting heart rate and overnight trends. All devices agreed within 1 bpm at rest. Buy nothing.

You expect it to change your training. More accurate heart rate is more accurate data. Whether that makes you faster depends entirely on what you do with it.