Wrist versus chest strap: what the measurements actually show

Why this study design is unusually strong

Most accuracy comparisons pit different brands against each other, which confounds two variables at once: the sensor and where it sits. You cannot tell whether Brand A beat Brand B because its sensor is better or because its case fits differently.

The 2025 Sensors study removed that confound. It put three identical Whoop 4.0 units on each participant simultaneously — 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 chest strap. Same sensor, same person, same session, three positions.

Whatever differences show up between those three Whoop units are caused by placement alone. That is a clean result, and it is why it does more work across this site than any other single paper.

The physics, briefly

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

Motion. The wrist is a joint that moves constantly and independently of the rest of the arm. Every movement shifts the sensor relative to the tissue underneath.

Tissue. A wrist is tendon, bone and comparatively little muscle. Perfusion is lower than in the muscle bulk of a forearm or upper arm.

Contact. A wrist is not round, and a watch strap that is tight enough for a good optical seal is uncomfortable. Most people wear one looser than the sensor wants.

A chest strap sidesteps all of this by not being optical at all. It reads electrical activity through skin contact — the same signal an ECG measures — so motion artefacts and perfusion simply do not enter the picture in the same way.

What each intensity tells you about your own training

Rest: buy nothing

Every device exceeded 0.99 concordance with bias under 1 bpm. If your interest is resting heart rate and overnight trends, your existing watch is already accurate enough and upgrading is a waste of money. We would rather say that than sell you something.

Steady aerobic work: an armband closes the gap

On the modified Bruce treadmill protocol — the most running-relevant test — the forearm Verity Sense recorded a bias under 0.1 bpm and concordance of 0.997, essentially matching the chest strap. Wrist devices sat at 0.85 or below. For steady running, a forearm band is genuinely strap-equivalent.

Intervals: the wrist starts failing

At the cycling warm-up, wrist devices showed limits of agreement spanning roughly ±10-19 bpm. That is the lag people notice: heart rate climbing through an interval while the watch catches up thirty seconds later, then overshooting on the recovery.

Explosive work: nothing optical survives

During burpees, every optical device dropped below 0.50 concordance, and the wrist-worn Garmin showed a bias of about 33 bpm. Not a lag — a systematic error large enough to put you in an entirely wrong training zone.

If your training is CrossFit-style, circuit work or anything stop-start, there is currently no optical product at any price that measures it properly. That is a hardware limit, not a brand problem.

Limits of this study, stated plainly

• n = 28, young and healthy (mean age 23.75 ± 1.11 years). Not a general population.
• The reference was a Polar H10 chest strap, not simultaneous ECG. Strong, widely accepted, but not the absolute gold standard.
• It tested a Whoop 4.0 and a Forerunner 55 — neither is a current model.
• It measured heart rate only. It says nothing about sleep staging, HRV or distance.

None of those undermine the core finding, because the placement comparison is internally controlled — the three Whoop units were identical, so any placement effect holds regardless of generation.