Wrist sensors read pulse from reflected light, which high frequency movement genuinely corrupts. Understanding why accuracy drops during intervals explains when to trust the reading and when to reach for a chest strap.

How optical heart rate works

A wrist sensor shines light into the skin and measures how much returns. Blood absorbs more light when a pulse of it passes through the vessels under the sensor, so the reflected signal rises and falls with each heartbeat. This technique, photoplethysmography, is cheap, comfortable, and suited to all day wear, which is why it dominates consumer wearables. Its weakness is built into the same mechanism that makes it convenient.

Why motion corrupts the signal

The light returning to the sensor changes not only with blood flow but with anything that disturbs the optical path. During vigorous movement the wrist tendons shift, the device slides against the skin, and ambient light leaks in. These produce changes in the reflected signal that look a great deal like a pulse but are not. The sensor is reading a genuinely contaminated signal, not simply failing to read a clean one, and that distinction matters.

Where accuracy breaks down

Steady state activity such as easy running tends to be handled well, because the motion is rhythmic and predictable enough to filter. The hard cases are rapid changes in intensity, interval sessions, weightlifting, and anything with irregular wrist movement. In these moments the algorithm can lock onto cadence instead of heartbeat, reporting a figure that tracks footstrike rather than pulse. The error is often largest exactly when accurate heart rate matters most.

What the algorithms do about it

Modern devices fuse the optical signal with motion data from the accelerometer, using the known movement to subtract its likely contribution from the light signal. This helps considerably but cannot fully recover information that motion has destroyed. The result is a steady improvement in difficult conditions rather than a solution, and performance still varies with fit, skin, and the specific activity.

When to reach for a chest strap

For sessions built around precise heart rate, intervals, threshold work, anything where a few beats change the prescription, a chest strap that measures the heart's electrical signal remains the more reliable choice. For all day trends, sleep, and steady efforts, optical sensing is usually good enough and far more convenient. Choosing between them is a question of how much the specific decision depends on accuracy, which is a recurring theme across this site.