What is the common name for a device worn to measure heart rate during exercise?

The common name for a device worn to measure heart rate during exercise is a heart rate monitor.

Heart rate monitors can display a person’s pulse in real time or record it for later analysis. Athletes use them to track training intensity, estimate recovery and stay within personalized heart-rate zones. Common formats include chest straps, upper-arm bands, wrist devices and smartwatches.

Chest-strap monitors generally detect the heart’s electrical activity using electrodes, much like a simplified electrocardiogram. Wrist-based devices usually use photoplethysmography, or PPG: LEDs shine light into the skin, and sensors detect blood-volume changes associated with each heartbeat.

A fitness tracker may include a heart rate monitor, but the terms are not identical: fitness trackers often measure steps, movement, sleep and other metrics as well. A pulse oximeter primarily measures blood oxygen saturation and pulse, usually through a fingertip, so it is not the standard exercise-monitoring answer. The first wireless ECG heart-rate monitor was developed by Polar Electro in 1977.