Heart rate monitor

Early models consisted of a monitoring box with a set of electrode leads which attached to the chest.

The first wireless EKG heart rate monitor was invented in 1977 by Polar Electro as a training aid for the Finnish National Cross Country Ski team.

As "intensity training" became a popular concept in athletic circles in the mid-80s, retail sales of wireless personal heart monitors started in 1983.

[2] ECG (Electrocardiography) sensors measure the bio-potential generated by electrical signals that control the expansion and contraction of heart chambers, typically implemented in medical devices.

PPG (Photoplethysmography) sensors use light-based technology to measure the blood volume controlled by the heart's pumping action.

Newer technology prevents one user's receiver from using signals from other nearby transmitters (known as cross-talk interference) or eavesdropping.

[3] In recent years, it has been common for smartwatches to include heart rate monitors, which has greatly increased in popularity.

Smartwatch displaying a reading of 92 beats per minute
X-ray image of a chest strap (left: frontal view; right: side view). Visible is the circuit board, the antenna for data transfer, the battery and the connections to the electrodes in the adjoining belt at picture top and bottom.
Welsh Government video: a smart phone heart rate monitor, 2016