The principle is that bioelectricity will be released during myocardial contraction. At the point of heartbeat, the current is the largest, which is a pulse signal. This signal can reflect the weak voltage between people's arms. There are two metal parts on the fitness machine that are held by hand. When measuring the heart rate, hold the measuring metal block with both hands, and the pulse voltage signal of the heartbeat will be loaded into the measuring circuit. The filter removes clutter and the amplifier amplifies the voltage signal. There are timers and counters to count the number of pulses measured per unit time.