Estimation of heart rate from a chest-worn inertial measurement unit

Recently, wearable devices for measuring cardiovascular functions have attracted increasing research attention. However, these devices must use electrodes or other sensors attached to the human body, which makes the wearer uncomfortable for long-term use. In this study, we demonstrated the feasibility of using a chest-worn device, called eButton, to measure heart rate without any skin-attachments. Instead of measuring electrical or optical signals, we use a 9-axis Inertial Measurement Unit (IMU) which contains a 3-axis gyroscope, a 3-axis accelerometer, and a 3-axis magnetometer to detect the mechanical vibration of chest due to heart movement. The signal acquired is called the ballistocardiogram (BCG). From an experiment with ten normal research participants, we have found that the BCG waveform is more observable in the output of the gyroscope than the output of the accelerometer. An algorithm is developed to estimate the heart rate from the gyroscope-acquired BCG.