Integration of Single-Foot Impedance Plethysmography and Ballistocardiography on a Shoe Insole for Robust Heartbeat Detection

Smart insoles are a promising tool for monitoring population health status in ambulatory settings. However, obtaining heart rate in these systems remains a challenge, as the most common techniques for measuring heart rate require direct contact with the skin. Ballistocardiography (BCG) has recently been shown to be a good alternative to overcome this drawback, as it is a cardiac mechanical signal that can be measured with a piezoelectric sensor embedded in a shoe insole without skin contact. However, a limitation of BCG is that it is sensitive to motion artifacts, especially while standing. On the other hand, impedance plethysmography (IPG) is a technique that has been shown to be effective in detecting the heartbeat in the foot also in insoles with better performance when standing rather than sitting, even through socks, as the weight of the body when standing favors a better contact which improves the measurement. This study explores the integration of the two approaches, BCG and IPG, in a shoe insole to improve the robustness of heartbeat detection. The results indicate a 10% improvement in heartbeat detection accuracy for both BCG and IPG when the subject is standing, and a 4% improvement when seated, compared to the single best method, BCG.