Wearable Ballistocardiography for Unobtrusive Respiratory and Heart Rate Monitoring

Respiratory and heart rate are accessible physiological signatures of human vitality. Unobtrusive insole ballistocardiography using highly sensitive piezoelectric sensors promises a wearable measurement system to monitor health throughout the day. In this work, the 3D-printed ferroelectret insole is worn by three participants while standing in upright posture. The beat-to-beat detection implements a continuous wavelet transform to filter the main frequency bands (6 to 8 Hz) in the time-frequency domain, offering accurate heartbeat timings. Amplitude and frequency variabilities in the heartbeat signal are evaluated separately to reveal the underlying respiration. Both instantaneous characteristics can be fused to obtain a robust breath-to-breath measurement, mitigating body movements and non-respiratory arrhythmia. The measured heart rate aligns well with the reference from a commercial chest strap, and the heart rate variability with ± 4 beats per minute is consistent with the literature values of respiratory sinus arrhythmia. In conclusion, the wearable insole concept with the presented signal processing chain qualifies for remote health monitoring.