Beat-wise comparison of in-ear photoplethysmography with finger pressure during walking

Aurora Rosato1 and Seraina Anne Dual2
11 Department of Biomedical Engineering and Health Systems, KTH Royal Institute of Technology, Stockholm, Sweden, 2KTH Royal Institute of Technology


Abstract

Continuous measurement of hemodynamic signals during walking remains a challenge, as signals include motion-induced hemodynamic effects and measurement-based artifacts. In-ear photoplethysmography (PPG) has recently attracted increasing interest, driven by its strategic physiological location resulting in widespread integration with consumer applications; however, walking distorts signal quality. We aim to investigate the morphological beat-wise effect on ear-PPG and finger pressure (fiAP) pulse waveforms during heart-paced walking, stepping at a variable delay from the R-wave of the cardiac cycle. Nine healthy subjects (43±22 years) participated in the study. Participants walked in synch with their hearts for a total of 15 minutes. We measured morphology, including normalized amplitude and area under the curve (AUC), using a linear mixed-effects model and ANOVA, controlling for HR, inter-subject variability, step timing, and sensor location. We observed a strong delay-dependent modulation of waveform morphology in ear-PPG and fiAP, indicating presence of motion-induced hemodynamic effects. fiAP showed a progressive increase in AUC at higher delays. For ear-PPG, AUC was most strongly reduced when stepping at 40-50\% delay and recovered later, indicating a U-shaped behaviour across delays. Amplitude was mostly modulated in the ear and not in the finger. Our results suggest that in-ear PPG exhibits stronger step timing-dependent modulation compared to the finger, suggesting higher sensitivity to motion-related hemodynamic effects on the cardiovascular system.