# The Impact of Contact Force on Signal Quality Indices in Photoplethysmography Measurements
###### Abstract  
Photoplethysmography (PPG) is widely used to assess cardiovascular health. Yet, its effectiveness is often hindered by external factors like contact force (CF), which significantly affects the accuracy and reliability of measurements. This study investigates how variations in the CF at the index fingertips influence six signal quality indices (SQIs)--including the perfusion index, skewness, kurtosis, entropy, zero-crossing rate, and relative power--using data from 11 healthy participants. Our analysis of normalized CF values reveals that lower CF ranges (0.2 to 0.4) may be optimal for extracting information about perfusion and blood flow. However, they may not be the best range to capture all the physiological details within the PPG pulse. In contrast, higher CF ranges (0.4 to 0.6) enable capturing more complex signals that could be physiologically representative. The findings underscore the necessity of considering viscoelastic tissue properties and individual biomechanical differences, advocating for both the normalization of CF for improved cross-subject comparison and personalized CF calibration to adapt PPG devices to diverse populations. These strategies ensure measurement reliability and consistency, thereby advancing the accuracy of cardiac and vascular assessments. Our study offers guidelines for adjusting the CF levels to balance signal detail and perfusion quality, customized to meet specific analytical requirements, with direct implications for both clinical and research environments. photoplethysmography; contact force; signal morphology; signal quality index; optimal contact force; signal calibration +
Footnote †: journal:  
represents the baseline blood volume level, affected by non-pulsatile tissue absorption and low-frequency variations associated with respiration, thermoregulation, and sympathetic activity. The PPG signal contains a wealth of information about the cardiovascular system, including waveform, amplitude, and timing characteristics [17]. The basic features are derived directly from the original PPG waveform, such as the anacrotic (rising) and catacrotic (falling) phases. These phases relate to the systole and diastole, respectively, with features like the dicrotic notch indicating healthy arterial compliance. The systolic amplitude reflects pulsatile blood volume changes and is linked to stroke volume and vascular distensibility. Pulse width, defined as the duration between the start and end of a pulse, provides insights into heart rate (HR), arterial elasticity, and vascular reactivity. The pulse area, measured as the total area under the PPG curve, can be segmented to provide a more detailed analysis, such as the \"inflexion point area ratio\" (IPA) [18; 19]. Combined features include indices like the perfusion index (PI), which represents the ratio between the pulsatile and non-pulsatile components of the signal and is used to assess peripheral perfusion and hypoperfusion states. Derivative features, such as the first derivative (VPG) and the second derivative (APG) of the PPG waveform, offer detailed temporal variations and can detect peaks and inflexion points that are difficult to identify in the original waveform. These derivatives are particularly useful for assessing cardiovascular health and have been correlated with ageing and other physiological changes [20]. PPG requires a light source and a photodetector (PD) to operate. The light source illuminates the tissue, and the photodetector senses the small variations in light intensity associated with the changes in perfusion in the capillary volume. As shown in Figure 1, PPG measurements can be performed in transmission and reflectance modes. The reflectance mode is more flexible and suitable for wearable devices due to its ability to utilize multiple wavelengths and its applicability to various body parts. This makes it ideal for non-invasive, long-term monitoring, enhancing the usability and effectiveness of wearable health monitoring devices [21; 22; 23].