# The Impact of Contact Force on Signal Quality Indices in Photoplethysmography Measurements
## 5 Discussion
Still, it may not be the best for peripheral perfusion. These findings are crucial for practical applications, such as monitoring the peripheral perfusion and SpO\({}_{2}\) measurement in critically ill patients, where an optimal \(P_{SQI}\) could be more relevant. While detecting cardiovascular abnormalities, continuous blood pressure estimation, and respiratory rate estimation, adjusting the CF within the range of 0.4 to 0.6 could be essential to obtain high-quality and accurate signals. The results highlight the importance of adjusting and validating the sensor placement on the skin as insufficient and excessive CF can negatively affect or suppress the PPG signal capture [38; 39]. A comparison among multiple SQIs reveals the complexity of assessing the quality of PPG signals. It underscores the importance of using multiple SQIs for accurate analysis and reducing the risk of misdiagnosis. SQIs such as \(P_{SQI}\), \(S_{SQI}\), \(K_{SQI}\), \(E_{SQI}\), \(R_{SQI}\), and \(Z_{SQI}\) offer varied perspectives on signal quality, addressing time-domain characteristics, waveform morphology, and aspects in the frequency domain. This comprehensive approach is essential to ensure the reliability of the monitoring systems in clinical settings. The analyzed SQIs, especially when combined, can comprehensively evaluate the quality of PPG signals regarding reliability, sensitivity, and error minimization. The appropriate use of these indices can significantly optimize cardiovascular monitoring and physiological research. The state of the art has focused on determining the optimal universal CF values using absolute values [26; 30; 31; 32; 33; 62; 63; 64]. However, it is not practical to determine the optimal CF values that apply to the entire population based on the absolute CF alone. The viscoelastic properties of human tissue play a crucial role in the interaction between the PPG probe and the skin. This means that its response to pressure can vary with time and the intensity of the force applied. This variability can significantly affect the quality of the PPG signal as the CF between the probe and the skin must be firm enough to capture pulsations without distorting due to excessive tissue compression. Furthermore, the biomechanical differences between individuals and measurement points are also important factors. Each person has a unique skin and subcutaneous tissue structure, varying in elasticity, thickness, and response to mechanical stress. Furthermore, different measurement points on the body (such as the finger, wrist, or earlobe) have different biomechanical characteristics that can influence the quality of the PPG signal. These differences underscore the need for custom CF standardization and calibration, which could enable consistent measurements in a diverse population, considering factors such as age, ethnicity, health status, and body mass index. A limiting aspect of the study could lie in the selection of the size of the analysis windows. Different window sizes can capture varying dynamics in response to CF changes. As shown in Figure 6 and Table 7, using the \(P_{SQI}\) as an example, the window size could affect the results. Small or very small windows often result in low PI values, while larger windows better represent the PI of the signal. However, large windows can also hide the influence of artifacts caused by sudden changes in CF. A pulse-by-pulse analysis would be ideal, but this adds complexity to the algorithm, which must be evaluated based on the final application. A 10-s window was used for all the SQI analyses to ensure a balance between the precision in assessing the SQIs and the ability to closely observe the evolution of the SQIs in the face of variations in CF. This enables a broader and more detailed view of the trends. Moreover, in the SQIs comparison, the number of samples per CF region varies for each data record due to the total duration of the measurements and manual CF adjustments during the experiment.