Key result
PPG sensor accuracy peaks at 60 mmHg pressure but degrades with speed and darker skin.
Why the study?
The measurement accuracy of photoplethysmographic sensors and factors interfering with their performance, such as contact pressure, activity level, and skin tone, are often neglected.
How do contact pressure, activity level, and skin tone affect the measurement accuracy of PPG sensors compared to a cardiac belt?
How do contact pressure, activity level, and skin tone affect the measurement accuracy of PPG sensors compared to a cardiac belt?
PPG sensor accuracy is significantly influenced by contact pressure, activity level, and skin tone, but optimizing contact pressure can improve accuracy to clinically acceptable levels.
Optimizing contact pressure may yield acceptable PPG accuracy; hypothesis-generating for activity and skin tone effects, warranting validation trials.
At present photoplethysmographic (PPG) sensors are rapidly spreading thanks to their multiple advantages: they are small, economic, and they can provide information on multidomain quantities related to cardiovascular health. For these reasons, research in this field is very active. Nonetheless, the measurement accuracy of this type of sensor and the factors that can interfere with their metrological performance are often neglected. The contact pressure between sensor and skin, the level of the activity performed by the subject, and also her/his skin tone are some of the quantities needing attention. The aim of this paper is to compare the measurement accuracy of a PPG sensor in relation to both contact pressure (20,60, and 75 mmHg) and walking/running speed (3,6, and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8 ~km / h</tex>) - using a cardiac belt as gold standard. Moreover, preliminary tests on dark skin subjects are performed to evaluate also the impact of this factor. The results show that the best performance is given at 60 mmHg and the uncertainty increases with activity level (passing from 8.2 bpm to 28.2 bpm when speed passes from 3 to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8 ~km / h</tex>, coverage factor <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k=2</tex>). Furthermore, it can be noticed that darker skin tone deteriorates the signal quality, increasing the mean absolute percentage error to values above 10% for speeds above <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3 ~km / h</tex>. Plus, by considering the subjective optimal contact pressure at each speed, it is possible to improve significantly the accuracy and to reduce the mean absolute percentage error for moderate speed (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6 ~km / h</tex>) at 1.89 %, which is known to be acceptable also for a clinical use. These results can be useful for researchers and producers of PPG-based wearable sensors, catching up on innovations potentially improving the quality of the measurement results. This could open new possibilities in terms of applications and stakeholders. In future, different wavelengths could be explored to expand the technology applicability.
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Scardulla et al. (2025) studied this question. Photoplethysmographic (PPG) sensor vs. Cardiac belt was evaluated on Measurement accuracy (uncertainty and mean absolute percentage error). PPG sensor accuracy was optimal at 60 mmHg contact pressure, with uncertainty increasing from 8.2 to 28.2 bpm at higher speeds (3 to 8 km/h) and darker skin tones deteriorating signal quality.
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