Contact pressure as influencing parameters for PPG sensors during physical activities

Photoplethysmographic (PPG) sensors are catching on in field of wearable sensors for the measurements of the heart rate, combining the advantages in costs, weight and size. Despite their potential, the accuracy of heart rate evaluation is affected by several influencing parameters, including the contact pressure between the skin and the sensor. Thus, a PPG sensor was tested in a cohort of 25 subjects at different speed of physical activity (i.e. 3, 6 and 8 km/h) and for different contact pressures (i.e. 20, 60 and 75 mmHg), with the aim of determining the optimal CP to produce a reliable signal during physical activity. Results show that the CP of 75 mmHg provides unreliable results at all speeds. The mean absolute percentage error presented its lower value at 3 km/h with a contact pressure of 20 mmHg while presented a value of 9.6% for a contact pressure of 60 mmHg and a speed of 6 km/h. Authors found that changes in the CP have a significant effect on PPG-HR signal quality. Although future studies on a larger cohort of subjects and for different wavelengths are still needed, this study could contribute a profitable indication to enhance accuracy of PPG-based wearable devices.