A preliminary investigation of the effect of contact pressure on the accuracy of heart rate monitoring by wearable PPG wrist band

The increase of safety and health monitor of workers has become a crucial objective to prevent excessive physical workloads, injuries, accidents and errors. Heart rate (HR) is a very important physiological indicator which could properly describe the workers' physical status. Recently, wearable photoplethysmographic (PPG) wristband trackers have been utilized to measure HR without hindering normal gesture of workers. However, the quality of PPG signals is highly affected by human physical motions, resulting in a poor reliable HR estimation. Specifically, during different activities and gestures, PPG sensor contact pressures may have an impact on the quality of the heart rate signal. To approach the problem, an instrumented adjustable bracelet equipped with a PPG sensor and a load cell has been designed. A simple test protocol consisting of three different activities (i.e. walking, jogging and running) has been realized to test PPG signal quality. PPG sensor has been subjected to different skin-sensor contact interface conditions to take into account the effect of contact pressure. Preliminary results showed that a weak contact pressure results in a poor quality of PPG signal as the activity becomes more and more intense. These results suggest that the contact pressure exerted by the sensor affects both the arterial compliance and the arterial volume, providing different results.