A novel flexible wearable sensor for estimating joint-angles

To circumvent current limitations of wearable sensors that can be used to assess and monitor joint movements, we developed an accurate, low-cost, flexible wearable sensor comprising a retractable reel, a string, and a potentiometer. This sensor is intended to estimate joint angles in correlation with the amount of skin stretch measured by the change in the length of the string. In this study, we validated the accuracy of the sensor against an optoelectronic system in estimating knee joint angles using a dataset obtained from 9 healthy individuals while they walk and run on a treadmill. By our simple calibration procedure, we could convert the voltage output of the potentiometer to the amount of skin stretch as subjects flex or extend their knee. Then, we incorporated a simple polynomial fitting model to estimate the joint angle. Using a leave-one-subject-out cross validation, we achieved an average root mean square error of 4.51 degrees. This work demonstrates the accuracy of the proposed system in estimating knee joint angles and provides the basis to develop more complex systems to assess and monitor joints having more degrees of freedom. We believe that our novel low-cost wearable sensing technology has great potential to enable joint kinematic monitoring in ambulatory settings.