A Convolutional Neural Network Approach to Classifying Activities Using Knee Instrumented Wearable Sensors

Wearable sensors permit convenient human activity data collection in diverse environments and collected data can be used to evaluate functional impairment or analyze recovery following surgical interventions such as knee replacement. Automated activity classification can be used for adding context to unscripted sessions for comparing identical tasks across subjects. In this study, twenty participants were instrumented with wearable inertial sensors placed above and below both knees while performing activities of daily living. Collected multivariate time series data were encoded as colour images and three convolutional neural networks were developed to classify activities into eleven classes. Performance was evaluated using twenty iterations of a leave-one-subject-out scheme. A first-stage classification model was able to differentiate static vs. dynamic activities almost perfectly and a second-stage model was able to further classify specific static activities performed with 99% accuracy. A separate second-stage model was developed to classify dynamic activities with 91% accuracy. Cycling and ascending/descending stairs were the most commonly confused activities. The current work has demonstrated that both static and dynamic activities of daily living can be classified using only leg instrumentation which is beneficial for applications studying knee performance in varying environments.