Adaptive Hierarchical Classification for Human Activity Recognition Using Inertial Measurement Unit (IMU) Time-Series Data

Human Activity Recognition (HAR) based on Inertial Measurement Unit (IMU) has become increasingly important in health and fitness applications. These systems can continuously and cost-effectively monitor human activity, regardless of the surrounding environment. However, the current dominant trend in HAR uses black box flat classification (FC) methods, which lack interpretability and do not consider the natural hierarchical relationship between activity classes. Such systems often achieve greater accuracy at the cost of increased complexity and are not suitable for critical decision-making applications. This paper proposes an Adaptive Hierarchical Decision Tree (AHDT) HAR system that recognizes human activities based on IMU measurements along with quasi-stationary inclination feature extraction. The proposed system generates a global classifier that classifies human activities according to a tree taxonomy structure. This approach maintains interpretability while considering the fundamental signal data features embedded in the natural hierarchical representation of the activity classes. In addition to the commonly used flat classification accuracy measures, we applied modified hierarchical accuracy measures to assess the exclusive characteristics of hierarchical relationships between the classes. We used seven publicly available datasets to evaluate the proposed system and compared its performance with other tree-based classifiers, including Random Forest, Gradient Boosting, XGBoost, and AdaBoost classifiers. Our results demonstrated that the AHDT system significantly improves the recognition performance of fine-grained activities and offers a balance between lower complexity and higher interpretability. Overall, the proposed AHDT system provides an interpretable and practical approach to HAR that can be valuable in critical decision-making applications.