One Model to Fit Them All: Universal IMU-based Human Activity Recognition with LLM-assisted Cross-dataset Representation

Human Activity Recognition (HAR) is essential for pervasive computing and intelligent interaction, with broad applications across various fields. However, there is still no one model capable of fitting various HAR datasets, severely limiting its applicability in practical scenarios. To address this, we propose oneHAR, an LLM-assisted universal IMU-based HAR system designed to achieve "one model to fit them all" --- just one model that can adapt to diverse HAR datasets without any dataset-specific operation. In particular, we propose Cross-Dataset neural network (CDNet) for the "one model," which models both the temporal context and spatial relationships of IMU data to capture cross-dataset representations, encompassing differences in device, participant, data collection position, and environment, etc. Additionally, we introduce LLM-driven data synthesis, which enhances the training process by generating virtual IMU data through three carefully designed strategies. Furthermore, LLM-assisted adaptive position processing optimizes the inference process by flexibly handling a variable combination of positional inputs. Our model demonstrates strong generalization across five public IMU-based HAR datasets, outperforming the best baselines by up to 46.9% in the unseen-dataset scenario, and 6.5% in the cross-dataset scenario.