MoPFormer: Motion-Primitive Transformer for Wearable-Sensor Activity Recognition

Hao Zhang, Zhan Zhuang, Xuehao Wang, Xiaodong Yang, Yu Zhang

Keywords: Human Activity Recognition, Wearable Sensors, Deep Learning

TL;DR: MoPFormer: Motion-Primitive Transformer for Wearable-Sensor Activity Recognition

Abstract: Human Activity Recognition (HAR) with wearable sensors is challenged by limited interpretability, which significantly impacts cross-dataset generalization. To address this challenge, we propose Motion-Primitive Transformer (MoPFormer), a novel self-supervised framework that enhances interpretability by tokenizing inertial measurement unit signals into semantically meaningful motion primitives and leverages a Transformer architecture to learn rich temporal representations. MoPFormer comprises two stages. The first stage is to partition multi-channel sensor streams into short segments and quantize them into discrete ``motion primitive'' codewords, while the second stage enriches those tokenized sequences through a context-aware embedding module and then processes them with a Transformer encoder. The proposed MoPFormer can be pre-trained using a masked motion-modeling objective that reconstructs missing primitives, enabling it to develop robust representations across diverse sensor configurations. Experiments on six HAR benchmarks demonstrate that MoPFormer not only outperforms state-of-the-art methods but also successfully generalizes across multiple datasets. More importantly, the learned motion primitives significantly enhance both interpretability and cross-dataset performance by capturing fundamental movement patterns that remain consistent across similar activities, regardless of dataset origin.

Primary Area: Applications (e.g., vision, language, speech and audio, Creative AI)

Submission Number: 17256