Heterogeneous multi-modal sensor fusion with hybrid attention for exercise recognition.

Exercise adherence is a key component of digital behaviour change interventions for the self-management of musculoskeletal pain. Automated monitoring of exercise adherence requires sensors that can capture patients performing exercises and Machine Learning (ML) algorithms that can recognise exercises. In contrast to ambulatory activities that are recognisable with a wrist accelerometer data; exercises require multiple sensor modalities because of the complexity of movements and the settings involved. Exercise Recognition (ExR) pose many challenges to ML researchers due to the heterogeneity of the sensor modalities (e.g. image/video streams, wearables, pressure mats). We recently published MEx, a benchmark dataset for ExR, to promote the study of new and transferable HAR methods to improve ExR and benchmarked the state-of-the-art ML algorithms on 4 modalities. The results highlighted the need for fusion methods that unite the individual strengths of modalities. In this paper we explore fusion methods with focus on attention and propose a novel multi-modal hybrid attention fusion architecture mHAF for ExR. We achieve the best performance of 96.24% (F1-measure) with a modality combination of a pressure mat, a depth camera and an accelerometer on the thigh. mHAF significantly outperforms multiple baselines and the contribution of model components are verified with an ablation study. The benefits of attention fusion are clearly demonstrated by visualising attention weights; showing how mHAF learns feature importance and modality combinations suited for different exercise classes. We highlight the importance of improving deployability and minimising obtrusiveness by exploring the best performing 2 and 3 modality combinations