Avaliação de representações simbólicas de movimentos articulares humanos com SAX e SFA a partir de dados de acelerômetro

Abstract

The monitoring and analysis of human joint movements using inertial sensors have become accessible and effective alternatives in fields such as rehabilitation, ergonomics, and telemedicine. However, the high dimensionality and individual variability of these signals hinder direct interpretation, calling for representation strategies that reduce complexity without discarding relevant information. This study investigates the symbolic representation methods SAX and SFA applied to time series acquired by smartphone accelerometers from eight healthy participants during the controlled execution of elbow flexion/extension, shoulder abduction/adduction, and shoulder circumduction. The experimental protocol comprised three bouts of ten cycles for each movement, with standardization and supervision by a physical education professional. We evaluated different symbolic configurations by varying the alphabet size (3, 7, and 10 symbols) and the strategies for generating the reference word, and performed a quantitative analysis based on the Hamming distance between symbolic sequences. Statistical significance was assessed using the Friedman test with Nemenyi post-hoc multiple comparisons. Results show that Standard SFA yielded, in most configurations, lower Hamming distances and greater between-subject homogeneity—especially with reduced alphabets (três and sete symbols)—confirming its suitability for more regular movements such as elbow flexion/extension and shoulder abduction/adduction. For shoulder circumduction, characterized by higher inter- and intra-subject variability, Standard SAX performed better with smaller alphabets, underscoring the importance of selecting the method according to movement complexity. Overall, the symbolic approaches preserved salient signal patterns at low computational cost, supporting their potential for deployment in rehabilitation systems and telemedicine e-health platforms. This work contributes a detailed comparative analysis between SAX and SFA across different movements and conditions, highlighting their limitations, strengths, and scenarios of best applicability.