Analyzing the Viscoelastic Correspondence Between Facial Electromyography and Facial Appearance During Smiling

Abstract

Quantitative modeling of facial expression dynamics is fundamental for human-centered sensing and interaction. While facial appearance provides a non-contact means of observation, assigning consistent intensity values remains challenging. Conversely, facial electromyography (fEMG) directly captures muscle activation but requires physical contact. This study investigates the dynamic correspondence between these modalities during smiling by evaluating whether an interpretable viscoelastic model can characterize their relationship. To overcome the challenge of fEMG electrodes occluding facial features, we utilized the bilateral symmetry of facial expressions, recording fEMG on one side of the face while capturing video of the contralateral side. Facial appearance was quantified using a comparison-based ranking method to derive ordinal smile intensities, while fEMG signals were decomposed into muscle synergies using non-negative matrix factorization. We hypothesized that the transformation from muscle contraction to visible tissue deformation follows viscoelastic principles. Specifically, we employed the standard linear solid (SLS) model to account for temporal dynamics and compared its performance against a static linear baseline. Experimental results from natural smiles elicited by comedic stimuli revealed strong correlations between synergy activation and smile intensity, with facial appearance consistently lagging behind muscle activity. The SLS model outperformed the linear baseline for the majority of participants, more accurately capturing delayed onsets and gradual relaxation phases. These findings suggest that incorporating viscosity provides a physically interpretable explanation of expression manifestation. This framework establishes an explainable bridge between physiological activity and visual dynamics, offering a foundation for more robust facial sensing technologies.

Keywords: Facial Expression, Smile Intensity, fEMG, SLS model, Viscoelastic Correspondence

DOI: 10.54941/ahfe1008111