Beyond Mass Movement: Decoding Functional Recruitment Saturation and Neuromuscular Dominance in Facial Synkinesis via Dynamic Surface EMG
Ji Hae Lee, Dae Rock Kim, Jin Han Kim, Ji Seong Park, Woo Shik Kim, Jin KimBackground:
Facial synkinesis is traditionally viewed as a generalized "mass-movement" phenomenon. However, the physiological mechanism behind why increased voluntary effort paradoxically worsens symmetry remains poorly understood. We aimed to quantify dynamic neuromuscular remodeling and identify objective predictors of patient-reported distress using multi-channel dynamic surface electromyography (dsEMG).
Methods:
Fifty-three patients with post-paralytic facial synkinesis (mean age, 51.57 ± 13.11 years) following peripheral facial nerve paralysis were evaluated between March 2025 and August 2025. The study followed the STROBE guidelines. A five-channel dsEMG protocol was used to assess the Frontalis, Orbicularis Oculi, Zygomaticus Major, Depressor Anguli Oris, and Platysma. We calculated the Recruitment Expansion Ratio (RER; peak/mean amplitude) and the Lip Amplitude Ratio (upper-lip elevator/lower-lip depressor). These metrics were correlated with the Synkinesis Assessment Questionnaire (SAQ) to identify hierarchical predictors of patient distress.
Results:
A significant "Weak Smile Paradox" was observed; Lip Amplitude Ratios were relatively preserved during submaximal effort but collapsed toward lower-lip dominance during maximal effort. This was driven by a profound depletion of the RER in affected elevators (1.38 vs. 2.64 on the normal side,
Conclusions:
Facial synkinesis is characterized by a "recruitment ceiling" in mid-facial elevators, where excess neural drive is diverted into synkinetic pathways. The ΔLip Ratio and functional saturation are critical determinants of recovery. These findings advocate for a paradigm shift from muscle strengthening to efficiency-based rehabilitation and provide a electrophysiological roadmap for precision chemodenervation and selective neurectomy.