Beyond Mass Movement: Decoding Functional Recruitment Saturation and Neuromuscular Dominance in Facial Synkinesis via Dynamic Surface EMG

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).

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.

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, p < 0.001), indicating "Functional Saturation." Affected-side Lower Lip Dominance showed the strongest correlation with SAQ scores (r = -0.492, p = 0.0013), far outweighing Pathological Co-activation intensity or patient age.

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.