Electromyographic activity of the orbicularis oris muscle in children with and without lip competence: A cross-sectional study

BACKGROUND: The "equilibrium theory" posits that the tongue and perioral muscles, including the orbicularis oris (OO) muscle, function synergistically to maintain balanced tooth positioning. Surface electromyography (sEMG) is a valuable, nonivasive method for assessing muscle activity. However, previous electromyographic (EMG) studies comparing lip muscle activity in children with and without lip competence (LC) have yielded inconsistent results. Therefore, further research is needed to clarify OO muscle activity patterns in this population. OBJECTIVES: The aim of the study was to evaluate the EMG activity of the superior (SOO) and inferior orbicularis oris (IOO) muscles in children with and without LC. MATERIAL AND METHODS: The sample comprised 30 children with lip incompetence (LI) (mean age 9.46 ± 1.76 years) and 30 children with LC (mean age 8.85 ± 1.52 years). Electromyographic recordings of the SOO and IOO muscles were obtained using a DAB Bluetooth Instrument (Zebris Medical GmbH, Isny im Allgäu, Germany) at clinical rest, during saliva swallowing, lip protrusion ("kissing" position), lip compression, and while articulating the syllables /pa/, /ba/, and /ma/. Statistical analyses were performed using Stata v. 11.0 (StataCorp, College Station, USA). The level of significance was set at p < 0.05. RESULTS: Electromyographic activity of the SOO and IOO muscles during saliva swallowing (p < 0.001, adjusted p (padj) = 0.002) and lip compression (p = 0.001, padj = 0.013 for SOO; p < 0.001, padj = 0.005 for IOO) was significantly greater in children with LI compared to those with LC. Similar EMG activity at rest and during speech production was observed in children with and without LC. CONCLUSIONS: Children with LI demonstrate increased SOO and IOO muscle activity during saliva swallowing and lip compression, suggesting greater muscular effort is required to achieve lip seal. This increased activity may disturb the muscular force balance essential for proper maxillofacial growth and could contribute to the development of malocclusion.