This single-group interventional study investigates the safety and physiologic effects of functional electrical stimulation (FES) applied to the tongue and submental muscles in adults at high risk for sleep apnea. Participants will receive five stimulation frequencies (15, 25, 35, 45, and 75 Hz) in randomized order during a single visit. Ultrasound and tongue pressure (IOPI) measurements, as well as subjective assessments of discomfort and perceived effects, will be conducted before, during, and after stimulation. The primary objective is to evaluate changes in muscle thickness, length, and tongue strength across different stimulation frequencies. Secondary objectives include assessing participant discomfort, safety, and subjective treatment effects.

Reduced tone and weakness of the genioglossus muscle are critical factors in the pathophysiology of obstructive sleep apnea (OSA). In addition to the genioglossus, other upper airway-related muscles, including the thyrohyoid and additional infrahyoid muscles, contribute to airway stability. These muscles differ in anatomical depth from the skin surface and in muscle thickness, which necessitates differentiated electrical stimulation approaches to ensure selective activation and safety.This clinical trial is designed to evaluate the safety and efficacy of functional electrical stimulation (FES) applied to the genioglossus muscle with modulation of stimulation frequency. The study aims to determine whether frequency adjustment influences muscle morphology and functional outcomes.The primary objective is to assess frequency-dependent changes in muscle length and thickness using ultrasonography, as well as differences in tongue strength measured by the Iowa Oral Performance Instrument (IOPI) before and after electrical stimulation.The secondary objective is to evaluate discomfort, safety, and subjective treatment effects through questionnaires administered before and after the electrical stimulation intervention.

Inclusion Criteria:

• Adults aged 19 years or older
• STOP questionnaire score ≥ 2
• Able to read and write in Korean
• Voluntarily provides written informed consent to participate in the study

Exclusion Criteria:

• Refusal to undergo study procedures or failure to provide informed consent
• Cognitive impairment or psychiatric disorders that interfere with the ability to follow study instructions
• Pregnant or breastfeeding women
• Known allergy or hypersensitivity to electrical stimulation pads or related materials (e.g., silicone, polypropylene, polyethylene)
• Skin conditions preventing application of electrical stimulation (e.g., hypersensitive skin)
• Medical conditions considered unsuitable for participation by the investigator (e.g., epilepsy, malignant tumor, high fever, diabetes mellitus, use of anticoagulants, phlebitis, thrombophlebitis, hemophilia)
• Infectious disease or local conditions at the stimulation site (e.g., inflammation, wounds, abscess, edema)
• Known cardiac disease or presence of a cardiac pacemaker or implanted electrical stimulation device
• History of surgery involving the submental area or upper airway region

This is an open-label study. Both participants and investigators are aware that functional electrical stimulation is administered during the study session. No sham stimulation or separate comparison group is included.