The purpose of this observational is to methodologically assess the standardisation and reliability of a protocol of ultrasound lingual markers, including tongue shear-wave elastography, attenuation indexes, and echo intensity in adult healthy subjects.

The main goals are to create a standardized protocol and its intra- et inter-rater reliability, in order to gather some data in a non-pathological population. This protocol will be used in further studies in the context of tongue fat accumulation, such as in obstructive sleep apnea syndrome.

Participants will be lying in supine position and after basic descriptive data collection (height, weight, neck circumference), submental ultrasound will be performed on them. This procedure is non-invasive, painless, safe (ultrasound does not involve ionising radiation) for the healthy participants.

Scientific literature is reporting increasing evidence that fat accumulation in the tongue and oropharyngeal region is linked to cardiovascular disorders in people with obesity. In this context, fat accumulation on the tongue is a contributing factor to Obstructive Sleep Apnea syndrome (OSA). The presence of OSA is a significant risk factor for the development of cardiovascular disease and is directly correlated with overall adiposity, while the percentage of neck fat relative to the abdomen is correlated with the severity of OSA in obese individuals, highlighting the relevance of studying body fat distribution. The links between increased tongue size, neck and oropharyngeal adiposity, BMI, and sleep-disordered breathing have been supported by magnetic resonance imaging (MRI) and cone beam computed tomography studies and are now well documented. It is interesting to note that OSA also affects people who are not obese, highlighting the relevance of studying its risk factors. Excessive lipid accumulation in peripheral tissues therefore appears to be a key feature of many metabolic diseases. Thus, early detection of excessive lipid accumulation in tissues and therapeutic monitoring are key to prevention and individualised, patient-centred care. Ultrasound has recently been used to establish a link between oropharyngeal fat and OSA: studies showed that changes in the dynamic properties of the tongue muscles measured by elastography, or the size and volume of the tongue, could be a biomarker for screening for OSA in adults. Shear wave elastography (SWE) is already used for fat accumulation in various organs, for example in liver or kidney diseases, and some authors have begun to work on the musculoskeletal system. This technique uses acoustic force to generate shear waves (which can propagate perpendicular to the ultrasound beam, causing transient tissue deformation that can be measured by the machine's module), providing a measure of the elastic properties of the tissue, which is reported as 'rigidity' or 'stiffness'. When muscles are infiltrated by lipid deposits, the histological content changes, leading to a modification of tissue properties. This ultrasound approach is a simple, non-irradiating and harmless technique that could be applied in the future therapeutic follow-up of patients with OSA.

Moreover, it is important to underscore that the lingual ultrasound markers analysed in this study are not solely indicative of fat content within the tongue. They are also likely to depict biomechanical characteristics of tongue tissues. Indeed, these markers can be influenced by various compositional and structural changes, including modifications in fat content, connective or fibrous tissue density, presence of edema or swelling... As such, variations in ultrasound-derived parameters may determine a range of tissue properties beyond adiposity, providing a more comprehensive insight into the biomechanical state of the tongue that is particularly involved in obstructive sleep apnea. This lingual ultrasound protocol could then be use in further studies by applying it in patients suffering from OSA.

Study objectives: The main objective of this study is to investigate ultrasound markers of fat accumulation in the tongue in healthy individuals, to assess the reliability and repeatability of the measurement, and to determine the distribution of tongue fat values in different regions of the tongue. The secondary objective will be to study the correlations of these values with the subjects' basic anthropometric data. The aim of obtaining values in healthy subjects will be to compare them with a future study on a population suffering from OSA.

Inclusion Criteria:

• Healthy adults aged 18-45 years
• Able to understand and provide informed consent

Exclusion Criteria:

• Any orthopaedic, traumatic, or rheumatic conditions affecting the head or neck
• Presence of metabolic disorders (e.g., diabetes, metabolic syndrome)
• Presence of cardiovascular conditions
• Diagnosed sleep disorders or known sleep apnea syndrome, as evaluated by the Stop-Bang and Berlin questionnaires
• Consumption of alcohol or drugs within 48 hours prior to the experiment

Healthy subjects (aged 18-45), male and female, recruited from physiotherapy students at ISEK-HE2B and health students at VUB (Brussels Health Campus site in 1090 Jette) and their relatives.