4. The midline sagittal plane of the tongue base was scanned with the transducer placed on the submental skin of the neck, between the hyoid bone and the symphysis of the mandible. The operator may tilt the transducer slightly aside to search the hyperechoic raphe of the tongue, which is the landmark of the midline septum of the tongue. The participant will be first instructed to remain still and silent during the measurements and to breathe normally and avoid tongue movement, swallowing or talking. The maximum distance between the submental skin and the dorsal surface of the tongue base, tongue base thickness (TBT eupneic), and subcutaneous fat thickness (SFT - eupneic), were recorded and measured on frozen ultrasound images at the end of expiration during eupneic breathing and during Muller's maneuver.