To evaluate the normal thickness of the thyrohyoid muscle, which is one of the key muscles related to swallowing, by ultrasonography.

The thickness of the left and right thyrohyoid muscles was measured in normal male and female adults ranging in age from 20 to 79 years by ultrasonography. The groups were classified according to age as follows: subjects ranging in age from 20 to 39 years were classified into group A, subjects ranging in age from 40 to 59 years were classified into group B, and subjects ranging in age from 60 to 79 years were classified into group C. The measurement level was the line that joins the upper tip of the superior thyroid notch and the oblique line of the thyroid cartilage. Also, a correlation with the thyrohyoid muscle was investigated by collecting information regarding height, weight, body mass index (BMI), age, and gender of subjects in the healthy group.

The number of subjects in each group was as follows: group A (n=82), group B (n=62), and group C (n=60). Also, the thicknesses of the left and right muscles were 2.72±0.65 mm and 2.87±0.76 mm in group A, 2.83±0.61 mm and 2.93±0.67 mm in group B, and 2.59±054 mm and 2.73±0.55 mm in group C, respectively. Thyrohyoid muscle had a correlation with height, weight, and BMI. The thickness of the left and right thyrohyoid muscles was greater in male subjects than in female subjects and the right side muscle was thicker than the left side muscle.

The average thickness of the left and right thyrohyoid muscles was 3.20±0.54 mm in male subjects and 2.34±0.37 mm in female subjects. The thickness of the thyrohyoid muscle was positively correlated with height, weight, and BMI, and the thyrohyoid muscle was thicker in male subjects than in female subjects and the right side muscle was thicker than the left side muscle.

To identify the differences in the movement of the hyoid bone and the vocal cord with and without electrical stimulation in normal subjects.

Two-dimensional motion analysis using a videofluoroscopic swallowing study with and without electrical stimulation was performed. Surface electrical stimulation was applied during swallowing using electrodes placed at three different locations on each subject. All subjects were analyzed three times using the following electrode placements: with one pair of electrodes on the suprahyoid muscles and a second pair on the infrahyoid muscles (SI); with placement of the electrode pairs on only the infrahyoid muscles (IO); and with the electrode pairs placed vertically on the suprahyoid and infrahyoid muscles (SIV).

The main outcomes of this study demonstrated an initial downward displacement as well as different movements of the hyoid bone with the three electrode placements used for electrical stimulation. The initial positions of the hyoid bone with the SI and IO placements resulted in an inferior and anterior displaced position. During swallowing, the hyoid bone moved in a more superior and less anterior direction, resulting in almost the same peak position compared with no electrical stimulation.

These results demonstrate that electrical stimulation caused an initial depression of the hyoid bone, which had nearly the same peak position during swallowing. Electrical stimulation during swallowing was not dependent on the position of the electrode on the neck, such as on the infrahyoid or on both the suprahyoid and infrahyoid muscles.