Dynamic change in hyoid muscle length associated with trajectory of hyoid bone during swallowing: analysis using 320-row area detector computed tomography.

Research on muscle activation patterns during swallowing has been limited. Newly developed 320-row area detector computed tomography (320-ADCT) has excellent spatial and temporal resolution, which facilitates identification of laryngopharyngeal structures and quantitative kinematic analysis of pharyngeal swallowing. We investigated muscle activity patterns by observing the changes in length of hyoid muscles. 320-ADCT was performed in 26 healthy males while swallowing. The following parameters were analyzed three-dimensionally: 1) origins and insertions of the stylohyoid, anterior and posterior digastric, mylohyoid, geniohyoid, and thyrohyoid muscles; and 2) movement of the hyoid bone. The stylohyoid, posterior digastric, and mylohyoid muscles began to shorten simultaneously during the initial stage of swallowing. The shortening of these muscles occurred during the upward movement of the hyoid bone. Subsequently, the geniohyoid, thyrohyoid, and anterior digastric muscles began to shorten, synchronizing with the forward movement of the hyoid bone. A significant correlation was observed between the shortened muscle lengths of the stylohyoid, posterior digastric, and mylohyoid muscles and the upward movement of the hyoid bone (r = 0.45-0.65). A correlation was also observed between the shortened muscle length of the geniohyoid muscle and the forward movement of the hyoid bone (r = 0.61). In this study, the sequence of muscle activity during pharyngeal swallowing remained constant. Serial shortening of the hyoid muscles influenced the trajectory of the hyoid bone. The stylohyoid, posterior digastric, and mylohyoid muscles initiated the swallowing reflex and contributed to upward movement of the hyoid bone. The geniohyoid is a key muscle in the forward movement of the hyoid bone.