The anatomical structures in the head motion and tongue depressor measurement. The displacement indices were the maximum displacement (distance B) at the facial surface due to the head-up and -down motion and the maximum displacement (distance C) at the midline due to the head-left and -right motion. For the evaluation of the tongue depressor function, the distances from the hard palate to the apex of the tongue (distance D), from the hard palate to the anterior part of the dorsal surface of the tongue (distance E), from the hard palate to the posterior part of the dorsal surface of the tongue (distance F) and from the posterior pharyngeal wall to the root of the tongue at the uvula tip (distance G) were measured, respectively.