2020 年 86 巻 888 号 p. 20-00123
During cervical lateral bending, cervical axial rotation occurs simultaneously; this phenomenon is known as coupled motion. To date, many studies have been conducted on coupled motion. However, it remains unknown how protagonists of cervical lateral bending affect lateral bending and coupled axial rotation. Therefore, in this study, we aim to construct a multibody model of the neck comprising the bones, muscles, and ligaments (including the intervertebral disc) and to analyze its effects on lateral bending and coupled axial rotation when one of the main protagonists is removed. The bone model, which included cervical vertebrae C1 to C7, was bound by 11 types of ligament models and intervertebral disc models constructed from 12 spring models. The sternocleidomastoid (SC), anterior scalene (AS), and levator scapulae (LS) were set as protagonist muscles of the right lateral bending of the neck, and the trapezius was set as the antagonist muscle. The condition under which all muscle models operate was set as the normal condition, and the condition under which one protagonist was removed was set as the removal condition. Under the normal condition, the right lateral bending angle was more than 30° and coupled axial rotation angle was 2.1° on right rotation. Compared with the normal condition, no changes were observed in the right lateral bending angle under the SC removal condition and AS removal condition, whereas the right lateral bending angle markedly decreased under the LS removal condition. Under the SC removal condition, coupled axial rotation resulted in a right rotation of 17.7° and right axial rotation markedly increased. However, under the LS removal condition, coupled axial rotation resulted in a left rotation of 11.2°, which showed a change to left axial rotation. These results indicate that the LS plays a key role in cervical lateral bending, whereas the SC and AS play ancillary roles. Furthermore, the SC and AS inhibited right coupled axial rotation and the LS inhibited left coupled axial rotation.