Comparison of Cervical Flexor Muscles Thickness During Cranial-Cervical Flexor Exercise According to Pressure Levels and Eye Directions in Healthy Subjects

Purpose: The purpose of this study is to investigate differences of cervical flexor muscle thickness (i.e., sternocleidomastoid muscle and deep cervical flexor muscles) depending on levels of pressure bio-feedback unit and eye directions during cranial-cervical flexor exercise in healthy subjects. Methods: A total of 30 subjects (12 males and 18 females) who had no medical history related to musculoskeletal and neurological disorders were enrolled in this study. They were instructed to perform cranial-cervical flexion exercise with adjustment of five different pressures (i.e., 22 mmHg, 24 mmHg, 26 mmHg, 28 mmHg, and 30 mmHg) using a pressure biofeedback unit, according to three different eye directions (i.e., $0^{\circ}$, $20^{\circ}C$, and $40^{\circ}C$). Muscle thickness of sternocleidomastoid muscle and deep cervical flexor muscles was measured according to pressure levels and eye directions using ultrasonography. Results: In results of muscle thickness in sternocleidomastoid muscle and deep cervical flexor muscles, the thickness of those muscles was gradually increased compared to the baseline pressure level (22 mmHg), as levels in the pressure biofeedback unit during cranial-cervical flexion exercise were increasing. In addition, at the same pressure levels, muscle thickness was increased depending on ascending eye direction. Conclusion: Our findings showed that muscle thickness of sternocleidomastoid muscle and deep cervical flexor muscles was generally increased during cranial-cervical flexion exercise, according to increase of eye directions and pressure levels. Therefore, we suggested that lower eye direction could induce more effective muscle activity than the upper eye direction in the same environment during cranial-cervical flexion exercise.