## 4 Discussion
### Limitations  
The findings of this study considering the dynamics of swallowing were consistent with the results of previous studies based on physiological measurement and structural analysis. However, there were some limitations to this study. First, the analysis was of only one healthy young volunteer. Even though the movement of the hyoid bone of the volunteer is a typical triangular trajectory, to generalize the results it would be better to increase the number of volunteers of various ages and genders because hyoid movement is slightly varied among individuals. In particular, to investigate the effects of aging is important, as muscle strength weakens with age and swallowing disorders are common in the elderly. Second, in relation to the first, we developed the musculoskeletal model using the CT data of the young volunteer, but the skeleton geometry also differs among individuals and that difference influences the dynamics calculation. Thus, a deformable standard skeleton model is required to represent different genders and ages. Besides, to analyze the sensitivity of subject-specific modeling to the uncertainties in the identification of muscle attachment site, muscle-tendon parameters and stiffness parameters of soft tissues are interesting because the geometry of head and neck parts is varied among individuals. Third, there are some simplifications. The effect of tongue deformation done by tongue muscles was not accurately simulated but it actually affects the kinematics and posture of the hyoid bone. Similarly, linear elastic and viscous behaviors were assumed for soft tissues surrounding the hyoid bone and thyroid cartilage. Thus, nonhomogeneity and anisotropy of their restrictions were not taken into account. In addition, the inertial effect of the muscles on the movements of the hyoid and thyroid was not considered, because Hill-type muscle model does not include muscle mass. However, the inertia of the attached muscles cannot be ignored as their masses are actually larger than that of the hyoid bone and thyroid. To overcome these simplifications, a 3D, finite element model of muscle and soft tissue should be constructed based on physiological literature. The 3D musculoskeletal model would also enable us to take into account the interaction between muscles and the effect of gravity on muscle deformation. Fourth, there are some assumptions and hypothesis due to the lack of physiological data. Most of the PCSA of the muscles derived from cadaver specimens was used as that of the young volunteer and the PCSA of the pharyngeal constrictor muscles were estimated from a 3D anatomical model. In addition, the estimated muscle forces in this study may not be a unique combination, because the estimated result depended on the objective function of the optimization process. Although the objective function considered physiological properties of muscles and is widely accepted in musculoskeletal studies, its general applicability is not fully accepted. Thus, the results of this study should be validated further by physiological manner. To investigate muscle activities in swallowing is difficult because swallowing is a complex behavior involving volitional and reflexive activities of many nerves and muscles. In general, EMG measurements are recognized as a reliable method to estimate muscle activities. At the same time, however, there are limitations in measuring deep muscles in the neck even with fine wire electrodes. Further, EMG signals do not necessarily reflect muscle strength. In comparing with EMG measurement, the advantage of the proposed method is that it is able to estimate muscle strengths that can satisfy dynamic consistency parameters with the movement of the hyoid bone and thyroid cartilage calculated using VF images, and as the activities of deep muscles can be estimated. As a result, this technique is only one method to investigate individual muscles in swallowing. If the musculoskeletal analysis of swallowing were validated physiologically, this technique would be useful for swallowing rehabilitation.