## 4 Discussion  
### Activity Pattern of the Muscles  
The major contribution of this study was to estimate the sequential activation pattern of the many muscles involved in swallowing based on the dynamics of swallowing, which is hard with conventional methods. In the same manner, the estimated activation pattern can be divided into three phases, corresponding to the sequence of events in swallowing. In the first time period, \(P_{\text{s}}\)-\(P_{1}\), the later oral phase, most of the suprahyoid muscles and a part of middle pharyngeal constructor muscles are activated to elevate the hyoid bone to push the tongue against the hard palate to hold bolus. The result indicates that PMH, AMH, GH, AD, PD, and SH contribute to elevating the hyoid bone and, particularly, PD and SH make significant effort. Subsequently, in the second period, \(P_{1}\)-\(P_{2}\), the early pharyngeal phase, those muscles are more activated to move the hyoid bone anteriorly and superiorly, in which GH and AD show a remarkable contribution to the anterior movement. In addition, TH, SP, and PT1 are activated to raise thyroid cartilage slightly behind the hyoid bone. In the third period, \(P_{2}\)-\(P_{\text{e}}\), the later pharyngeal phase, all the muscles tend to relax, returning the hyoid bone and thyroid cartilage to resting positions. During the pharyngeal phase, the larynx rises along with the antero-superior movements of the hyoid bone and thyroid cartilage, and the epiglottis folds backward to cover the entrance of the larynx to keep bolus from entering windpipe. At the same time, the pharynx would extend to transport bolus into the esophagus.  
To estimate the muscle activity, electromyography (EMG) was conventionally used as a reliable technique. In particular, fine wire EMG was widely used to evaluate swallowing-related muscles individually and to investigate sequential muscle activation patterns [43; 44; 45; 46; 47; 48]. In these previous studies, activities of typical muscles involved in swallowing were investigated, but not the deep muscles, which are difficult to approach by wire electrode. The following findings were obtained:  
Figure 8: Ratios and directions of maximum force of each muscle. (**a**) Anterior-posterior, (**b**) superior-inferior.  
Some studies have indicated that activation of AD, GH, MH, and SN varied with food characteristics and that there were no obvious consistent activation patterns [43; 44; 45]. However, a more recent study suggested that there was a typical sequence of muscle activation [46]. That is, the masseter (MA) is first, the next is GH and AD, and the last is SN.
* SN was rarely activated or showed quite low values in comparison with the other muscles [43; 46].
* The onset of muscle activation of MH before that of TH was significant [47].
* Normal activity of CP exhibits an inhibition phase in swallowing: Preswallowing and rebound bursts across the pause [48].  
Similar findings were also observed in this study. That is, GH and AD are activated at an early stage in addition to MH (AMH, PMH), PD, and SH to move the hyoid bone anteriorly and superiorly. The onset of MH (AMH, PMH) precedes the activation of TH, which mainly contributes to the movement of thyroid cartilage, and the activity of SN is quite low. Besides, CP (CP1, CP2) and the lower region of PT (PT3, PT4) slightly exhibited twice activation. However, note that the number of muscles that can be measured by conventional methods are limited, unlike our approach.