When chewing solid food, part of the bolus is propelled into the oropharynx before swallowing; this is named stage II transport (St2Tr). However, the tongue movement patterns that comprise St2Tr remain unclear. We investigated coronal jaw and tongue movements using videofluorography. Fourteen healthy young adults ate 6 g each of banana, cookie, and meat (four trials per foodstuff). Small lead markers were glued to the teeth and tongue surface to track movements by videofluorography in the anteroposterior projection. Recordings were divided into jaw motion cycles of four types: stage I transport (St1Tr), chewing, St2Tr, and swallowing. The range of horizontal tongue motion was significantly larger during St1Tr and chewing than during St2Tr and swallowing, whereas vertical tongue movements were significantly larger during chewing and St2Tr than during swallowing. Tongue movements varied significantly with food consistency. We conclude that the small horizontal tongue marker movements during St2Tr and swallowing were consistent with a "squeeze-back" mechanism of bolus propulsion. The vertical dimension was large in chewing and St2Tr, perhaps because of food particle reduction and transport in chewing and St2Tr.

Few studies have reported the activation sequence of the swallowing muscles in healthy human participants. We examined temporal characteristics of selected hyoid muscles using fine wire intramuscular electromyography (EMG). Thirteen healthy adults were studied using EMG of the anterior belly of digastric (ABD), geniohyoid (GH), sternohyoid (SH), and masseter (MA, with surface electrodes) while ingesting thin liquid, banana, tofu, and cookie (3 trials each). Onset timing was measured from rectified and integrated EMG. Data were analyzed using repeated-measures ANOVA with Bonferroni correction. When drinking thin liquid, MA, GH, and ABD were activated almost simultaneously, but SH was activated later (using GH onset as 0 s, MA -0.07 (-0.20 to 0.17) second [median (interquartile range)]; ABD 0.00 (-0.10 to 0.07) second; SH 0.17 (0.02 to 0.37) second; P < 0.01). With solid foods, MA contraction preceded GH and ABD; SH was last and delayed relative to liquid swallows (GH 0 s; MA -0.17 (-0.27 to 0.07) second; ABD 0.00 (-0.03 to 0.03) second; SH 0.37 (0.23 to 0.50) second; P < 0.01). The role of the MA differs between solids and liquids so the variation in its timing is expected. The synchronous contraction of GH and ABD was consistent with their role in hyolaryngeal elevation. The SH contracted later with solids, perhaps because if the longer duration of the swallow. The consistent pattern among foods supports the concept of a central pattern generator for pharyngeal swallowing.

Stage II transport (St2Tr) is propulsion of triturated food into the pharynx for storage before swallowing via tongue squeeze-back against the palate. To clarify the phenomenology of St2Tr, we examined the effects of food consistency and the number of chewing cycles on the number of St2Tr cycles in a chew-swallow sequence. We recorded chew-swallow sequences in lateral projection with videofluoroscopy of 13 healthy volunteers eating 6 g of hard (shortbread cookie), and soft foods (ripe banana and tofu) with barium. We counted the number of chewing and St2Tr cycles from food intake to terminal swallow. We used the Friedman test for bivariate analyses and negative binomial regression for multivariable analyses. On bivariate analysis, food consistency had a positive association with the number of chewing cycles (P = 0.013), but not with the number of St2Tr cycles (P = 0.27). Multivariable analysis, however, revealed a greater number of St2Tr cycles with hard than soft food (P ≤ 0.01) and a trend toward negative correlation between the numbers of St2Tr and chewing cycles (P = 0.083). The number of chewing cycles needed to clear the mouth differs among food consistencies as demonstrated previously. Greater numbers of both St2Tr and chewing cycles were elicited with the hard than with the soft foods. Given the trend toward negative correlation, the association between the number of St2Tr cycles and that of chewing cycles deserves further study.