Abstract
The purpose of the present study was to investigate the development of the masticatory function in infants from sucking or weaning to adulthood with maturated chewing, by evaluating the changes in the EMG activities of the peri-oral muscles (TM: temporal, MM: masseter, OM: orbicular oris and SM: suprahyoid muscles). A total of ninety-one subjects were divided into the following seven groups, the sucking group (G 0: as a control, mean age 3.7 month), G 1 (early weaning stage,7.1months), G 2 (middle weaning stage,9.1 months), G 3 (late weaning stage,10.8 months), G 4 (completion of weaning,1.2 years), G 5 (3.8 years) and G 6 (adults,25.1 years). The transition of the EMG activities of the muscles while ingesting cooked rice as a test food were analyzed.
1) When the four muscles were grouped into three according to their function such as the TM and MM for the jaw closing muscles, the OM for the facial muscles, and the SM for the jaw opening and tongue moving muscles, characteristic changes in the EMG activities were found.
2) The muscle activities of the TM and MM increased from the initiation of weaning, whereas no change was found in the activity of the SM except for G 5.
3) The total muscle activity (TM+ MM+ SM: TMA) increased from G 0 to G 2 and once decreased in G 3, but after that it significantly increased again.
4) The ratio (%) of the TM to the TMA increased significantly in G 1 and it significantly increased after G 3 again, whereas the ratio of the SM significantly decreased in G 1 and increased in G 3, and then it decreased continuously to G 6.
5) The masticatory function developed from the G 1 to G 4, but there was no significant relationship between the tooth eruption and the EMG activities of the muscles in G 1 and G 4. However, EMG activities of the jaw closing muscles increased greatly in G 5 which exhibited completion of the primary teeth occlusion.
These findings suggest that there is a turning point in the masticatory function during weaning in infants of G 3 and this point is suggested to be the start of the matured type mastication pattern.
