Association between tongue muscle composition and acoustic properties of articulation in healthy adults: An ultrasonographic study

(Introduction)

Although tongue muscle composition affects articulatory function, previous studies have only compared it with oral diadochokinesis*. We investigated the relationship between tongue muscle composition and articulation by analyzing tongue muscle structure, tongue pressure, oral diadochokinesis, speech rate, and the acoustic properties of tongue articulation.

(Methods)

Fifty-four healthy adults (15 males, 39 females; mean age: 22.0 ± 3.3 years) aged over 18 years were included, while individuals with a history of any disease associated with motor speech disorders were excluded. Tongue muscle thickness was measured from the coronal slice of the ultrasound. Tongue muscle quality was analyzed using two methods: (1) brightness obtained from ultrasound images and (2) the tongue muscle quality index, defined as the maximum tongue pressure divided by the tongue muscle thickness. From recordings of the long sentence “The North Wind and the Sun”, we analyzed speech rate and the second formant transition (F2 slope)** . Regarding the F2 slope, we focused on the /ai/ transition in /gaito:/. We identified the minimum and maximum points of the second formant and measured their travel widths and durations. The F2 slope (unit: Hz/ms) was calculated by dividing the movement extent by the movement duration. We also examined oral diadochokinesis.

(Results)

Tongue muscle thickness showed a significant correlation with speech rate (r = -0.34, p = 0.02). Tongue muscle quality index was significantly correlated with oral diadochokinesis for /t/ and /k/ (r = 0.29, p = 0.04; r = 0.35, p = 0.01). However, tongue brightness showed no significant correlation with the other parameters. Similarly, no significant correlation was found between the F2 slope and the other parameters.

(Conclusion)

We found that tongue muscle composition was related to speech rate in healthy adults. However, in normal subjects with preserved speech intelligibility, our results showed that acoustic properties, such as the F2 slope, were not significantly affected by tongue muscle composition. As for tongue muscle quality, the quality index may be a better indicator than brightness.

* Nakao Y, et al. Journal of oral rehabilitation 2021; 48(12): 1347-1353.

**Tamura T, et al. 2022; PLOS ONE 17(3): e0264995