| Children with neuromuscular disorders are frequently characterized as having abnormal muscle tone or stiffness, even though no reliable means of quantifying stiffness of the oral articulators has been identified. Moreover, the role of stiffness in speech impairment is unknown. The present investigation was designed to determine whether children with suspected muscle tone abnormalities present with jaw stiffness differences and how these differences may relate to speech impairment. The measures included jaw kinematics during stress production, jaw stiffness, and muscle activation patterns. Children between the ages of 3 years, 11 months and 10 years with suspected muscle tone abnormalities (i.e., Down syndrome and spasticity) were compared with their typically developing peers. The kinematic correlates of stress production were obtained using video-based kinematics and included jaw displacement, peak velocity, and movement duration. Children with suspected hypo- and hypertonicity generally demonstrated the expected kinematic correlates of stress production in three-syllable tokens. In two-syllable tokens, the children with Down syndrome did not modulate displacement and duration similarly to the typical children. Two measures were used to test for differences in jaw stiffness: the stiffness ratio and the electromechanical delay (EMD). The stiffness ratio, an index of mass-normalized stiffness, was derived from the slope of the regression of the peak velocity-displacement relationship (Kelso, Vatikiotis-Bateson, Saltzman, & Kay, 1985; Ostry, Keller, & Parush, 1983). Analysis included comparison of the slope values across the groups, as well as across stressed and unstressed syllables, as unstressed syllables are produced with greater stiffness than unstressed syllables by adults (Ostry et al., 1983; Vatikiotis-Bateson & Kelso, 1993). Stiffness ratio differences between stressed and unstressed syllables were observed in three-syllable productions, but not in two-syllable productions. The EMD was derived by determining the interval between the onset of suprahyoid muscle activity and onset of jaw depression. No group differences were found for either of the stiffness measures, suggesting that muscle tone abnormalities do not affect orofacial muscles sufficiently to influence speech production. Comparisons of patterns of activation of the jaw muscles indicated greater co-activation by the children with Down syndrome, and suggest a possible method of compensation for stiffness deficits. |
