Amyotrophic lateral sclerosis (ALS) affects the speech motor system and causes a substantial decline in intelligibility. Clear speech strategies have been found to benefit intelligibility in talkers with dysarthria and the purpose of this study was to examine clear speech effects on tongue, lip, and jaw movements in persons with ALS and healthy controls. For this purpose, participants produced 10 repetitions of the sentence 'I owe you a yoyo' using both habitual and clear speech. Movement variability as well as maximum speed, range, and duration of movement of the articulators were obtained for both conditions. The results show significantly lower jaw movement variability during habitual speech but greater variability for clear speech in ALS relative to controls. The kinematic data was consistent with expectations for individuals with dysarthria. Based on these results, clear speech may be considered a mode of speech that produces high movement variability in talkers with ALS.
Keywords: Clear speech; dysarthria; kinematics; movement variability; speech motor control.