Mean (error bars = SD) time constants derived from fitting a curve with a single exponential term to the constant-effort pressure curves. All data are for the tongue performing the constant-effort task beginning at 50% of maximum pressure (strength), and then removing visual feedback with participants concentrated on maintaining a constant sense of effort. The left panel illustrates data from two separate groups of young neurologically normal adults performing the task before and after a fatiguing task (repetitive brief maximal efforts to exhaustion; Solomon et al., 1996, 2002). The middle panel contains results from 16 pairs of matched participants with PD and without neurologic disease (Solomon & Robin, submitted). The right panel plots preliminary data from 6 participants in each of the PD and control groups from an ongoing study. In this study, participants performed the constant-effort task, with a bite block placed unilaterally between the molars, before and after 1 hour of rapid syllable repetitions containing lingual-alveolar consonants and diphthongs (speech-like exercise).