Induced fatigue effects on velopharyngeal closure force.

This investigation studied the effects of induced velopharyngeal fatigue in speakers with normal mechanisms. Five adult female and 5 adult male subjects were used. A force sensing bulb was placed in the velopharynx to measure velopharyngeal closure force and intramuscular electrodes were inserted in the levator veli polatini muscle to sample muscle activation levels. The subjects' task was to repeat the syllable /si/100 times while an external load was placed on the velopharyngeal mechanism. The external load consisted of various levels of air pressure (0 as a control, 5, 15, 25, and 35 cm H2O relative to atmospheric pressure) delivered to the nasal passages via a tube and nasal mask assembly. Fatigue was defined as a declination of force across the series of syllables within a pressure condition and was depicted as the slope of a linear regression line that was fit to the data. The more negative the slope, the greater was the rate of fatigue. Within each experimental pressure condition, small cyclic variations in force were noted about each regression line that corresponded to individual breath groups. This type of declination, within breath groups, has been reported in the literature previously. Overall declination in force over an entire series of syllables and over several breath groups is a new finding. It was possible to induce such fatigue in most subjects, and greater rates of fatigue generally occurred at the higher levels of external loading, i.e., at 25 and 35 cm H2O. Two subjects, 1 male and 1 female, reached exhaustion. The female subject could not perform the syllable repetition task at 25 cm H2O, and the male subject could not complete the task at 35 cm H2O. Three subjects, 1 female and 2 males, exhibited virtually no force declination even at the highest level (35 cm H2O) of external loading. There were no discernable differences in patterns of fatigue or in initial velopharygeal closure force values between the male and female subjects.