Ability to control directional lip-closing force during voluntary lip pursing in healthy young adults.

Background It is considered important to evaluate the ability to control lip-closing force (LCF). Objective This study aimed to investigate the ability to control directional LCF. Methods The experimental system included an apparatus developed to measure LCF during lip pursing in eight directions (upper, lower, right, left and the four directions in between) and a display showing the exerted LCF and a target value in each direction in real time. Twenty subjects (10 men and 10 women) were instructed to maintain the LCF at a specific target value using visual feedback. Based on our preliminary experiments, the target value was set as 50% of the maximum LCF, and the range was set at the target value +/- 8%. The accuracy rate was defined as the ratio of the matched time, in which the subject was able to keep the LCF in the target range, to the total 3 seconds. Results The accuracy rate of men was higher than in women in the lower, lower left and lower right directions. The accuracy rate of the directional LCF differed significantly depending on the direction. In assessing the accuracy rate for each directional LCF, the rates of upper and lower directional LCF were significantly higher than those of oblique directional LCF. No significant relationship was observed between the accuracy rate and the maximum LCF except for one direction in men subjects. Conclusions Our findings suggest that the ability to control directional LCF is affected by sex and the force direction.