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Forces generated by facial muscles during speech production have previously been estimated to involve up to 20% of maximum voluntary closing force. The gross nature of this estimate is due, in part, to the complex relationship between muscle contraction and three-dimensional tissue conformation and to the lack of data concerning maximum force. The objective of the present study was to determine the maximum voluntary closing forces for the upper and lower lips in male and female adults. The results indicate the maximum force capabilities for the lower lip to be approximately three times greater than for the upper lip. Male subjects generated significantly greater lip closing forces than female subjects. The large difference between the upper and lower lips as force plants is discussed in relation to speech function and skilled motor behavior.

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