Abstract
To investigate performance of M. orbicularis oris, one of the articulatory muscles, computer processed measurements of EMG activity were examined. This muscle can protrude lips and also can tighten lips to prevent air escape. Subjects could monitor the displacement of upper lips or intraoral air pressure levels and they were requested to maintain their lip position or oral pressure at either one of six or five predetermined levels.
EMG data were recorded. Records of stationary activity period of 4.1 seconds were processed with a computer, which could detect occurrence of peaks, peak amplitude, and duration of activity beyond a certain level, to calculate following two indices. Index A takes into account both amplitudes and duration of peaks, and is related to integrated EMG values. Index B reflects peak amplitude only, and is similar to “amplitude” proposed by Fitch.
Comparing these two indices with displacement and oral pressure, the following findings were obtained.
(1) Both two indices were almost proportional to displacements of upper lips.
(2) Values of both indices increased steeply as intraoral air pressure increased at lower pressure levels, but increments were less steep if pressure was increased more. The rate of increment was different between Index A and B.
These results suggest that both indices well reflect the muscular works, and that synchronization and interference of motor unit activities occurred if high air pressure was produced, although they were rare or absent when protruding of lips or low pressure levels. Possible clinical applications of these indices were also discussed.
