Comparison of EMG activity and blood flow during graded exertion in the orbicularis oris muscle of adult subjects with and without lip incompetence: a cross-sectional survey.

Background/Objectives: The peri-oral muscles-including orbicularis oris-are critical in maintaining equilibrium in tooth position. Lip incompetence (LI) can thus be a factor in malocclusion. We therefore aimed to validate a technique to evaluate not only muscle activity via electromyography (EMG) but also muscle endurance and fatigue via blood flow (BF) for LI. Subjects/Methods: Subjects were classified into increased muscle tension/lip incompetent (experimental) and normal muscle tension/lip competent (control) groups. Each subject then exerted force on a custom-made traction plate connected to a tension gauge. Using laser speckle imaging and electromyographic measurements, we characterized muscle activity and corresponding BF rates in these subjects in various states of resting, loading, and recovery. Results: Results showed a significant difference between the experimental and control groups, notably in the rate of change in BF to the inferior orbicularis oris muscle under conditions of increasing load (graded exertion). Furthermore, the data suggested that the muscles in the control group undergo a more prolonged (and therefore presumably more complete) recovery than muscles in the experimental group. These factors of reduced BF and short recovery may combine to accelerate muscle fatigue and produce LI. Limitations: The sample used here was controlled for malocclusion (including open bite) to eliminate this type of confounding effect. Conclusions/Implications: From these findings, we conclude that reduced BF and inadequate recovery in the orbicularis oris muscles may be more significant than EMG activity in the assessment of LI.