An electromyographic study of whether the digastric muscles are controlled by jaw-closing proprioceptors in man

Whether in the oral system the digastric muscles (which lack muscle spindles) are under the control of proprioceptive information from the masseter muscles (which contain muscle spindles) was investigated by analysing whether and how the masseters and digastrics showed coordinated behaviour during a static, forceful bite. Subjects were asked to maintain a 100-N force for 15 s with and without visual guidance; bite force exerted, and masseter and digastric electromyograms (EMGs) were recorded. Under visual guidance all subjects co-contracted their digastric muslces during the isometric bite. They held the force for a short time, followed by periods with fluctuations (peak-to-peak force amplitude about 15-20 N). Fluctuations in bite force correlated with the masseter EMGs, the maximum in the correlogram occurring at about -50 ms with the force lagging the masseter. In 75% of the subjects a significant periodic component in the masseter and in the force spectra was found at about 4 Hz. This was also seen in the amplitude spectra of the forces, which showed in 80% of the subjects a significant elevation between 7-10 Hz as well. No correlation between the digastric EMGs and the bite forces, and between the EMGs of masseter and digastric could be detected. Spectra of digastric EMGs showed no prominent maxima. When subjects were deprived of visual feedback, maxima at -50 ms in the cross-correlation functions of the masseters and the forces were reduced considerably; periodicities of +/- 250 ms disappeared. In the force amplitude spectra the 3-5 Hz components became much smaller or were not significant; elevations between 7-10 Hz were not affected, however. Cross-correlation of the digastrics and forces, and of the masseters and digastrics showed no significant correlation whatsoever in the absence of visual feedback. Thus, the digastrics (although co-contracting during the bite) do not contribute actively to the control of an isometric bite; they do not receive information from jaw-closing muscle receptors during an isometric bite. Large amplitude fluctuations in the bite force arise from the jaw-closing muscles. These fluctuations may be caused by both resonances of high-order feedback loops (involved in decoding the target force engram), and from visual feedback.