Journal of Dental Research, Vol 79, 950-956, Copyright © 2000 by International & American Associations for Dental Research Online Journals
EMG power spectrum and motor unit characteristics in the masseter muscle of the rabbit
S. J. Turkawski and T. M. van Eijden
Department of Functional Anatomy, Academic Centre for Dentistry Amsterdam (ACTA), The Netherlands. S.J.Turkawski@AMC.UVA.NL
Masticatory muscles contain a large variety of motor units with different
physiological and morphological properties. In this study, we tested the
hypothesis that a relationship exists between the mechanical and
myo-electric properties of single motor units in the masseter muscle of the
rabbit. It was expected that faster-contracting motor units, which usually
have a relatively large number of fibers with large diameters, should have
faster action potentials with larger amplitudes than slower motor units.
Single motor units were stimulated. A two-dimensional force transducer
registered mechanical parameters of the units. EMG electrodes were used to
determine amplitude and frequency parameters of the action potentials of
the same units. The results showed that faster-contracting motor units
indeed produced action potentials with higher conduction velocities.
However, faster motor units had no significant larger amplitude of the
action potential. Small but significant positive correlations were found
between the tetanic peak force and the amplitude of the action potentials.
Little difference was found among the various frequency and amplitude
parameters, respectively, making them equally suitable to describe the
action potential. Surprisingly, a negative correlation between the
amplitude and frequency parameters of the action potential was found, which
may result from variability in arrival times of action potentials at the
electrode site. Regional differences in the frequency parameters were found
between the anterior and posterior parts of the superficial masseter.
