D. Junge et Gt. Clark, ELECTROMYOGRAPHIC TURNS ANALYSIS OF SUSTAINED CONTRACTION IN HUMAN MASSETER MUSCLES AT VARIOUS ISOMETRIC FORCE LEVELS, Archives of oral biology, 38(7), 1993, pp. 583-588
The jaw-closing muscles differ from peripheral limb muscles in that th
e maximum attainable force does not decline following a sustained isom
etric contraction. Also, the averaged electromyographic (EMG)/force ra
tio in these muscles does not change with sustained contraction, where
as it increases in fatiguing limb muscles. The present study analysed
EMG records from masseters in healthy male subjects. No statistically
significant difference was seen between average rectified EMG signals
at the beginning or the end of a sustained isometric contraction at 25
-100% of maximum voluntary contraction (MVC). However, when the number
of turns, or reversals of direction, was taken at various percentage
MVC levels, a significant decrease was seen after 60s or at the end of
the contraction. The turns/force ratio decreased monotonically with p
ercentage MVC, but the ratio was not significantly different at the en
d of a contraction from that at the beginning. This result confirms an
earlier suggestion that neuromuscular fatigue does not accompany sust
ained contractions of these muscles.