Integrated electromyographic activity and biting force during rapid isometric contraction of fatigued masseter muscle in man.

Abstract The slopes of the curves relating integrated electromyographic activities to biting forces during rapid isometric contractions ( voltage tension curve) were investigated before and after fatiguing contraction of the masseter muscle. The slope of the curves depended on the speed of isometric contraction in non-fatigued masseter muscle. When the muscle was fatigued, the speed became slower and the slope became steeper. This increased slope was not solely due to a decrease in the speed of isometric contraction, and it was shown that more electrical activity was associated with exertion of a tension in the fatigued state, as was evident from the voltage factor calculated in this experiment. This phenomenon could be due to fatigue in the intracellular mechanism of the muscle fibre contraction.