The relationship between jaw muscle activity and fiber type composition

Objectives: Skeletal muscles have a heterogeneous fiber type composition, which reflects their functional demand. However, it remains mostly unclear to what extent various muscle activity characteristics affect the fiber type composition. The examination of this relationship in adult rats was the purpose of this study. Methods: Six Wistar strain male rats (400 to 450 g) were used. A radio-telemetric device was implanted to record muscle activity continuously from the superficial and deep masseter, digastric and temporalis muscles. The degree of daily muscle use was quantified by the total duration of muscle activity per day (duty time) exceeding specified levels of the peak-EMG (2, 5, 20, and 50%). Fiber type composition of the muscles was examined by the myosin heavy chain content of the fibers by means of immunohistochemical staining. Results: At lower activity levels (exceeding 2% and 5% of the peak-EMG), the duty time of the digastric was significantly (p<0.01) higher than those of the other three muscles. The digastric muscle also contained the highest percentage of slow type fibers (ca. 11%). An intermuscular comparison showed a positive, but not significant, relationship between the duty time at the lower activity levels and the percentage of slow type fibers. However, the peak-EMG of the various muscles was correlated (p<0.05) with the percentage of fast type fibers. Conclusions: The present results demonstrate that fiber type composition in the adult rat jaw muscles is determined by the intensity of muscle activity, and to a lesser extent by its total daily duration.