Fiber Type Composition in Relation to Daily Jaw Muscle Activity

Skeletal muscles contain a mixture of fibers with different contractile properties, such as maximum force, contraction velocity and fatigability. Muscles adapt to altered functional demands by changing their fiber type composition. This fiber type composition is changed by the frequency, duration and presumably the intensity of activation. Objective: The aim of this study was to analyze the relation between the spontaneous daily muscle activation and the fiber type composition in rabbit jaw muscles. Methods: Using radio-telemetry, the daily activity of five jaw muscles was characterized by the number of the activity bursts. Fiber type composition of the muscles was classified by analyzing the myosin heavy chain content of the fibers. Results: Including all muscle behavior the temporalis showed much larger daily burst numbers (205,000) than the digastric (120,000), medial pterygoid (115,000) and the superficial and deep masseter (90,000). For activations during powerful behaviors the superficial masseter and medial pterygoid showed significantly larger burst numbers and duty times than the other muscles. The jaw muscles contained a relatively large amount of fast type fibers ranging from 53.4 - 84.7%. The percentage of slow type fibers ranged from 13.0 - 40.3% in the tested muscles. The amount of slow type fibers was positively correlated to the number of bursts only for activations exceeding 20% of the maximum activity per day. Conclusion: The present data indicate that the amount of activation above a threshold (>20% peak activity) is important for determining the fiber type composition of a muscle. Activation above this threshold occurred only around 2% of the time suggesting that contractile properties of muscle fibers are maintained by a small amount of powerful contractions per day.