Adaptation to chronic lengthening of the rat masseter and temporalis muscles.

This study examines the potential of skeletal muscle for adaptation to chronic lengthening, in the presence and absence of function (range of motion). Adult female rate wore an intraoral bite-opening appliance which lengthened the masseter and temporalis muscles, either with (group L/M) or without (group L/IM) immobilization, for periods of one, two and four weeks. Longitudinal change was assessed by measuring muscle fiber length, sarcomere length, sarcomere number per fiber, and serial connective tissue. Cross-sectional fiber area, percent composition and percent total area of muscle fiber types were evaluated. Change in the physiological parameters of maximum bite force, gape-tension relation, individual whole muscle maximum tetanic tension, length-tension relation and time-to-peak tension were examined. Muscle fibers stabilized at lengths significantly longer than control by one week, via sarcomere addition, in both the mobile and immobilized groups. Few changes were found in fiber cross-sectional area or histochemical composition of the muscles. Maximum active bite force declined most markedly in group L/IM, but increased in group L/M by four weeks after an initial decline. Individual whole muscle active force production in both groups returned to near control levels by four weeks, while passive tension exerted by lengthened and immobilized deep masseter muscles remained significantly higher than control. Speed of contraction was significantly increased by four weeks in both group L/M and L/IM. The results of this study indicate that increase in muscle fiber length in response to chronic muscle lengthening occurs whether or not isotonic contraction is permitted. Neither treatment consistently produced muscle fiber atrophy or hypertrophy. These findings are reflected in the fact that force production in individual whole muscles did not differ from control by the end of the experimental period. The fact that deep masseter muscle passive tension remained higher than control in group L/IM suggests that the experimental period was not sufficient to permit equilibration of connective tissue response when immobilization was applied. The results of this study indicate that the main difference in response between mobile and immobilized chronically lengthened muscles lies not in the type or magnitude of adaptation, but in the temporal progression of change. (Abstract shortened with permission of author.)PhDMorphologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/161840/1/8812953.pd