Date of Award


Document Type

Dissertation - Restricted

Degree Name

Doctor of Philosophy (PhD)


Biological Sciences

First Advisor

Robert H. Fitts

Second Advisor

James B. Courtright

Third Advisor

Jerry Hall

Fourth Advisor

Bela Piacsek

Fifth Advisor

Elliot Stein


It is well established that the motor activity pattern regulates physiological and molecular processes in mammalian skeletal muscle. Adaptations to disuse have been extensively studied, but observations of contractile function have yielded conflicting results. Much of this has been due to the manner in which the muscles were studied. The most significant shortcomings of previous investigations include the use of heterogeneous muscles and a variety of immobilization techniques. Because of the differences known to exist between the various muscle fiber types and their specific adaptations to increased muscular activity, it was very important to make a comprehensive evaluation of muscle properties in both fast and slow skeletal muscles and their adaptations to disuse. The present study was therefore undertaken to characterize the disuse-mediated alterations in both isometric and isotonic contractile properties and selected biochemical parameters in the slow, type I, soleus (SOL); the fast, type IIA and B, extensor digitorum longus (EDL); and the fast, type IIB, superficial vastus lateralis (SVL) muscles. Judging from the present findings, it is apparent that the tonically active SOL atrophies to a greater extent than the fast EDL and SVL following hindlimb casting. The preferential effect is evident in both morphological and contractile properties. It is important, however, to point out that despite preferential type I atrophy, the present data fail to substantiate fiber type switching or the notion that fast muscles are somehow unaffected by disuse. Despite elevated catabolic processes, impaired contractile function at rest, and an increased dependence on glycolysis, contractile properties of atrophied muscles, both slow and fast, differed little from controls in their pattern of change with continuous contractile activity. The present results also suggest that skeletal muscle is a highly mutable tissue with the ability to completely recover from extensive atrophy. Adaptations incurred by chronic immobilization as well as those observed during remobilization proceed in a fiber-type specific manner and emphasize the importance of motor activity on the functional integrity of skeletal muscle.



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