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To study the detrimental effects of hypokinesia, many models of reduced muscular activity (i.e., reduced-use), including alterations in the size of the living environment, have been developed. Although significant structural and functional changes have been documented, the effect of reduced-use on muscle fatigability remains unclear. This project was designed to study the effects of cage-size on selected properties of rat hindlimb muscle, with particular emphasis on fatigue. Further, in view of the lack of information on the potential effect of gender, both males and females were studied. The rats were raised in either a small, conventional cage or one approximately 133 times larger. Subsequently, terminal experiments were performed to characterize the contractile properties, fiber-type composition and oxidative potential of two hindlimb muscles of the small- and large-cage-reared rats. The test muscles, soleus and extensor digitorum longus, were selected on the basis of their pronounced differences in function, usage and fiber-type composition. The results suggest that reductions in cage-size can influence the properties of skeletal muscle (specifically, muscle mass, force and fatigability) and that this effect is a function of both gender and interanimal differences. However, due to the large variability exhibited by all three factors, their overall effect will be minimal. An analysis of the response of the test muscles to a 6 min fatigue test revealed three findings. First, both muscles exhibited a wide range of fatigability, an unexpected finding particularly for soleus but in keeping with the multiplicity of factors discussed above. Second, the association observed between whole-muscle force and the electromyogram (e.m.g.) was found to be dependent upon the measure used to quantify the e.m.g., the fiber-type composition of the muscle and its degree of fatigability. And third, a coexistence of twitch potentiation and muscle fatigue was observed which also was dependent upon the fiber-type composition and the extent of fatigue. Finally, a comparison of qualitative and quantitative histochemical analyses revealed broad, overlapping ranges for oxidative enzyme activity for each of the three muscle-fiber types. This suggests that differences in fatigability usually attributed to different fiber types are not due solely to differences in oxidative potential.

Reliability analysis of series structural systems with emphasis on problems typical for metal fatigue is addressed. Specific goals include the following: (1) Given the distribution of strength of the components and the distribution of external loads on the system what is the probability of failure of the system? (2) Given the target safety index for the system, what would be the target safety index for the components? Exact solutions in the analysis of series structural systems only exists for some special problems. Some of these special problems are investigated. In particular some special cases of the problem of unequal element reliabilities are considered and some interesting observations are made. Numerical integration is in general required even when an exact solution exists. A correction or adjustment factor is developed for an important class of problems. This factor makes it possible to relate element and system probabilities of failure without numerical integration. However in most cases no exact solution to the structural series system problem exists. Approximations by for instance bounds on the probability of failure or Monte Carlo simulation has been the only way of approximating solutions. These two methods are generally not good approximation schemes since they are either too crude or too expensive. In this dissertation an approximation scheme for analysis of series systems where no exact solution exists is developed. The method only requires a simple numerical integration if the component safety index and the correlation coefficient between failure modes is known. Numerous examples are used to verify the method against known exact results and excellent estimates are obtained. Applications by practical examples is also given. In the appendix the problem of convergence of fatigue life distribution is also summarized.

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