The long range objective of this research program is to establish the structural, material and microstructural mechanics of bone in response to various loading histories and relate these characteristics to growth and remodelling phenomena. The knowledge gained will improve our understanding of the contribution of mechanical forces and exercise activity to normal bone growth, turnover, osteopenia due to bed rest or "strain shielding" of bone with implants, hypertrophy in response to exercise, and fatigue fracture. In addition, an understanding of bone mechanics which is based on microstructural and molecular events may influence our interpretation of the causes of deformities and fractures observed in pathologic bones. The specific plans of this research proposal include: (1) conduct creep, creep-recovery and creep-fatigue tests of small, devitalized human bone tissue specimens, (2) conduct creep and creep-fatigue tests of whole human and rabbit femora specimens, (3) develop mathematical models to explain the creep, fatigue and impact deformation and fracture response of devitalized bone tissue and whole bones, (4) adapt and improve an in vivo cost-immobilized rabbit femur model to relate bone atrophy and recovery to bone strain histories and alterations of bone structural characteristics. The experimental and theoretical studies on devitalized bone will be conducted within the framework of continuous damage theory. This approach will be an innovation in constitutive and failure modelling of bone which is based on the premise that microstructural damage events provide an important mechanism for energy dissipation. Microdamage events may constitute one class of cellular stimuli for affecting structural adaptation in bone. The rabbit animal model will be used to test some of the current hypotheses of bone remodelling in response to loading changes. Unilateral casts will be applied to create bone loss in the femur. The casts will then be removed to allow recovery of bone mass. These processes will be related to mechanical demands and bone structural characteristics using in vivo strain recordings, in vitro mechanical testing and histomorphometric analysis.