Project Summary Chronic tendinosis, the degeneration of tendon which leads to tendinopathy, is believed to be initiated by mechanical overuse. Tendinosis and tendinopathy are notoriously difficult to treat, and as tendon is a major organ of movement, can be physically debilitating to those affected. The timeline of overuse tendinopathy is not fully elucidated; there is limited work on the progression between acute overuse and end-stage chronic tendinopathy. Current treatments are not rigorously established in physiology due to this lack of information. In order to evaluate the structural and mechanical changes, the use of novel multiscale modes of imaging and mechanical testing is necessary. Tendon has a complicated hierarchical structure, with damage and remodeling occur at multiple different length scales, making multiscale analysis necessary. The objective of this proposal is to use novel imaging and multiscale mechanical testing with a novel animal model to investigate the progression of overuse tendinosis. I will apply the synergistic ablation model to overload rat plantaris tendon and use my lab’s expertise in multiscale imaging and mechanical testing to apply novel techniques to this problem. Aim 1: Determine the multiscale structural changes occurring in vivo during the progression of overuse tendinosis. I hypothesize that tendon will show altered collagen fibril structure, altered collagen fiber structure, and gross pathological hallmarks of tendinopathy by 4 weeks. Throughout the rest of the time points, I hypothesize that structure will continue to deteriorate, though it is possible that tendon healing may successfully be able to reverse overuse, and the healing response may be evaluated. Aim 2: Quantify the multiscale mechanical changes occurring in vivo during the progression of overuse tendinosis. I hypothesize that microscale damage will initiate at 4 weeks in the form of nonrecoverable sliding between fibers and increased fiber strain. I also hypothesize that as tendinosis progression, molecular collagen and microscale fiber damage will be followed by impaired tissue-level mechanical properties. This study will establish key changes in the structure and mechanical function of tendon during the progression of overuse tendinopathy. Understanding these changes will be an important step forward in being able to treat this prevalent musculoskeletal disorder. My career goals as an aspiring PI are to reduce the burden of musculoskeletal degenerative injuries. The training I will receive during this project will make me a valuable investigator, with proficiencies in sophisticated mechanical testing, novel imaging and image analysis, and preclinical animal models. The Elliott Lab at the University of Delaware is part of an active and highly ranked network of programs and faculty centered on musculoskeletal research, and is ideal for this project.