PROJECT SUMMARY/ABSTRACT The control of muscle mass is impaired in older adults, and is exacerbated by periods of muscle disuse common to lengthy hospital stays, immobilization, and physical inactivity. The recovery following disuse atrophy is incomplete in older adults, which contributes to functional decline. During the recovery of muscle mass in older adults, there is an accumulation of intramuscular collagen, which contributes to functional deficits. Lost strength following disuse atrophy in older adults is clinically important due to its tight correlation with morbidity and loss of independence, which combined with the rising population of older adults, equates to a large burden on the United States healthcare system. Therefore, it is critical therapies be made available to regulate excessive collagen deposition in muscles from older adults. We propose massage in the form of cyclic compressive loading may present as a useful alternative in regulating collagen deposition in muscles from older adults, as it is a well-tolerated clinical tool. Using a rat model and our laboratory's custom-built robotic massage device, we show massage is able to initiate the turnover of collagen in muscles of old rats undergoing muscle regrowth. However, it is currently unclear how massage initiates collagen turnover or how aging affects collagen regulation during the recovery of muscle mass in older adults. Therefore, the purpose of this application is to (1) determine if massage impacts collagen remodeling by using isotope tracer methods and mass spectroscopy and (2) identify how aging and massage impact the gene expression of collagen- remodeling cell populations using single cell RNA sequencing. Results from the proposed experiments will define the use of massage as a complementary health intervention for mitigating excessive collagen deposition during the recovery period following disuse atrophy in older adults. Apart from assessing gaps in knowledge of collagen regulation during muscle regrowth in old rats, this proposal is an excellent training platform to accomplish my long-term career goals of becoming an independent scientist. The innovative research plan prepared for this application is overseen by experts in the field of skeletal muscle aging, who have each committed their time in training me to accomplish the goals set out in this application. Combining principles of biomechanics and molecular biology, this is a unique training plan that not only benefits my training, but also identifies massage as a complementary health intervention for older adults recovering from disuse atrophy. The combination of innovative research, mentorship, and research environment each contribute towards training to become an independent scientist in the field of skeletal muscle aging.