SUMMARY Older Veterans have a higher burden of comorbid chronic diseases compared to the general population, resulting in more frequent inpatient hospitalizations. A consequence of frequent and recurring hospitalizations is the acceleration of age-related loss of skeletal muscle mass and strength (sarcopenia). The combined effect of sarcopenia and hospitalization exacerbates muscle dysfunction since older adults do not adequately recover from bedrest placing them on an accelerated trajectory toward loss of independence. Therefore, the period around hospitalization is a critical period to intervene to prevent disability, loss of independence, and frailty in Veterans. It is not clear why skeletal muscle in older individuals is resistant to regrowth after a period of atrophy. Further, current strategies to enhance regrowth in older skeletal muscle remain largely ineffective. The proposed work is significant because it provides evidence that mTOR activation is the cause, not the solution, to the failure to recover muscle after disuse. The current proposal is innovative in that it goes against current therapies that were based on data from young animals; it proposes three independent but overlapping mechanisms; and it uses sophisticated combined isotope labeling and proteomic approaches to study the periods around disuse atrophy. The overall hypothesis of this proposal is that that chronically activated mTOR in aged skeletal muscle impairs the recovery of muscle mass/function after a period of disuse. To test this overall hypothesis, we will use old (28 mo) F344/BN rats with and without an mTOR inhibitor (rapamycin (Rapa)) to address three specific aims that determine if inhibiting chronic mTOR activity in older muscle: 1) corrects the mitochondrial dysfunction that impairs muscle recovery after a period of HU, 2) improves the proteostatic stress that impairs muscle recovery after a period of HU, and 3) helps resolve fibrosis that impairs muscle recovery after a period of HU. Our hypotheses are that suppression of chronically active mTOR activity will: 1) increase the selective translation of mRNA to improve mitochondrial protein remodeling, improve the deterioration of network connectivity, and improve mitochondrial function, 2) improve proteostatic maintenance during RE, and 3) reduce muscle fibrosis to improve mechanotransduction during RE. For these specific aims we will use our innovative targeted and discovery-based kinetic proteomics along with novel (e.g. mitochondrial imaging) and well-established secondary outcomes, including functional outcomes, to support our proteomic outcomes. It is hoped that these experiments will clarify the underlying cause of failed recovery in muscle of older Veterans, and whether we should fundamentally change the approach to enhance recovery after disuse in aged muscle. This knowledge could help prevent the accelerated progression to a disability threshold for a significant number of older Veterans.