Sarcopenia refers to the decline in muscle mass and function that occurs with advancing age. It is driven by alterations in multiple cellular processes. AMP-activated protein kinase (AMPK) is a cellular energy sensor that opposes many age-related changes, making it an attractive target for the treatment of sarcopenia. This study aimed to test the effect of chronic treatment of old mice with the AMPK-activating prodrug, AICAR, on treadmill running capacity and muscle mass, force production, gene expression, and intracellular markers relevant to sarcopenia. Old (23 months) mice were tested for treadmill running capacity, then randomly assigned to receive daily treatment with AICAR (OA
300 to 500 mg/kg, delivered via subcutaneous injection) or an equivalent volume of saline vehicle (OS) for 31 days. Young (5 months) saline-treated mice (YS) served as controls. Treadmill posttesting was performed after 24 days, and the mice were euthanized after 31 days of treatment. Extensor digitorum longus (EDL) muscles were tested for force generation and RNA sequencing, RT-PCR, and western blotting were performed on quadricep muscles. Treadmill running capacity declined from pre- to posttesting by 24.5% in OS mice. This decline was not observed in YS or OA mice. Quadricep weight was ~8% higher, and tetanic force production by the EDL muscle increased by 26.4% in OA versus OS. These phenotypic improvements with AICAR treatment were accompanied by changes in gene expression in OA/YS versus OS muscles consistent with the "rejuvenation" of gene ontologies associated with connective tissue, neurodegenerative disease, Akt signaling, and mitochondrial function, among others. AICAR increased the mitochondrial markers cytochrome C by ~33%, and citrate synthase by ~22%. Serum insulin-like growth factor-1 levels increased, and Akt phosphorylation tended (