The purpose of this research study is to determine if Metformin taken after anterior cruciate ligament reconstruction (ACLR) surgery can decrease muscle fibrosis, cellular senescence and improve leg strength. ACL injury and surgery increase muscle collagen content and inflammation which slows the recovery of muscle and function and ultimately impacts the quality of life. Individuals enrolled in the study will be randomized to receive metformin (or placebo) for 4 months after surgery. The investigators will measure muscle strength and inflammatory markers in muscle biopsy samples after the intervention period. Investigators will also follow up with participants 8 months later to determine the lasting effects of the intervention.