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.

Inclusion Criteria:

1. Age between 40y and older
2. BMI: <30 kg/m2
3. Full thickness ACL tear
4. Less than 6 months between ACL injury and scheduled ACLR

Exclusion Criteria:

1. Prior ACL tear in limb scheduled for reconstruction or current bilateral ACL tear
2. Total knee dislocation
3. Anticipated quadriceps ACLR grafts
4. Current infection near the lower limb
5. Inability to attend physical therapy sessions
6. Post-operative loading restrictions
7. History of cardiovascular disease (e.g., CHF, CAD, MI, CVA)
8. History of endocrine or metabolic disease such as hypo/hyperthyroidism and diabetes (Treated hypo/hyperthyroid for at least 6 months will be permitted)
9. History of stroke with motor disability
10. History of respiratory disease (acute upper respiratory infection, history of chronic lung disease)
11. Self-reported pregnancy
12. Bleeding disorder
13. Implanted electronic devices (e.g., pacemakers, electronic infusion pumps, stimulators)
14. Cancer or history of successfully treated cancer (less than 1 year) other than basal cell carcinoma
15. Chronic systemic corticosteroid use (≥ 2 weeks) within 4 weeks of enrollment and for study duration (intra-articular/topical/inhaled therapeutic or physiologic doses of corticosteroids will be permitted)
16. Androgens or growth hormone within 6 months of enrollment and for study duration (topical physiologic androgen replacement will be permitted)
17. Currently taking estrogen products (topical estrogen products will be permitted)
18. Taking anticoagulant medication (warfarin/Coumadin, heparin) that may complicate the muscle biopsy
19. Any staff members who report directly to the principal investigators

IPD can be requested by the PI, Dr. Micah Drummond.