Quadriceps muscle dysfunction persists for years after anterior cruciate ligament reconstruction (ACLR) and is related to poor self-reported outcomes, altered movement patterns and joint loading associated with post-traumatic knee osteoarthritis (OA), and higher risk of reinjury. Emerging evidence indicates that central drive (pathway from the brain to the muscle of interest, i.e corticospinal excitability) to the quadriceps muscle is reduced as early as 2 weeks after surgery and can persist for years after ACLR, meaning that current rehabilitation strategies may not be addressing potential maladaptive changes in central drive. Anodal tDCS is a neurostimulation technology that increases brain excitability (i.e. central drive) and has the potential to address alterations in central drive and quadriceps muscle performance. The purpose of this study is twofold: 1) to determine the effects of anodal tDCS on central drive and quadriceps muscle performance in patients after ACLR, and 2) determine the relationship between central drive and quadriceps muscle performance in patients after ACLR. Central drive will be defined by two measures: 1) active motor thresholds, and 2) slope of a stimulus response curve. Quadriceps muscle performance will be defined by two measures: 1) isometric quadriceps strength, and 2) rate of torque development (RTD). For purpose 1 the investigators hypothesize that measures of central drive and quadriceps muscle performance will increase with administration of active anodal tDCS compared to no change with sham tDCS. For purpose 2 the investigators hypothesize that both measures of central drive will be associated with both measures of quadriceps performance, with a stronger association between central drive and RTD. Following a cross-over design patients 3-6 months from ACLR will receive active and sham anodal tDCS at different sessions separated by 7-10 days while they ride a stationary bike for 20 minutes. Bike position and intensity will be standardized for all patients to maximize quadriceps activity. Findings from this study will expand our basic science knowledge on how tDCS effects different aspects of corticospinal excitability and quadriceps strength, and lead to subsequent studies to determine the effects of multiple sessions of tDCS on corticospinal excitability and quadriceps muscle performance in patients recovering from ACLR.

Inclusion Criteria:

-Underwent primary ACL reconstruction within 3-6 months

Exclusion Criteria include:

• multiple ligament reconstruction
• osteo-chondral procedures
• any previous lower extremity surgery
• previous ACL injury
• Metal or implants in the head or neck
• history of neurological disease, seizures, severe migraines, and concussion within the last 6 months