The overall objective of the proposed research is to conduct a prospective, randomized controlled trial to investigate whether the addition of BFR therapy to standard post-operative rehabilitation regimen significantly improves clinical outcomes following anterior cruciate ligament reconstruction (ACLR) standardized with respect to both surgical technique and graft selection. Patients indicated for ACLR will be screened and offered enrollment into this prospective, randomized controlled trial. MRI of bilateral thighs will be performed within 7 days of ACLR. Patients will be randomized to either REHAB or REHAB + BFR (study intervention) using a block randomization scheme. Patients will undergo repeat MRI of bilateral thighs pre-surgery, at 16 weeks, and at 56 weeks post-operatively (primary outcome measure.) Patient reported outcome (PRO) instruments will be administered pre-operatively to establish pre-operative debility related to ACL injury and subsequently after ACLR at 28 weeks, 40 weeks, and 56 weeks post-operative (secondary outcome measures).

The overall objective of the proposed research is to conduct a prospective, randomized controlled trial to investigate whether the addition of BFR therapy to standard post-operative rehabilitation regimen significantly improves clinical outcomes following ACLR standardized with respect to both surgical technique and graft selection. The investigator's central hypothesis is that the addition of BFR therapy has the potential to accelerate surgical recovery, expedite return-to-duty, and maximize medical readiness following ACLR. In the proposed trial validated isokinetic and isometric quadriceps strength testing will be used to quantify the effect of therapy on quadriceps strength, patient reported outcome measures will be used to assess for functional clinical improvement, while quadriceps musculature volume as measured on MRI will be used as an effort independent measure of quadriceps recovery. The rationale for using this modality is based upon the fact that the measurement of quadriceps CSA and muscle volume as surrogates for overall quadriceps function is well described, and a number of studies have convincingly demonstrated positive correlations between these parameters and quadriceps strength4, 17. However, in spite of these reports, the acceptance acknowledging MRI as a surrogate for quadriceps strength remains limited. Therefore, isokinetic and isometric quadriceps strength of both the operative and non-operative knee extensors (quadriceps) will be measured pre-operatively within 7-10 days approximately prior to ACLR and post-operatively at approximately 16 weeks, 28 weeks, 40 weeks, and 56 weeks. MRI of bilateral thighs will be performed immediately prior to ACLR to establish a baseline CSA in the operative and non-operative lower extremity. Then a repeat MRI will be performed following the conclusion of a standardized 12-week (approximately) post-operative rehabilitation regimen among patients randomized to either a standard rehabilitation regimen (REHAB) or a rehabilitation regimen with BFR (REHAB + BFR.) Finally, MRI will be repeated at 56 weeks post-operative to examine for latent changes in CSA once patients have been returned to unrestricted physical activities.

AIM 1: To determine if the addition of BFR therapy to a standard post-operative rehabilitation protocol increases functional strength and quadriceps muscle volume as measured with magnetic resonance imaging (MRI) after ACLR.

Hypothesis: The addition of BFR therapy to a standard post-operative rehabilitation regimen following ACLR will increase quadriceps strength and muscle volume when compared to a standard post-operative rehabilitation regimen measured at approximately 16-weeks post-operative.

AIM 2: To determine if any observed increases in quadriceps strength and muscle volume are sustained beyond the cessation of BFR therapy at approximately 56 weeks post-operative.

Hypothesis: Observed increases in quadriceps strength and muscle volume will be sustained beyond the cessation of BFR therapy at 56 weeks post-operative.

AIM 3: To determine if CSA measurements obtained by MRI reliably predict knee extensor strength.

Hypothesis: CSA measurements obtained by MRI reliably predict knee extensor strength as measured by isokinetic and isometric testing.

AIM 4: To determine if the addition of BFR therapy to a standard post-operative rehabilitation regimen following ACLR results in significantly improved functional outcomes when compared to a standard post-operative rehabilitation regimen at 28, 40, and 56 weeks post-operative using patient reported outcomes (PRO).

Hypothesis: The addition of BFR therapy to a standard post-operative rehabilitation regimen following ACLR will result in significantly improved PRO when compared to a standard post-operative rehabilitation regimen at 28, 40, and 56-weeks post-operative.

Inclusion Criteria:

• Active duty member or Department of Defense Military Health System beneficiary
• 15 to 45 years of age
• Indicated for ACL reconstruction with quadriceps tendon autograft with evidence of skeletal maturity; no open physes/growth plates

Exclusion Criteria:

• Patients undergoing a concomitant surgical procedure that would otherwise require a period of immobilization and/or restricted weight bearing (i.e. meniscal repair, meniscal allograft transplantation, osteochondral allograft transplantation, high tibial osteotomy) will be excluded.
• Undergoing combined multiligamentous knee injury reconstruction
• Unable to consistently participate in the prescribed post-operative rehabilitation regimen
• No patients with a history of recent lower extremity deep vein thrombosis, within the 12 months or on active treatment with anticoagulants, a history of ipsilateral lower extremity lymph node dissection or a history of endothelial dysfunction.
• Patients that are unable to obtain MRI secondary to either intolerance and/or implanted medical devices that preclude the safe completion of the MRI.

The operating surgeon will remain blinded to the post-operative rehabilitation regimen to which subjects are randomized to post-operative rehabilitation regimen (REHAB vs. REHAB + BFR.)