Adding high-frequency transcutaneous electrical nerve stimulation to the first phase of post anterior cruciate ligament reconstruction rehabilitation does not improve pain and function in young male athletes more than exercise alone: a randomized single-blind clinical trial. PURPOSE: Knee ligamentous trauma and the following pain is one of the most prevalent athletic injuries. Transcutaneous electrical nerve stimulation is one of the conservative approaches in controlling pain which is low cost, noninvasive, and safe with low complications. Our purpose was to study whether transcutaneous electrical nerve stimulation (TENS) could help athletes perform better during the first phase of rehabilitation (0-4 weeks) after anterior cruciate ligament (ACL) reconstruction surgery and until the follow-up. METHODS: This randomized single blind (exercise instructor and assessor) clinical trial was performed on 70 male athletes, undergone ACL surgery. After the surgery, patients were randomly divided into two groups: the first group received semi-supervised exercise plus high-frequency TENSTENS for 35 min a day and the second group only performed exercises. Treatment duration continued for 20 sessions, 4 weeks. The visual analog scale (VAS) score (100 mm), International Knee Documentation Committee (IKDC) questionnaire and knee flexion Range of motion (ROM) were evaluated for all patients after the surgery (before commencing the rehabilitation program), after 4 weeks and 14 weeks from the surgery. RESULTS: Mixed ANOVA was used to explore the interaction effects of time and group on outcome measures and post hoc additional tests were performed on the data. The VAS, IKDC questionnaire score and knee flexion ROM increased in both groups over time, but the amount of improvement did not differ between the two groups, implying no additional improvement in the group receiving TENS along with exercises. CONCLUSION: The findings of the present trial shows that adding TENS to a specific protocol of semi-supervised exercise in the first phase of rehabilitation after ACL reconstruction is not efficient on improving knee function and pain more than exercise alone. Implications for Rehabilitation Injury to the anterior cruciate ligament of the knee is a common injury specifically in athletes. Incorporating effective pain relieving strategies during the post anterior cruciate ligament reconstruction surgery rehabilitation could lead to improvement in regaining the range of motion and function of the knee which is of crucial importance after the surgery. Applying transcutaneous electrical nerve stimulation alongside a specific protocol of semi-supervised exercise in the first phase of rehabilitation after anterior cruciate ligament reconstruction did not have any additional effect to exercise alone.