Anterior cruciate ligament injury is very common in recreational and elite athletes. It is considered the second most frequent pathology seeing in Sports Medicine Services. After the injury and ACLR, there is a marked decrease in the strength of the knee extensors/flexors muscles. Strength exercise programs are aimed at recovering strength and functionality, however, a low percentage of patients manage to achieve the optimal return-to-exercise criteria. Therefore, it is relevant to design and evaluate exercise programs that allow early recovery of muscle strength and knee functionality. The main purpose of this study is to investigate a neuromuscular exercise program compared with a standard institutional program in the recovery of the strength and functionality of the knee extensor/flexor muscles after ACLR.

Study design: this is a 22-week, randomized trial. 62 participants will be recruited from the medical services of the National Institute of Rehabilitation (NIR).

Participants: adults between 20-40 years old, with a body mass index <30.0 kg/mˆ2, history of sports practice immediately prior to the injury, post-operated unilaterally for ACL with the hamstring autograft technique (semitendinosus-gracilis tendons), with full ranges of motion and no edema will be recruited. Participants with multi-ligament knee injuries, with more than 12 months from the injury to surgery or more than 8 months after surgery, with uncompensated metabolic diseases or acute illnesses/infections under treatment will not be recruited.

Intervention: participants will be randomized to 1) neuromuscular or 2) institutional exercise program with computer-generated random numbers. Both programs consist of isotonic open and closed kinetic chain exercises for the hip and knee. The neuromuscular program also includes exercises to improve vastus medialis and lateralis strength, hamstrings, hip flexors and extensors, proprioception, and core stability. On the other hand, the institutional program includes concentric and eccentric knee exercises. Both programs will be performed 3 days/week, 3 sets, 18-22 repetitions for each exercise; the intensity will initiate at 65-75% (1RM), and it will be increased to 75-85%; the workload will be increased 5-10% from the fifth week according to perceived exertion. The exercises will be done with both legs; however, the injured leg will be exercised complementary. The last 6 weeks of the programs correspond to functional exercises according the sport. Every training session will consist of 15 minutes of warm-up, 40-60 minutes of resistance training, and 15 minutes of cool-down; all training sessions will take place in the NIR Sports Medicine therapeutic gym.

The knee flexors/extensors muscle strength will be determined by isokinetic dynamometry (at 60°/s). The knee function will be evaluated by hop tests and agility T-test. The patient's perception of daily functional activities after ACL reconstruction will be assessed by the Lysholm knee score.

For each participant, the study length will be 22 weeks with 66 exercise sessions. The beginning of the study corresponds to session 1 of exercise. Initial evaluation will be performed 5 days prior to the start of the intervention, while final evaluations will be conducted at the end of 22 weeks of follow-up (2 days after the last day of the intervention). Data will be recorded by study investigators.

Inclusion Criteria:

• History of sports practice immediately prior to the injury
• Post-operated unilaterally for ACL with the hamstring autograft technique (semitendinosus-gracilis tendons),
• Full ranges of motion and no edema
• Body mass index < 30.0 kg/mˆ2

Exclusion Criteria:

• Multi-ligament knee injuries,
• More than 12 months from the injury to surgery
• More than 8 months after surgery
• Uncompensated metabolic diseases
• Acute illnesses/infections under treatment