The immediate postoperative rehabilitation begins 2–7 days after surgery depending on the amount of preoperative rehabilitation and education provided. The goals of phase 1 are to (1) prevent ROM loss, (2) promote activation of the quadriceps, (3) reduce the inflammatory process in the knee (swelling and pain), and (4) ensure safe and effective ambulation. Once these primary goals have been achieved, postoperative physical therapy aims to normalize ROM, strength, and daily activity.

After ACLR, the knee joint is significantly inflamed, as evidenced by swelling and postoperative pain. Swelling is associated with both impaired quadriceps activation and limited flexion ROM. Slight knee joint flexion is the position of minimal joint contact and is typically the preferred position of comfort for individuals with pain and swelling. Therefore, patients after ACLR may be unable to achieve full active extension due to poor quadriceps activation and discomfort.

The initial focus of rehabilitation is to achieve active knee joint extension to neutral (0^°) with a quadriceps contraction that produces a superior patellar glide. The patient should be positioned sitting on the ground (or a long table) with their legs straight in front of them and nothing underneath the knee joint. The heel may be propped to allow for full extension or mild joint hyperextension. Patellar mobilizations are effective in increasing the available ROM of the patellofemoral joint. Quadriceps activation can be facilitated with simultaneous contraction of the contralateral quadriceps, manual facilitation of a superior patellar glide, or neuromuscular electrical stimulation (NMES) [11]. Care should be taken to avoid common compensations for an inability to activate the quadriceps. Compensations include contraction of the gluteus maximus to extend the femur or dorsiflexion of the ankle joint to create a sensation of tension across the posterior capsule (i.e., perceived stretch by the patient). Low-load, prolonged stretch exercises may be of benefit to the patient who struggles with extension ROM. These exercises may include prone positioning with a weight on the heel to promote extension or a long sitting position (as described above) with the heel propped on a bolster. These exercises may be used but care should be taken to ensure that the patient is able to relax enough to allow extension to occur. Patients who do not achieve neutral extension by the end of postoperative week 2 or extension within 3^° of the contralateral limb by 4 weeks postoperative should be referred back to their treating surgeon for evaluation.

Flexion ROM can be increased with active ROM exercises and active assisted ROM exercises (e.g., heel slides). For individuals with hamstring autograft ACLR, care should be taken early after surgery to avoid resisted or strenuous activation of the hamstrings while healing occurs. For individuals who are unable to increase ROM independently with active and active assisted ROM exercises, patellofemoral joint mobilizations are beneficial. Specifically, inferior joint mobilizations replicate the normal arthrokinematics of knee joint flexion. Medial patellar joint mobilizations may also be helpful when knee joint effusion forces the patella to deviate laterally during knee joint flexion. Patients who do not achieve 90^° flexion by the end of postoperative week 2 or 120^° by 4 weeks postoperative should be referred back to their treating surgeon for evaluation, if a regularly scheduled appointment does not already exist.

To promote increases of ROM, it is also important to reduce the overall amount of swelling in the knee joint. Aside from potential pharmacological intervention from the medical provider, the rehabilitation professional can assist the process through the application of compression and elevating the limb. Cryotherapy is also of benefit to decrease inflammation and control pain. Effusion should be tracked with the modified stroke test [40], with an expectation that the patient achieve a grade of 2+ between weeks 2 and 4 postoperative and a 1+ by week 8.

Patients with noncomplicated ACLR (i.e., no meniscus repair or chondral surgery) should be encouraged to assume a normal gait as soon as possible after surgery. Depending on surgeon preference and recommendation, patients may be issued a postoperative brace and/or assistive device. To initiate the normal gait cycle, exercises such as terminal knee extension to simulate the midstance phase of gait and weight shifts to simulate initial contact and weight acceptance can be implemented. As the patient demonstrates a consistent, normal step through gait without pain or excessive aberrant movements, they can discontinue crutch use. Until gait is completely normal, they should use the crutches with weight bearing as tolerated to practice a normal walking pattern. Patients are recommended to use the postoperative brace in crowded or uncertain situations (e.g., inclement weather).

The primary strength focus of postoperative rehabilitation is for the quadriceps muscle in both weight-bearing and non-weight-bearing exercises. Non-weight-bearing exercise is crucial to isolate the quadriceps muscle. There is concern that non-weight-bearing knee extension exercises may put excess stress on the reconstructed ACL due to anterior tibial shear. However, limiting the ROM to between 90 and 45^° of knee flexion limits that anterior strain [9]. Weight-bearing exercises in the 45^° to 0^° ROM also produce limited strain on the graft. Therefore, these range restrictions are imposed for the first 8–12 weeks after ACLR, after which the ranges of motion are slowly increased. Weight-bearing exercises typically reflect the demands of daily activities to improve performance in those tasks (e.g., step-ups, step-downs, sit to stand). Various versions of squats and leg presses may also be used. Any irritation to the patellofemoral joint should be treated with specific patellar mobilizations to increase mobility, stretching exercises for the quadriceps especially the rectus femoris, and additional strengthening of the quadriceps. All exercises should be pain-free, and the patient should not complain of pain after therapy.

In addition to the quadriceps, the other lower-extremity muscles affecting the knee joint should be strengthened as needed. The hip abductors, external rotators, and extensors are important for dynamic control of the femur. The calf muscles, especially the muscles to dynamically support the arch of the foot, are also important to control the tibia. As a general treatment approach, abdominal and lumbar strength should be targeted with specific exercise to limit aberrant trunk motions which influence the demands on the knee joint.

Running on a treadmill or track provides controlled environment to systematically increase the load placed on the knee joint between 3 and 5 months after ACLR. Progression to the running phase is only allowed after mastery of phase 1 has been determined. This includes symmetrical ROM, trace or less knee joint effusion, and minimal gait deviations during fast treadmill walking. Gait deviations including decreased stride length, contralateral pelvic drop, femoral internal rotation, and medial collapse of the knees while walking indicate the patient is not ready for running. The patient must then complete a Y-balance test composite score of at least 90 %, 30 step and holds, and 10 consecutive single-leg squats on the involved leg to at least 45° of knee flexion without compensatory patterns. Finally, the individual must demonstrate 80 % quadriceps muscle strength symmetry.

When the patient achieves the criteria without increased pain or inflammation, a run-walk progression is implemented with progressive increases in distance (see example in Adams et al. [1]). The authors advocate a distance-based progression rather than a time-based progression to more accurately monitor knee joint loading during this phase. When an appropriate running gait pattern is consistently observed, the individual can complete the running progression independently.

To demonstrate mastery of phase 2, the individual must be able to run 2 miles continuously without any complaints of pain, signs or increased swelling, and without gait deviations. Neuromuscular control is tested with 10 consecutive weighted single-leg squats to 45° of knee flexion without aberrant movements with a limb symmetry index of greater than or equal to 75 % and a Y-balance test with a composite score of at least 100 %. Individuals must also demonstrate greater than or equal to 85 % LSI for quadriceps strength.

Once these tests are passed, the individual can begin agility training. Basic agility drills include straight plane movements (anteroposterior and lateral), such as shuttle running, side shuffling, carioca (lateral shuffling while crossing your trail leg over the lead leg), and agility drill ladder exercises or small agility hurdles in forward and lateral directions. Direction changes with appropriate hip and knee flexion to absorb the load are emphasized. To promote safe movements, effort begins at approximately 50 % speed and progresses as performance improves. Compensatory patterns should be quickly resolved with cueing from the physical therapist.

To demonstrate mastery of phase 3 (low-level agility drills), the patient must complete forward/backward shuffling, side shuffling, carioca, and ladder drills at full speed without compensation patterns. Individuals must also demonstrate adequate neuromuscular control by performing ten consecutive weighted single-leg squats to at least 60° of knee flexion with a limb symmetry index of at least 85 % and demonstrate quadriceps muscle strength symmetry of greater than or equal to 90 % LSI.

Phase 4 of rehabilitation includes double-limb jumping, in which the individual begins with forward jumps, lateral jumps, and rotational jumps. Progression to ascending and descending box jumps is at the discretion of the physical therapist. Rebounding jumps and combination movements are the final stage of progression. The patient must avoid abnormal frontal and transverse plane movements (dynamic valgus) and should be cued to exaggerate hip and knee flexion with a soft and quiet landing with equal weight distribution for takeoff and landing [8, 15, 26, 32].

The patient must demonstrate mastery of rebound and combination jumps without compensations. To demonstrate neuromuscular control, individuals must perform ten consecutive weighted single-leg squats to at least 60° of knee flexion with a limb symmetry index of greater than or equal to 90 % and demonstrate quadriceps muscle strength symmetry of greater than or equal to 90 % LSI (Table 47.1).