The goals of phase II during this phase are to regain neuromuscular control of the quadriceps musculature, improve lower extremity strength, normalize knee ROM, and reduce abnormal forces on the knee during activities of daily living. A baseline level of quadriceps strength is critical to regain early in rehabilitation [29]. In conjunction with strength, neuromuscular training is important to emphasize early within rehabilitation as, in conjunction with strength training, it can decrease compensatory changes in muscle activation patterns and facilitate joint stability [16].

ROM continues to progress as tolerated in this phase with the goal of attaining full knee ROM by the end of week eight compared to the uninvolved side. Patients are permitted to discontinue crutch use when they are able to demonstrate a good quad contraction (minimal to no extensor lag), full knee extension, minimal effusion, minimal pain with gait, and proper weight acceptance onto the involved limb. Patients are weaned from crutch use by a gradual progression from two crutches , to one, and ultimately to no assistive device (Fig. 18.4).

Exercises are progressed at this time as weight bearing continues to progress. Open kinetic chain exercises can now begin to work within new ranges of motion. Early data suggested limiting full extension with resisted OKC quad strengthening until 4–6 months after ACLR [30]. More recent work suggests increasing knee extension ROM at week 5, with a progression to 90-30°, 90-20° at week 6, and 90-10° in week 7 and full ROM in week 8 [16]. It is important to once again highlight the need to delay adding weight to hamstring strengthening in OKC until week 8–12 to protect the graft from elongating [16]. Finally, a slow and gradual progression of balance and proprioception activities is introduced within this phase in alignment with weight bearing status (Fig. 18.5). Cardiovascular conditioning activities can be progressed at this time to include biking, treadmill walking, and aquatic exercises (not involving lap swim).

When the patient is able to demonstrate good quad control with a MMT of 4−/5, full knee ROM, good patellar mobility, minimal knee effusion, and no patellofemoral pain, he or she is able to progress to phase III of the protocol.

The dynamic strengthening phase of rehabilitation after ACLR focuses on continued development of the foundation of strength and an initiation of more dynamic movements. The goals during phase III include improvement of muscular endurance and strength, introduction of proper biomechanics and neuromuscular control with functional movements while protecting the ACL graft and patellofemoral joint.

During phase III, the baseline strength and proprioception activities initiated in phase II of rehabilitation are progressed. Strengthening activities are progressed through the addition of weights, sets, repetitions, and focus on the involved, single limb activities (Figs. 18.6, 18.7, 18.8, and 18.9). These activities should continue to be executed on both limbs, but can be done with each leg individually to increase the demand placed on the target muscle group. Balance and stability activities can be progressed through the incorporation of more dynamic surfaces, progression from double limb support to single limb support, and the addition of functional movements. Conditioning activities should be advanced at this time in both intensity and duration. The elliptical machine is another cardiovascular option for conditioning towards the end of this phase. Successful completion of phase III of the program occurs when quad control is good (4+/5 MMT), ROM and patellar mobility are WNL, no effusion is present, no patellofemoral pain and isokinetic peak torque to body weight side to side deficit of 30–35% or less is present by the end of this phase.

The main goals of phase IV are to maximize strength and endurance in preparation for the initiation of impact activities for the patient’s specific return to sport needs. Continued progression of strength and endurance are necessary until the patient is an isokinetic peak torque to body weight deficit of 20% for both strength and endurance [31]. A sufficient foundation for strength is necessary to safely participate in dynamic movements without developing abnormal compensatory movement patterns. In addition to strength, it is suggested that the patient also have no pain, full knee range of motion, no swelling, can balance with eyes open and closed for 30 s, and has good movement patterns prior to initiating plyometric activity [31].

Once strength is adequate and all other criteria are met, initiation of impact and plyometric activity can occur. Currently, there is little consensus on the best way to progress plyometric activity [31]. Therefore, it is important to remember all of the training variables that must be considered when creating any basic plyometric progression . These variables include neuromuscular overload (change of direction of a limb or body without external load), spatial overload (muscle activation and stretch reflex initiated within a specific range of motion), temporal overload (short amortization phase, timing), intensity (amount of effort required to perform the activity), volume (total work performed in a single session), frequency (number of exercise sessions that take place during the rehabilitation cycle), recovery time (time between sets and rehabilitation sessions), and specificity (plyometric movements specific to their sport) [31].

Specific to the pediatric ACL patient, it is important to highlight volume and proper recovery time. When initiating impact, a slow and gradual approach is recommended to minimize overuse stress to healing tissues. With respect to recovery, there is limited research on the optimum recovery times following plyometric exercise. Despite this lack of consensus, the muscle physiology literature recommends an optimal recovery time between training sessions of 48–72 h [31]. This recovery time is sufficient to allow lower extremity musculature to recover from the load of the plyometric exercise, but also minimized repetitive stress on the graft. Emerging basic science research has begun to question the ability of the healing graft to endure repetitive micro-stress and ultimately recover to its native length. Subsequently, the concept of necessary time to recover to minimize chronic elongation of the graft suggests at least 24 h may be necessary to minimize creep of the graft [32].

An ideal plyometric activity to initiate impact activity is a double leg broad jump or a double leg wall jump. Both activities can leverage the use of a double leg impact activity to balance forces through the involved and uninvolved limb and provide an opportunity to master technique in such a way to minimize high risk asymmetries with these movements. Asymmetries with double limb tasks have been identified as predictors of future ACL injuries in this population [33]. The progression of impact activities should be deliberately slow (50 foot contacts) [31] with a high focus on proper biomechanical form versus repetition and submaximal effort prior to max effort. Until the patient can demonstrate proper technique with takeoff and jump landings, they should not be advanced to higher level impact activities.

When proper double leg jump form is demonstrated, the patient can begin transitional jumps that allow the patient to develop neuromuscular control from a double leg jump to a single leg hop. Around this same time straight line jogging can begin. A combination walk and jog program is suggested as a way to slowly reintegrate the patient into unrestricted running. It is important to note that differences in training environments such as treadmill running vs outdoor running may need to be considered. It is important to remind the patient that unrestricted recreational play running is not recommended at this time in the rehabilitation process.

Once a return to running program is established, a further jump progression should continue to work on developing SL hops in a single plane that are, once again, completed with good biomechanical form. Some may argue that a single leg hop should be demonstrated prior to initiating a return to running program. With the pediatric population, some patients may struggle with a single leg hop as it is a novel isolated skill, who are able to return to running, which is a known skill, with ease. Either way, a deliberately slow progression with high emphasis on low repetition and proper biomechanical form is still warranted and needs to be of the highest priority.

The athlete is ready to progress to phase V pending successful attainment of phase IV goals. These goals include continued resolved pain and effusion during all activities, full AROM compared to the uninvolved LE, MMT of 5/5 for all relevant musculature, isokinetic strength ≥85% of the noninvolved limb, IKDC score of ≥85, SL hop tests with a limb symmetry index of ≥85%, and demonstration of appropriate mechanics during additional screens per clinical judgement.

The main focus on phase V is to transition the individual from the rehabilitation setting and begin the gradual return of the athlete to their specified activity or sport once they have sufficient baseline strength and technique as outlined in the goals for entry into phase V. During this phase, the patient will participate in therapeutic activities that functionally progress and optimize their strength, muscle performance, and neuromuscular control in multiple planes of motion to focus on pivoting and cutting maneuvers and then begin to transition from the rehabilitation setting into restricted activity participation.

In order to safely progress the patient, strength and muscle performance activity progressions that alter planes of movement are now appropriate for the patient to attempt. A focus on lateral, rotational, and transitional activities along with the addition of unanticipated perturbations or an unstable surface are now appropriate. Activities that challenge multiple trunk and lower extremity muscle groups simultaneously are now appropriate. It is also important to think about making multiple activities occur simultaneously or make movements sequential to make the task more challenging for the patient.

Neuromuscular control should be progressed to position specific sport activities and drills to help promote the transfer of skills from the clinic to the field or court. A focus on high level plyometric activities, specifically related to power generation during takeoff and force attenuation during landing, can be initiated. Activity progressions should include a progression of impact loading, single leg to double leg transitions and vice versa, altered planes of movement that focus on cutting and pivoting and transition activities, sport-specific perturbations, more challenging support surfaces, or adding sequential or simultaneous activities.

In order to safely reintegrate into the desired sport, a progressive reintegration is needed into the desired activity. This reintegration must include a mastery of movement technique as well as a progression of cardiovascular endurance to insure a prevention of fatigue and the potential decrements in performance that can result. As such, activities in the clinic must focus on maintaining appropriate performance technique as well as advancing cardiovascular and sport-specific endurance.

Initial return to play consists of noncontact drills and conditioning activities. Modifications to the amount of participation time along with speed/demand of participation (ex; 50% effort progressing to 100% effort) can be made. Once this is safely done, the patient can be progressed to contact drills and full practice. Modifications to the amount of participation time, speed, and demand of participation can be made. Finally the patient can be progressed to a scrimmage and game time setting. Modifications to the time of participation can be made.

The athlete is ready to progress to phase VI pending successful attainment of the following goals. These goals include continued resolution of pain and effusion during all activities, isokinetic strength ≥90% compared to the noninvolved limb, IKDC score of ≥90%, SL hop tests with a limb symmetry index of ≥90%, and demonstration of appropriate mechanics during activity-specific maneuvers and drills. In addition, the patient must pass one or more of the following criteria: (1) demonstrates appropriate mechanics on the drop vertical jump assessment, (2) is able to pass a tuck jump assessment with less than six flaws [34], or (3) is able to complete the Star Excursion balance test with a composite reach distance of ≥94.

The patient can be cleared for unrestricted activity participation once they have achieved the goals for entry into phase VI. Discharge from therapy is based on clinical judgement, attainment of goals, and successfully participating in their desired activity. Several authors in multiple systematic reviews have highlighted the wide variation and lack of standard assessments used to determine readiness to return to sport after ACLR in an adult population [16, 35–37]. Van Melick et al. suggested, based on their systematic review and expert opinion, an extensive battery of tests should be used to dynamically assess readiness to return to sport [16]. Further, higher criteria to successfully pass these functional assessments should be considered to best identify those patients specifically ready to return to sports [16]. For example, limb symmetry values of 85% compared to the uninvolved limb on strength tests and functional hopping tests may be sufficient to return to ADL’s, but fall short of readiness to participate in pivoting and cutting activity. Perhaps, LSI values of 90–95% or greater on these assessment are necessary to help insure success with return to dynamic, cutting activity [16]. This may be even more appropriate in young athletes who very often seeks to return to the highest level of activities, which also pose the greatest risk for future injury. Ultimately, once the patients achieve these necessary criteria to return to sport, a functional transition to acclimate and ultimately released to desired activity is most appropriate. Follow-up with the physical therapist to ensure successful reintegration and participation in the unrestricted activity participation is also recommended.