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October 2004

By Gregory D. Myer, MS, CSCS; Mark V. Paterno, PT, MS, SCS, ATC; and Timothy E. Hewett, PhD

Successful completion of the end stages of physical therapy during the 3- to 6-month postoperative period following anterior cruciate ligament (ACL) reconstruction is a major challenge for both the athlete and the therapist.

The athlete may be "released to full activity," especially with "accelerated rehabilitation" protocols.¹ However, at this point the athlete may be prepared to begin more functional training in preparation for sport competition, yet may not be ready to begin competitive play. The physical therapist or ATC must now bridge the gap in the athlete's perceived and actual sports readiness. Although the surgical reconstruction of the ligament may have created a mechanically stable joint, the athlete may not yet have a functionally stable knee during dynamic movements.²

Assessment of functional stability is difficult. After ACL reconstruction for the restoration of mechanical stability, the goal is to maximize dynamic functional stability. Return of the athlete to high level sports before functional stability has been achieved may lead to a new injury or failure of the reconstruction. The decision to return the athlete to unrestricted activity usually is based on a combination of muscle strength measurements, functional tests, and time from surgery. Use of quadriceps and hamstring strength measurements alone as criteria for return to sports is an inadequate assessment, because patients can have functional instability even when strength has returned.³ In addition, it has been reported that commonly used functional performance tests using one-legged hopping and jumping maneuvers have low sensitivity for detecting functional limitations.⁴

The purpose of the present report is to outline general guidelines for the objective assessment and progression of athletes to aid the therapist/trainer with decision making for return-to-sport training for competitive athletes. Our ACL return-to-play strategy is a four-phase, criterion-based outline designed to guide the advanced stages of rehabilitation and athletic development training following ACL reconstruction and ensure successful progression back to sports. Introduction into the program as well as progression to each subsequent phase of the program depend on successful attainment of objective criteria. If baseline criteria are not met, it is recommended that the athlete continue to train using the protocol for the current phase.

PHASE ONE: FUNCTIONAL BALANCE AND CORE STRENGTHENING
Prior to initiation of a return-to-play program, patients should meet baseline measures to ensure safe entry into the program. These may include: a) a minimum postoperative period (eg, 12 weeks), b) no giving way symptoms, and c) minimum quadriceps strength to body weight (eg, 40% to 55% for men and 30% to 45% for women). It may be possible to return athletes to pivoting, twisting, and rotational sports as early as 3 to 4 months postoperatively.¹

However, the safety of aggressive ACL rehabilitation and early return to play frequently comes into question due to the weakened mechanical properties of the reconstructed graft during the first several months postoperative and decreased proprioceptive function and neuromuscular control of the joint. The loads required to rupture the reconstructed graft in a knee with mechanical and neuromuscular deficits are decreased for up to 2 years. If proprioceptive deficits exist at this point, regardless of strength and range of motion (ROM), clinicians may be subjecting their athletes to a high risk of graft failure or reinjury.

To safely begin an advanced neuromuscular training program, the athlete must demonstrate a baseline level of strength. Strength assessment is one objective measure that can be used in combination with other measures to assess readiness to return to activity. Decreased baseline strength will result in an inability to dynamically control forces at the joint. Therefore, a baseline ratio of knee extension peak torque to body weight (eg, 40% to 55% for men and 30% to 45% for women) at more functional speed (eg, 300 degrees/second) is necessary to initiate a return-to-sport program.

When subjects meet the above criteria, they are ready to begin a return-to-play protocol. Return-to-play training following ACL reconstruction must develop the proprioceptive abilities of the athlete without exposing the athlete to further risk of injury. The goal of the first phase of return-to-play training (functional balance training and core strengthening) is to develop a baseline level of core stability and coordination for the athlete that allows them to properly reduce excessive force, maintain balance and posture, and generate force in the desired positive direction.

Proficient walking gait should be established prior to return to play, though running gait may not be corrected to the preinjury form. A running gait retraining program should be designed to improve symmetry of the lower extremity musculature recruitment, which may prevent abnormal loading of the ligaments and soft tissue and increase strength and endurance during sports competition.⁵ Gait retraining is best performed on a treadmill in front of a mirror to provide simultaneous visual and verbal trainer feedback to the athlete. An early goal in running gait retraining is to normalize ROM in the involved and the noninvolved limbs. The involved limb often has limited joint ROM. Simultaneous gait retraining and a progressive plyometric program teach the athlete to properly initiate, control, and decelerate ground reaction forces that he or she will encounter in competitive play when jumping, landing, and cutting.

PHASE 2: FUNCTIONAL STRENGTH
Prior to initiation of phase 2 of a return-to-play protocol, the athlete must demonstrate bilateral symmetry in balance and a single leg squat. Proprioceptive and neuromuscular balance deficits exist after ACL rupture and well into the postoperative rehabilitation period. Clinicians should be aware of these deficits and focus rehabilitative interventions in these areas.

Preoperative single-leg stability of subjects with ACL deficiency can show a significant deficit of the involved limb and the noninvolved limb when compared with the control group.⁶ This significant deficit in both limbs may persist to at least 9 months postoperative. The mean values for the single-leg stability at 12 months postoperative may show significant deficits in the noninvolved limb; however, no significant difference may be present in the involved limb. A standard level of proficiency in postural balance before return to sport is important to protect athletes from reinjuring the affected joint. Balance board proprioceptive training should be utilized well past the acute postsurgical rehabilitation phase, not only for restoration of function, but for the potential prophylactic effect on ligament reinjury.^6-8

Leg dominance is the imbalance between muscular strength and recruitment on opposite limbs, with the involved limb often having weaker and less coordinated musculature. During single-leg landing, pivoting, or deceleration, the returning athlete may have a lack of dynamic muscular control of the involved limb, which may predispose the knee to injury.⁹ The patient should be able to demonstrate a single-leg squat adequately (eg, to 45° bilaterally within 15%- ± 7°-of contralateral limb).

PHASE 3: POWER PHASE
Following the functional strength phase of the return-to-play program, the patient needs to demonstrate a progression of strength prior to advancement to the power phase of the program. The power phase of the program is a progression of challenging, multidirectional strength and power exercises. Typically, quadriceps strength is the limiting factor in progression, and failure to possess adequate strength could potentially result in increased risk for future injury, specifically overuse injuries such as anterior knee pain. Therefore, the patient should demonstrate peak torque symmetry (eg, within 85% of the contralateral limb for both quadriceps and hamstring strength at 300 degrees/second) to progress to phase 3 of a program. Phase 3 of training should begin to incorporate power movements including double and single leg plyometrics.

PHASE 4: SPORT-SPECIFIC SYMMETRY
The final level of training to prepare an athlete to return to sport is the high torque landing and cutting. Use of high-difficulty movements like unanticipated cutting is warranted. Sagittal plane-only training and conditioning protocols that do not incorporate cutting maneuvers will not provide the levels of external varus/valgus or rotational loads that are seen during sport-specific cutting maneuvers.¹⁰ Neuromuscular training programs that incorporate safe levels of varus/valgus stress may induce more muscle dominant neuromuscular adaptations.¹¹ Such adaptations can better prepare the athlete for multidirectional sport activities that can improve performance and reduce risk of lower extremity injury.^12-14 Valgus loads can double when performing unanticipated cutting maneuvers similar to those utilized in sport.¹⁵ Thus, the endpoint of training designed to reduce ACL loading via valgus torques can be gained through the use movement techniques that produce low abduction moments at the knee.¹¹ Training that incorporates unanticipated movements can reduce knee joint loads.¹⁶ Additionally, training individuals to preactivate their musculature prior to ground contact may facilitate appropriate kinematic adjustments and ACL loads may be reduced.^{15, 17}

RETURN TO SPORT
Limb asymmetries with athletic tasks have been identified as a potential risk factor for ACL injury and, therefore, must be minimized prior to return to sport. Focus throughout all phases of the ACL return-to-play program attempts to minimize these asymmetries, not only with strength, but also with athletic maneuvers. Asymmetries in vertical ground reaction force are a potential means of determining an athlete's readiness to safely return to sport. Therefore, one should assess a patient's ability to demonstrate symmetry in vertical ground reaction force during landing maneuvers prior to return to sport.

Additionally, the vertical jump exercise has been show to demonstrate a high level of reliability in normal subjects.¹⁸ An athlete prepared to return to sport should be able to perform three successive vertical jumps with high repeatability (eg, ± 15%) of their median maximum jump. An athlete who fails to perform at this level may be affected by residual pain or a neuromuscular deficiency that warrants further training prior to return to play.

CONCLUSIONS AND APPLICATIONS
ACL injury and reconstruction should not end an athletic career. Surgical management with appropriate early physical rehabilitation can bring an athlete back to a baseline functional level. Progressive multi-phase return-to-play neuromuscular training that bridges the gap from the clinic to the playing field should allow the athlete to safely return to competitive play. Be aware that bilateral asymmetries may continue to be present during dynamic sports maneuvers beyond 2 years in competitive athletes if not appropriately assessed and treated.¹⁹ Therefore, training focused on maximizing strength and balance and minimizing limb asymmetries is necessary to ensure successful rehabilitative management and return to sport for the competitive athlete following ACL reconstruction.

Gregory D. Myer, MS, CSCS, is a sports biomechanist for Cincinnati Children's Hospital Research Foundation's Sports Medicine Biodynamics Center.
Mark V. Paterno, PT, MS, SCS, ATC is coordinator of orthopedic and sports physical therapy for the Cincinnati Children's Hospital Research Foundation.
Timothy E. Hewett, PhD is director of the Cincinnati Children's Hospital Research Foundation's Sports Medicine Biodynamics Center.

REFERENCES

Shelbourne KD, Klootwyk TE, Wilckens JH, De Carlo MS. Ligament stability two to six years after anterior cruciate ligament reconstruction with autogenous patellar tendon graft and participation in accelerated rehabilitation program. Am J Sports Med. 1995;23:575-9.
Lephart SM, Fu FH. Proprioception and Neuromuscular Control in Joint Stability. Champaign, Ill: Human Kinetics; 2000.
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Hewett TE, Paterno MV, Noyes FR. Differences in Single Leg Balance on an Unstable Platform Between Female and Male Normal, ACL-Deficient and ACL-Reconstructed Knees. 1999.
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Hewett TE, Stroupe AL, Nance TA, Noyes FR. Plyometric training in female athletes. Decreased impact forces and increased hamstring torques. Am J Sports Med. 1996;24:765-773.
Besier TF, Lloyd DG, Ackland TR, Cochrane JL. Anticipatory effects on knee joint loading during running and cutting maneuvers. Med Sci Sports Exerc. 2001;33:1176-81.
Myer GD, Ford KR, Palumbo JP, Hewett TE. Comprehensive neuromuscular training improves both performance and lower extremity biomechanics in female athletes. J Strength Cond Res. In press.
Neptune RR, Wright IC, van den Bogert AJ. Muscle coordination and function during cutting movements. Med Sci Sports Exerc. 1999;31:294-302.
Moir G, Button C, Glaister M, Stone MH. Influence of familiarization on the reliability of vertical jump and acceleration sprinting performance in physically active men. J Strength Cond Res. 2004;18:276-80.
Paterno MV, Ford KR, Myer GD, Heyl R, Hewett TE. Biomechanical limb asymmetries in female athletes 2 years following ACL reconstruction. Submitted to American Physical Therapy Association Combined Sections Meeting, New Orleans, 2005.

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