Sport-specific training and return to sport after ACL reconstruction in elite athletes: a narrative review

Introduction

Background

When collegiate and professional athletes undergo an anterior cruciate ligament (ACL) reconstruction, the rehabilitation process is reverse engineered from the demands of the athlete’s sport and position. The athlete’s and medical staff’s goal and definition of success is that they return to sport (RTS) participation at their prior level of function. This requires meeting objective criteria such as lower extremity strength measurements ≥90% limb symmetry index (LSI), passing scores on psychological readiness assessments, and performance benchmarks such as running speed and jump metrics. What makes this population unique is that coaches, teammates, and management also have a vested, and possibly financial, interest in the athlete’s safe and timely RTS. Scholarship retention, depth chart position, roster decisions, financial compensation through name, image, and likeness (NIL) and contract extensions are a few of the items affected when a collegiate or professional athlete sustains a long-term injury. Unfortunately, being considered an elite athlete does not guarantee a successful RTS, as recent evidence suggests that the successful return to prior levels of competition for elite athletes ranges from 83–95% (1-3).

Before beginning any functional training, it is imperative that the early phases of post-operative rehabilitation be successful. This includes achieving a “quiet knee” [full active and passive extension, trace to zero effusion, normal patella mobility, and no lag with a straight leg raise (SLR)], and returning quadriceps strength to ≥80% LSI. Without meeting these foundational criteria and normalizing activities of daily living (ADL) function, the athlete will have a difficult time progressing through the later phases of rehabilitation. For the purpose of this paper, we will operationally define sport specific training as rehabilitation and training that occurs once the athlete is cleared to begin a running progression. Clearance to initiate a jogging progression is a major milestone in the rehabilitation process, and one that is poorly defined. In a scoping review of 201 studies, Rambaud et al. identified the median time to return to running occurred 12 weeks after surgery but that less than 20% of the studies used objective performance testing to clear patients to run (4).

Rationale and objective

A “gold standard” ultimate RTS clearance test also does not exist. Currently, rehabilitation professionals use various combinations of time after surgery, strength measurements, hop testing, functional runs/movements, force plate testing, and player analytics via Global Positioning System (GPS) devices (5-20). The purpose of this narrative review is to discuss and outline the sport specific progression of elite athletes following ACL reconstruction. This will encompass the comprehensive approach taken by the sports medicine and performance teams, which include but are not limited to the surgeon and physicians, physical therapist, athletic trainer, strength and conditioning coach, and sports scientist. The comprehensive post-operative rehabilitation approach blends the expertise of the surgeon focusing on biological healing, the sports medicine team resolving musculoskeletal impairments and completing functional testing, and the sports performance team testing and restoring on- and off-field metrics to preinjury levels. In the absence of a “gold standard” clearance criteria, we aim to utilize frameworks such as the Control-Chaos Continuum (21,22), RTS Clearance Continuum (RTSCC) (23), and the RTS continuum which differentiates the return to participation from RTS and return to performance (24). The principles and strategies employed in this review can be applied to all athletes recovering from ACL reconstruction. Due to participation levels and incidence of ACL injuries, we have paid special attention to football (25), soccer (26), basketball (27), and skiing (28). We present this article in accordance with the Narrative Review reporting checklist (available at https://aoj.amegroups.com/article/view/10.21037/aoj-25-27/rc).

Methods

A narrative review was written on the rehabilitation and RTS procedures following ACL reconstruction in elite athletes. The PubMed database was searched for entries within the past five years for articles related to rehabilitation and RTS following ACL reconstruction. Studies related to specific interventions and testing criteria, as well as sport-specific rehabilitation were included for review. Selected studies were cross-referenced to find additional related studies. Final selection occurred after reviewing the full article. A summary of the search strategy can be found in Table 1.

Rehabilitation to return to participation

Regardless of the diagnosis or the patient’s level of function, discharge planning starts on day one, and this holds true even for elite athletes. The physical therapist must evaluate what the patient needs to safely RTS and reverse engineer a rehabilitation program that prepares the athlete for the physical demands of their sport. These demands will vary depending on the sport, as the physical stressors for sports such as skiing and soccer for example are different. Rehabilitation is also position dependent, as an offensive lineman has different responsibilities and movements compared to a defensive back in football. The rehabilitation process should therefore be individualized to reflect the demands and movement requirements of the athlete’s sport and position. The Return to Participation phase consists of activities related to functional training that serves as the foundation for sport specific movement patterns. They are always performed in a controlled environment and under the supervision of the sports medicine team. Guidelines for functional training, starting in the Return to Participation Phase, are presented in Appendices 1-4 for football, soccer, basketball, and skiing, respectively.

While time after surgery for biological healing must be considered, the goals of rehabilitation to achieve successful RTS participation include strengthening the quadriceps, completing a running progression, and initiating low impact plyometric exercises. Fortunately, the rehabilitation plan and criteria to begin and progress through a running program look very similar for the majority of athletes. At this point (3–5 months post-op), range of motion (ROM) should be full, quadriceps strength should be ≥80% LSI (4,16,29) and/or a peak torque to bodyweight ratio (PTBW) of >1.45-2.0 Nm/kg (30), quadriceps exercises should shift from hypertrophy to strength training focus (31), and the athlete should not display any reactive effusion following running and involved lower extremity strengthening.

Regarding the initiation and progression of running, the physical therapist must determine the distance and speed ran, individual session and weekly volume, and running schedule. If available, running progressions may initially start in an antigravity treadmill or in a pool to reduce the impact of ground reaction forces. Specific programs that provide a framework for interval running progressions following lower extremity injuries exist in the ACL literature (32,33). The athlete should constantly be monitored for an increase in effusion and soreness after running (32). This is more important once they start to run on a treadmill or flat ground, as each step produces ground reaction forces approximately 2–8 times their body mass (34,35). As the athlete continues to progress with strength training with higher loads and intensities, they will require rest days between strengthening sessions. Thus, running should occur on the days they strength train. The “rest days” can be used for motor control exercises, adjunct muscle group strengthening, and recovery. A sample progression of running in an anti-gravity treadmill is provided in Table 2.

Running, by definition, is a plyometric activity as it utilizes the stretch shortening cycle. Following ACL reconstruction, we are evaluating if the post-operative knee can tolerate the increased amount and speed of load imparted to the tibiofemoral joint. As the athlete resumes running, this will be their first encounter with faster movements. In this phase, the rehabilitation team should include exercises that have similar physical demands and forces as compared to running to improve load tolerance performance. Some examples include, but are not limited to, A-skips and B-skips, where the athlete is cued to “put force into the ground” (Video 1), performing forward and lateral quick feet drills in a ladder (Video 2), wall drills (Video 3), and assisted pogo jumps where the emphasis is on a quick ground contact time (Video 4).

Some clinicians utilize the benchmark of 80% LSI for hop testing as one component to determine readiness to initiate running (36-38) while some surgeons and clinicians prefer to delay maximum distance hopping until the later stages of rehabilitation. However, sub-maximal hopping can be a valuable training exercise in this phase. Single leg quick repeated hopping forward/backward and medially/laterally over a line or piece of tape on the floor primarily uses an ankle strategy that mimics the lower leg function during running (39,40). Single leg vertical hopping performance has been found to better reflect quadriceps strength status compared to horizontal (forward) hops (41-45). In this phase, the athlete can focus on the propulsion portion of the hop, where they stand on the involved extremity, perform a countermovement and hop vertically onto a box, landing on one or both feet. This is advantageous to focus on the propulsion portion in this phase as more work (J/kg) occurs at the knee joint compared to the work distribution with horizontal hops (12,40,41). A hallmark of this phase is developing the neuromuscular system to control deceleration with running and plyometric tasks, and submaximal hopping facilitates this adaptation.

The running progression is athlete and sport dependent. When the athlete demonstrates that they can run without increased effusion and soreness in an antigravity treadmill, they can transition to the field/court with a strength and conditioning coach to build running capacity. A sample running progression is provided in Table 3. The progression is performance based and the athlete will never run more than three times per week. The athlete will continue to run the same distance and total volume until they reach the velocity goal to progress to the next phase. When the athlete completes the structured linear running, they will progress to angular running and frontal plane change of direction (COD). GPS tracking is especially useful, as it monitors variables such as total distance ran, total high-speed distance ran, and maximum velocity. Although running is not a component of a sport like skiing, it remains a valuable exercise for these athletes as part of the plyometric training progression. Other sports run significantly more but vary among position groups. Elite men’s soccer players run 6,230–15,040 yards/match (46) and collegiate men’s and women’s soccer players run 9,700–10,800 and 6,000–8,200 yards/match, respectively (47-49). Wide receivers and defensive backs, on the other hand, run 4,700–5,500 yards/game (50) but the offensive and defensive linemen only run ~2,300 yards/game (51). The volume during the return to running and conditioning phases should reflect the athlete’s sport and position. During offseason conditioning, modifications can be made to allow the recovering athlete to participate with their team and position group but run off to the side if they cannot complete the distance or speed required for that day’s training plan. Once they can, it is beneficial to include the athlete in team functions even if modifications are made.

Rehabilitation to RTS

During the Return to Sport Phase, the athlete will focus on skill development (23), progressing to higher impact plyometric training, transitioning from a strengthening program to one focusing on power development, and normalizing sport specific movement patterns. The athlete will also progress the complexity of their position and sport specific movement patterns during their field/court training sessions. Physical therapy looks to introduce and ensure safety with these new activities, and resolve any remaining musculoskeletal impairments, as poor biomechanical and neuromuscular control with landing and pivoting movements is a known risk factor for injury (17,52-54). The strength and conditioning coach and sports scientist will continue to track and progress variables such as distance ran, velocity, and player load (volume metric of a player’s activity level during a training session or game), and improve the efficiency of movement on the field/court. As the athlete progresses the speed and intensity of sport and position specific movements, this is a perfect opportunity to involve the coaching staff to observe and provide insight on the athlete’s movement patterns.

Neuroplastic changes occur following ACL reconstruction and a specific post-operative rehabilitation strategy must be employed to achieve proprioceptive reweighting (55-59). Following ACL reconstruction, athletes aim to RTS, which is an everchanging, chaotic environment that the athlete must navigate. Recent evidence and attention have been given to sensory reweighting and the principles of motor learning within the rehabilitation process (60,60-64). Throughout the rehabilitation process, external focuses of attention and implicit learning can be used to instruct the athlete on proper ways to perform exercises (56,62,64). The use of decision-based tasks can be used for both simple and complex rehabilitation exercises. For example, visual or auditory cues can be used to indicate which leg to step with during a lunging exercise, or they can be used to determine in which direction a jump should occur during various plyometric drills.

Throughout this phase, the athlete should continue their resistance training program with the strength and conditioning coach and at this time, there should not be any limitations in the weight room aside from plyometric training. In the collegiate and professional setting, the athlete will have preinjury data such as maximum lifts (e.g., squat, deadlift and bench press), countermovement jump (CMJ) metrics and running metrics via GPS tracking (e.g., typical distance ran in practice, maximum velocity, and typical player load in a practice). The athlete should meet or be close to meeting their preinjury numbers for lifts such as squat and deadlift, and now progress towards completing these movements at their preinjury velocities (65). In conjunction with the strength and conditioning plan, the physical therapist should continue to ensure the quadriceps strength is ≥90% of both current (LSI) and estimated preinjury capacity (EPIC) of the uninvolved extremity (66) and a PTBW ratio >2.5–3.0 Nm/kg (67). The physical therapist should complete regular isometric or isokinetic testing at 60 and 180 °/s using an electronic dynamometer (41,44,68), or an in-line dynamometer, depending on availability (69-72). Another variable to assess is the quadriceps rate of torque development (RTD) in the 0–200 ms window, which decreases after injury and surgery but reaches pre-surgery levels around nine months post-surgery (73-76). RTD has been linked to landing mechanics with functional tasks and must be restored to normalize the absorption of peak forces imparted on the knee as well as the amount of knee excursion with landings (73-80). The athlete will be returning to activities and movements that are linked to ACL injury, such as COD and agility combined with deceleration, and landing or pivoting near full extension, making it imperative that the strength and RTD is sufficient to control these motions (17,81-88). Improvements in RTD are also directly correlated with improved athletic performance (89-94) and interventions such as weightlifting, power training, and plyometrics have been found to improve quadriceps RTD (95-97).

Deceleration and movement retraining will progress to full speed/effort in this phase. This includes, but is not limited to COD drills, multiplane plyometrics, sprinting, cutting and pivoting. Moving outside the sagittal plane at faster speeds, changing direction, and performing unanticipated tasks all impart greater load to the knee (98-101). When beginning the running progression, the athlete will initially focus on acceleration and take as much distance needed to gradually slow down. The progression to deceleration training in this phase involves running a predetermined distance and stopping at a designated spot, requiring double leg deceleration in a shorter period of time. The demands on the knee are higher with decelerations compared to accelerations and sprinting, so the speed and distance ran will be the progression and the athlete must demonstrate the ability to decelerate without a limb dominant compensation strategy (102-105). Single leg horizontal and vertical hopping will help to retrain this motor pattern. Single leg vertical hopping for height utilizes more quadriceps muscle contribution than horizontal hops (40,41) but single leg landing from horizontal hops emulate the eccentric control needed with deceleration for COD tasks (10,106).

When progressing COD training, the athlete must demonstrate braking control regardless if the next acceleration is laterally or backward by maintaining a more posterior center of mass in order to maximize athletic performance (103). Specific and familiar movements such as lateral shuffling between two cones or executing a 45° cut, should be introduced at submaximal speeds due to the greater loads placed on the knee (98,101,107). As performance improves, the athlete will transition to agility training (performing COD drills on reaction as opposed to pre-planned) and higher angle cutting such as 90° and 180°. If available, two-dimensional (2D) video analysis can be used to qualitatively evaluate the movements (108). Video analysis reveals that ACL tears occur in sport when complex tasks and movements occur, both pre-planned and unplanned (53,109). All COD drills in this phase should employ the principles of motor learning to best accelerate the learning process and transition it to sport (56,62,64,110).

Without a specific “gold standard” clearance test, the components of RTS testing are highly variable (24). However, completing the full course of rehabilitation, including functional training and passing a RTS test, results in a reduced risk of reinjury (11,111). The authors advocate assessing strength, functional movements, and psychological readiness. Throughout their course of rehab, the athlete must be tested regularly in an isometric or isokinetic fashion on an electrical dynamometer (41,44,68) and achieve ≥90% LSI (111-116). Values should be compared to preinjury data when available. In addition to isokinetic dynamometry, hamstring strength can be tested eccentrically with a NordBord (Vald Performance NordBord, Version 1.0, Australia) (117-119) and hip musculature can be assessed with a ForceFrame (Vald Performance, Brisbane, Australia) (120-122). Hop tests continue to be a clinically friendly test following ACL reconstruction due to the lack of equipment required and ability to use the contralateral leg as a benchmark. Examples include the single hop for distance, timed 6-meter hop, triple hop for distance, triple crossover hop for distance, single medial and lateral hop for distance, single leg vertical hop for height, and the medial/lateral hop test (9,10,12,13,18,40,41,106,123-127). Despite its widespread use in RTS testing (125,128,129), hop testing can overestimate knee function following ACL reconstruction (66) and the LSI for distance does not provide the necessary information on movement quality or recovery of knee function (130-132). For those reasons, we advocate using the EPIC values for hop testing when available.

Collegiate and professional athlete will most likely have access to force plate testing, which has been found to reveal kinetic asymmetries in multiple phases of the double- and single-leg CMJ (17,85,133-135). While single-leg CMJ (SLCMJ) provides better inter-limb assessments, double-leg CMJ (DLCMJ) provides insight on jump strategy, deceleration performance, jump height, and landing forces that can be compared to preinjury data (136-138). The SLCMJ interlimb differences are associated with COD capability and can be used to objectively assess countermovement depth, jump height, deceleration, propulsion, and landing forces, formulate concentric and eccentric force velocity profiles, and better detect asymmetries compared to horizontal hops (12,139-142). Force plate testing should resume when the athlete begins their plyometric training and the information should be used to modify the strengthening program to address specific deficits (e.g., eccentric focused activities). The athlete should reach preinjury metrics in DLCMJ and resolve any interlimb differences to <10% with the SLCMJ at the end of the RTS phase.

Elite athletes often have access to equipment and technology to assist in the rehabilitation and training process. Force plates can detect asymmetries with various jumps and balance testing. They can also be used to assess rate of force development and peak force produced with an isometric mid-thigh pull, which is a reliable way to assess for asymmetry (143-145). Technology such as the 1080 Sprint (1080 Quantum Synchro; 1080 Motion AB, Sweden) is a motorized resistance device that tracks force, power and velocity of horizontal movements such as sprinting, COD, deceleration, and hopping. Based on impairments or asymmetries discovered, training can be modified to specifically address them, such as including loaded snap downs to target the eccentric deceleration and isometric mid-thigh pulls combined with vertical hopping to target the concentric propulsion phase. The athlete will also undergo “return to sport” testing each time they are on the field/court. GPS tracking will allow the performance team to assess their distance ran, maximum velocity, and player load and compare it to preinjury data and position specific norms. Utilizing this data to provide a graded exposure to the volumes and intensities that occur during practice and competition is necessary to reduce the risk of injury (146-149).

These individual and team activities also allow for the evaluation of the athlete’s movements in position-specific drills and modified team activities. Their performance will help to determine if the athlete can progress to more complicated and chaotic team activities (21) (see Appendices 1-4 for specific examples) The final components of the RTS phase include performing sport and position specific drills with the inclusion of opposition and contact. These activities begin in controlled, pre-planned fashion where strength and conditioning coaches and teammates often assist to provide opposition and contact for each drill (see Appendices 1-4 for specific examples). In addition to evaluating movement quality with each new activity, the athlete should be assessed for psychological readiness. The ACL Return to Sport Injury Scale (ACL-RSI) assesses emotions, confidence in performance, and risk appraisal regarding returning to sport and has been found to be reliable and predictive of successful RTS in athletes following ACL reconstruction (150-153). In the event that psychological readiness is deemed suboptimal, counseling may be advised throughout the rehabilitation process to minimize the risk of reinjury (152).

Rehabilitation to return to performance

The difference between Return to Sport and Performance phases varies for each athlete and is influenced by factors such as the sport that they play and the time of season. If the athlete is returning at some point during the offseason, it’s easier to transition fully into team activities and resume in-season training alongside teammates. However, if the athlete returns mid-season, a more gradual integration is typically required. Returning to performance is a shared decision between the surgeon, and the rehabilitation and performance staff (154,155). While passing RTS criteria does not guarantee a safe RTS without reinjury (156), failing to pass the RTS test battery, decreased psychological readiness, and abnormal movement patterns seen on the field/court can justify restricting an athlete’s participation in new or more complex activities. This important distinction assists in the process of allowing the athlete to return to unrestricted practice and ultimately competition.

In the Return to Sport Phase, the athlete will participate in various individual drills and progress to ones that include teammates which help to incorporate reaction and contact. For example, soccer players can progress to 3v3 play that includes block tackles and slide tackling and football offensive and defensive linemen can progress to run and pass blocking (see Appendices 1,2). The Return to Performance phase aims to incorporate the athlete back into practice situations without modification. This includes the whole team (e.g., 10 players on the court in a basketball game, 22 players on the football field) playing at full speed, with contact, and incorporates decision-making scenarios that come with the athlete’s individual sport and position. In this phase, the rehabilitation staff is evaluating the athlete’s movements to look for compensatory patterns, the knee’s response to increased activity (post-practice swelling), and the athlete’s report regarding their mental state. The performance team and coaching staff is simultaneously evaluating the athlete’s movements and overall performance. While a specific volume of modified practice participation before full clearance is unknown to determine ultimate injury risk reduction, the medical staff makes the decision to notify the coaches when the athlete is fully cleared and available. If the athlete is returning to performance during the competitive season, once full medical clearance is given, the return to game participation rests in the coach’s hands. They determine if the performance on the field/court warrants getting playing time during a game. When this happens, it is important for the athlete to know that the medical and performance teams are still available to support them. This includes, but is not limited to, meetings with mental health professionals, pre- and post-practice medical treatment, and supplementary workouts/training.

This narrative review is not without its limitations. The review serves as a framework that clinicians can utilize when providing physical therapy to athletes following ACL reconstruction. It does not account for concomitant injuries and surgeries nor does it address the rehabilitation process that should begin immediately following the ACL injury. The review includes a level of bias, as the evidence provided reinforces our thought processes in using this framework. The review references tests and measures which require equipment that is not readily available in outpatient physical therapy clinics. It also highlights the on-field/court training that occurs with a strength and conditioning coach, which also may be a barrier to applying this framework to all patients following ACL reconstruction.

Conclusions

This narrative review outlines the progression of post-operative care in elite athletes following ACL reconstruction. Sport participation for elite athletes is a yearlong process. As soon as the competitive season ends, offseason training and conditioning begins. This increases the number of exposures the athlete encounters when they are cleared for full participation. Along with time after surgery (11), the rehabilitation, reconditioning, and testing process looks to identify and resolve asymmetries and impairments to safely return the athlete to sport. Throughout the rehabilitation process, increasing the complexity of the exercises/drills is a form of progression. Athletes can perform strengthening exercises and sport-specific movements in controlled environments with strict parameters and add elements of “chaos” by requiring the athlete to answer questions during the drill, starting/stopping based on reactions and cues, including a ball to catch or kick, and/or including a teammate or opponent within the drill. The athlete must pass all testing that the sports medicine and performance team requires, and they must also demonstrate the same standard of performance in a chaotic environment. Unfortunately, the gold standard RTS test does not exist. Future research is needed to refine objective benchmarks to identify which test and measures are most specific to support a safe and successful RTS decision following ACL reconstruction in elite athletes. This review outlines the key principles of a criterion-based progression can be used in athletes following ACL reconstruction and illustrates how task complexity increases throughout the rehabilitation process, culminating in a safe return to the athlete’s prior level of sports participation.

Acknowledgments

None.

Footnote

Provenance and Peer Review: This article was commissioned by the Guest Editors (Jeremy Burnham, Brian Godshaw and Patrick Cook) for the series “Evaluation and Treatment of ACL Injuries in High Level Athletes: The Continuum of Care” published in Annals of Joint. The article has undergone external peer review.

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at https://aoj.amegroups.com/article/view/10.21037/aoj-25-27/rc

Peer Review File: Available at https://aoj.amegroups.com/article/view/10.21037/aoj-25-27/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://aoj.amegroups.com/article/view/10.21037/aoj-25-27/coif). The series “Evaluation and Treatment of ACL Injuries in High Level Athletes: The Continuum of Care” was commissioned by the editorial office without any funding or sponsorship. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All videos are published with the participant’s consent.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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