Return-to-Sport Testing in Young Athletes After Anterior Cruciate Ligament Reconstruction

Abstract

Implementing return-to-sport (RTS) testing should be an integral component of rehabilitation for young athletes who have undergone anterior cruciate ligament (ACL) reconstruction, but there are no universally accepted standards for such testing. In this article, we highlight our institution’s use of a structured and evidence-based approach to guide RTS decision-making for athletes, coaches, surgeons, therapists, and parents, with an emphasis on reducing the likelihood of reinjury after ACL reconstruction surgery.

Introduction

Anterior cruciate ligament (ACL) injuries are among the most common and debilitating sports-related injuries, affecting young athletes worldwide [2]. These injuries may be due to overuse, faulty movement mechanics, a lack of functional strength, or a lack of a regimented strength and conditioning programs. As youth increasingly participate in high-intensity and year-round sports, the incidence of ACL tears and surgical interventions to correct them has risen [40]. Despite the need for measures addressing safe return to sport (RTS) for young athletes following ACL reconstruction, there is no consensus in the rehabilitation community on a single testing protocol.

Traditionally, ACL rehabilitation has focused on restoring knee stability and function. Studies have shown that young athletes who RTS before 9 months after injury had a 7 times greater chance of re-injury than those who delayed [4]. In addition, as youth sports become faster, more explosive, and more competitive, it is even more essential for young athletes to reach certain milestones before RTS. This has prompted a shift toward incorporating rigorous RTS testing protocols for this population.

In addition to physical recovery, psychological readiness and the risk of reinjury are critical elements that demand attention. Young athletes often face immense pressures to RTS swiftly, putting them at risk for reinjury [40,42]. We have found that incorporating objective assessment tools and testing into the rehabilitation of young athletes after ACL reconstruction, including both the physical and psychological aspects of RTS, aids everybody involved in the decision-making process.

In this article, we propose 4 pillars of testing based on our clinical experience that may allow for the safe RTS of young athletes who have undergone ACL reconstruction: assessments of strength, rate of force development (RFD), power, and movement as integral components of rehabilitation. We also explore the potential benefits of advanced technologies, such as motion analysis systems, force plate assessments, and functional testing, in objectively evaluating an athlete’s RTS readiness [30,31]. By leveraging such tools, sports medicine practitioners and other clinicians can make informed decisions, mitigate the risk of reinjury, optimize rehabilitation outcomes, and ultimately facilitate successful RTS for the youth athlete.

Strength Assessment

It can be argued that because it is the area in which many athletes are likely to fall short, strength is the most important assessment in RTS for young athletes. This assessment should involve the entire lower extremity; the quadriceps can be the most challenging muscle group to return to full strength. “Full strength,” however, is a relative term. Because the athlete’s preinjury strength numbers may be difficult to attain, we base our benchmarks on the uninjured limb. Although there is currently an industry-wide effort to develop normal strength numbers based on torque production and bodyweight, this approach remains in its infancy [27, 37].

Isolated strength deficits often persist long after initial injury, and poor quadriceps strength has been associated with deficient performance [34, 36]. Grindem et al [13] found that more than half of patients failed to achieve a limb symmetry index (LSI) of >90% at 9 months postsurgery and that for every 1% increase in strength symmetry, there was 3% reduced injury rate. Brunst et al [6] found an LSI of <85% of quadriceps strength showed early markers of cartilage degeneration at 5-year follow-up.

The appropriate timetable for an athlete’s return to running after ACL reconstruction has been debated. In the past, athletes were required to perform functional tests that included a forward step down using an 8-inch box, a single leg (SL) squat, and an SL bridge. While we use these tests, we also rely on more objective strength measurements. After the athlete has achieved no effusion in the knee, the next hurdle is to have an Internal Knee Documentation Committee score of >64, after which we perform quadriceps strength testing. The athlete will need >65% LSI of the quadriceps and >1.7 Nm/Kg bodyweight force generation to be cleared to run [9, 14, 15, 33]. These strength metrics are obtained using isokinetic dynamometry at 60°/s. When isokinetic dynamometry is not available, isometric testing can be completed at 90° knee flexion using a crane/pull-style dynamometer, allowing for a torque/bodyweight calculation using the athletes’ bodyweight and moment arm.

Open kinetic chain knee extension after ACL reconstruction has long been a controversial topic. Some fear that it stresses the reconstructed ACL graft and causes laxity over time. However, Forelli et al [12] recently evaluated open chain knee extension and knee flexion isokinetically and found no increase in graft laxity. They also correlated this finding with decreased quadriceps and hamstring strength deficits at 3 and 6 months postoperatively. The authors concluded that open chain knee flexion and extension strengthening is superior to closed chain exercises alone in restoring quadriceps and hamstring strength.

Rate of Force Development

High levels of athletic performance require high levels of muscular strength to be generated over short intervals. Rate of force development is the amount of strength an athlete is able to generate over a defined time, typically over the first 100 to 250 ms. After ACL injury, deficits in RFD have been demonstrated to lag behind deficits in peak isometric strength [1] and to have stronger associations with asymmetrical biomechanics in tasks such as running and jumping [8,24]. Assessment of RFD can be clinically useful but requires technology—typically, force plates, isokinetic dynamometers, or fixed dynamometers that sample at high frequencies. Rate of force development testing can be used for single joint motions, such as isometric knee extension at 90° knee flexion, or with compound movements such as an isometric mid-thigh pull. The role of RFD testing after ACL reconstruction demands further investigation [8,38].

Power Testing

Restoring lower-extremity power and reactive strength can be difficult after ACL reconstruction. Deficits often persist, despite normalized quadriceps strength [17,18,19,20, 21,22,23]. In recent decades, a battery of hop tests has been used to assess power after lower-extremity injury [3,7]. Traditional hop testing uses horizontal distance as the primary outcome measure for assessing dynamic lower-extremity function, but using hop distance alone has a number of limitations [25,26,41]. Kotsifaki et al [25,26] demonstrated that the propulsive phase of a forward hop is poor at assessing knee function, as 85% of power can be generated from the hip and ankle, and that compensatory movements can be adopted while still achieving >90% LSI on horizontal hop for distance testing. A shift to a hip strategy for power generation can mask deficits in dynamic knee extensor function [25,26,41]. While horizontal hop testing has its advocates [3,28], studies have demonstrated that traditional hop tests have high agreement with one another and are less sensitive than an SL vertical jump at detecting deficits after ACL reconstruction [11,16,29]. Recent studies have shown that deficits in SL jump height and SL reactive strength can persist after quadriceps strength deficits have resolved [25,26,35]. It is our opinion that vertical jump height and reactive strength are better measures of dynamic knee function in RTS testing.

Technology such as force plates, contact mats, and iPhone applications can be used to calculate jump height and reactive strength index with good accuracy and reliability [38]. Countermovement jumps and drop jumps allow for the assessment of slow and fast stretch–shortening cycle activity. We use double and SL variations of both jumps to assess inter-limb asymmetries in force production and absorption.

To allow more in-depth analysis of force-time variables, we recommend the use of dual force plate technologies for looking at performance metrics, asymmetry metrics, and analysis of the force time curve (Table 1).

Table 1.

Examples of performance and asymmetry metrics used to evaluate lower extremity function as part of the return to sport continuum.

Jump type	Performance metrics	Asymmetry metrics
Countermovement jump (CMJ)	Jump height RSI modified	Eccentric impulse asymmetry Concentric impulse asymmetry Peak landing force asymmetry
Drop vertical jump (DVJ)	Jump height RSI	Eccentric impulse asymmetry Concentric impulse asymmetry Peak landing force asymmetry Drop landing RFD asymmetry
Single leg vertical jump (SLCMJ)	Jump height	LSI
Single leg drop vertical jump (SLDVJ)	Jump height RSI	LSI for both jump height and RSI

RSI, reactive strength index; RFD, rate of force development; LSI, limb symmetry Index.

For additional information on these metrics please see the Vald Forcedecks User Guide (https://support.vald.com/hc/en-au/articles/17160684077977-ForceDecks-User-Guide) or Comfort P, Jones PA, McMahon JJ. Performance Assessment in Strength and Conditioning. New York, NY: Routledge; 2018.

Some level of asymmetry (<10%) is normal and expected when performing bilateral and unilateral jumping tasks [32]; indeed, 0% asymmetry is unlikely with double leg jumps, and uninjured athletes often show natural variability when they can load each leg asymmetrically over a series of jumps. Jordan et al [18,19] demonstrated that those without ACL injury often have natural variability in the loading asymmetries with repeated jumping tasks, while those who have undergone ACL reconstruction shift to the nonoperative side. Further analysis of jump strategies and phase specific asymmetries can be used to evaluate force-time curves [18,19,35]. Deficits in the eccentric deceleration phase of a countermovement jump may be predictive of horizontal breaking ability—a key performance indicator for athletic performance and a good proxy for evaluating this ability.

The addition of vertical jump testing has been a crucial part of RTS testing in young athletes at our institution. We believe that the resolution of deficits in neuromuscular qualities such as power, explosiveness, and reactive strength are critical for injury reduction and RTS after injury.

Movement Testing

Assessing how a young athlete moves throughout rehabilitation after ACL injury is essential for RTS. Movement occurs as the result of repeated muscular contractions regulated by neural control mechanisms and by motor learning pathways within a predetermined structural boundary. All segments working at the appropriate time results in a conservation of momentum and an unleashing of explosive power, which is known as “kinetic linking” [10]. During the initial stages of rehabilitation of a patient with lower-extremity injury, alterations, and compensations in movement strategies become apparent; some may have been present before injury and may have contributed to it. In a study of 3-dimensional biomechanics, patients who underwent ACL reconstruction had reduced energy absorption contribution from an ankle strategy when landing from a drop vertical jump [5]. Comprehensive movement testing enables the detection of these imbalances and provides information necessary to individualize a rehabilitation program.

One aspect of kinetic linking is that appropriate biomechanical forces and motions result in desired athletic movement. Research has shown how a loss of range of motion, strength, or sequencing at one joint or limb segment can affect a proximal or distal joint or segment. In 2007, Wainner et al [39] coined this “regional interdependence” [10].

Assessment of movement is twofold. First, a qualitative assessment ensures that proper movement patterns are being implemented with higher level movements such as running, jumping, and cutting, as well as with fundamental movements such as squatting and eccentric step-down training. Making sure that a young athlete has the proper movement patterns, such as a hip hinge pattern and alignment during SL activities, is a precursor to higher level movement patterns. Skilled observation to correct movement faults is one of a rehabilitation professional’s best tools.

Quantitative assessment of movement is also necessary and can be performed with 2-dimensional video, including video capture using smartphone and tablet applications. In addition, software is available for portraying joint angles and estimating landing forces along with RFD measures. Recent advancements in computer vision and machine learning have led to the development of markerless motion capture systems that use depth cameras and sophisticated algorithms to track and analyze human movement in real-world environments. This makes it easier for a clinic—especially one without an advanced motion capture lab—to assess and quantify movement in a young athlete.

Both qualitative and quantitative data can identify the source of abnormal movement patterns. For example, inefficient depth during an SL squat can result from quadriceps weakness or from ankle immobility. If the latter is present, providing the athlete with concrete data on joint angles can help them visualize and execute optimal movement.

Safe RTS Requires Collaboration

A young athlete’s readiness to RTS must include reconditioning, a standard that ensures that they have met the fitness requirements to play safely, competitively, and explosively. This is the least understood part of the process. It should address skill development, force, and load volume tolerance and include an RTS plan that exposes the athlete to workloads replicating those of their sport without exceeding workload capacity [10] (Fig. 1).

Fig. 1.

Return to Sport Clearance Continuum (RTSCC) as described by Draovitch et al [10]. Highlights the importance of looking at decision-making as a continuum; progressive, regular testing including qualitative and quantitative movement aid in assessing specific neuromuscular qualities and provide a criterion-based approach to return-to-sport decision-making.

In this phase, the focus shifts from the injury to the athlete. An exercise physiologist or a strength and conditioning coach can guide the young athlete through the progression of fitness and the advancement of strengthening necessary for them to return to full performance in their sport. Depending on the patient’s age and experience, however, this phase can seem long and arduous. Collaboration among surgeon, physical therapist, strength and conditioning coach, athlete, parents, and sports coach makes it more likely that a young athlete will return to initial play and then to safe and full performance of sports activities.

At our institution, the testing of a young athlete after ACL reconstruction occurs in collaboration with the surgeon and starts at 6 weeks postsurgery, with repeat testing at 3-, 6-, and 9-month follow-ups. Education of all stakeholders is critical to RTS, especially to the young athlete, for whom these periods are not called “testing intervals” but rather “check-ins” to assess strength, power, movement, and fitness. If an athlete is falling behind, frequent testing can put them back on track. Conversely, if the athlete is reaching milestones, it boosts their confidence. This testing process also helps the athlete know what milestones must be reached to attain their RTS goal.

In conclusion, successful rehabilitation of a young athlete after ACL reconstruction requires a patient-centered approach, with the primary goals of RTS while minimizing re-injury risk. Unlike professional or collegiate athletes, young athletes often do not have the organizational, multidisciplinary support to help guide RTS. With the increasing demands of youth sport, it is critical that they understand that RTS is a process that requires completion of milestones in reaching toward their optimal strength, power, and movement capabilities.