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ACL Rehabilitation: Re-injury and Return to Sport Tests

Original Editor - Mariam Hashem Top Contributors - Mariam Hashem, Wanda van Niekerk, Kim Jackson, Jess Bell, Tarina van der Stockt, Uchechukwu Chukwuemeka, Rucha Gadgil, Chelsea Mclene, Leana Louw, Olajumoke Ogunleye and Carin Hunter  

ACL Reinjury Rates

Approximately 81% of people return to some form of sport or activity after ACL reconstruction (ACLR); 65% return to their previous level of sport, but only 55% to competitive sport.[1] For those who successfully return to sport, reinjury remains a significant risk.

The average incidence of reinjury is 15%, split roughly equally between the contralateral knee (8%) and the ipsilateral (or grafted) knee (7%). This figure rises to 23% in athletes under 25 years of age who return to sport.[2]

Note that a second ACL injury can either be a reinjury to the previously injured ligament or a contralateral ACL injury.

Overall, the contralateral ACL has a higher rupture rate than the ipsilateral ACL in athletes, though this finding is driven primarily by female athletes; male and adolescent athletes do not show the same pattern.[3] Passing return-to-play testing appears to reduce re-rupture risk on the ipsilateral side, but does not appear to affect the contralateral side.[4] The risk of reinjury extends up to 5 years following injury.[5][6] For example, in professional male football, one in five players sustains a second ACL injury over an average of five years.

Reinjury Risk Factors

Risk factors for second ACL injuries have been widely investigated in the literature.

Neuromuscular Risk Factors

Persistent neuromuscular deficits, including muscle weakness, reduced joint mobility, and impaired movement control, are commonly identified following ACLR and may persist for months or years despite formal rehabilitation.[7][8] Neuromuscular control deficits are considered a key modifiable risk factor for a second ACL injury.[8]

Specific biomechanical risk factors include: altered movement patterns at the trunk and lower limb; knee, hip and trunk control deficits during landing tasks; muscle imbalance, and side-to-side asymmetry.[9][10][11]

Abnormal movement patterns are often present bilaterally, with changes in kinetics and kinematics showing in both knees.[12] Compensatory strategies in the uninvolved hip are considered a primary predictor of risk in athletes who sustained a second ACL injury within the first year of return to play.[13] Therefore, rehabilitation should involve both limbs.[12]

Quadriceps muscle weakness commonly persists after ACLR and can lead to abnormal loading patterns during gait and sports activities.[12][14] Research has found that knee extensor strength is reduced by more than 10% at one year post-ACLR compared with the contralateral limb, and around 20% compared with uninjured controls.[14] There appears to be limited improvement in strength after this time.[14] Similarly, a difference in thigh circumference of more than 2.5 cm at one-year follow-up was also found to be a significant predictor of reinjury.[15] Early restoration of quadriceps strength is, therefore, a clinical priority.

The earlier quadriceps strength is restored, the better a patient's functional performance at the time of return to sport.[16][17]

Demographic and Surgical Risk Factors

Young athletes are at a higher risk of both reinjury and contralateral injury.[12] The odds of reinjury after ACLR increase by 7% for every one-year decrease in age.[18] Athletes younger than 18-20 years are 2.5 times more likely to reinjure their ACL compared with athletes older than 18-20 years.[18]

The evidence on sex as a risk factor is mixed, with no consistent difference in reinjury rates between males and females following ACLR, though females have a higher risk of contralateral ACL rupture.[19][18][20][21]

Graft type may influence reinjury risk, with research showing an advantage for autograft over allograft and patellar tendon graft over hamstring graft.[22]

Psychological Risk Factors

Psychological factors such as fear of reinjury, kinesiophobia, lack of confidence in performance after ACLR, and negative mood states all increase the risk of reinjury.[23] Kinesiophobia is associated with altered movement patterns, reduced physical activity and poorer functional outcomes after ACLR. Athletes who return to sport after ACLR show significantly higher psychological readiness, greater self-efficacy and lower kinesiophobia than those who do not return to sport, even when both groups have similar objective knee function scores.[24]

Prevention of Reinjury

Reinjury and the need for revision surgery are associated with poorer long-term outcomes, including an increased risk of osteoarthritis, persistent knee instability, and reduced likelihood of returning to sport.

Achieving side-to-side quadriceps symmetry before returning to sport is considered essential for reducing re-injury risk. The hamstrings-to-quadriceps (H/Q) strength ratio is another important factor to consider in rehabilitation. The aim is to achieve at least 90% strength symmetry before returning to sport.[25] Return to sport criteria are also important. One study found that athletes who passed return to sport criteria had a 5.6% reinjury rate. Those who failed these criteria had a 38.2% reinjury rate.[26]

Does Timing Play a Role in Preventing Reinjury?

The role of time in reducing ACL reinjury risk after reconstruction is an ongoing debate. Early opinion in the late 1990s favoured a 6-month return to sport (RTS) timeline. However, a landmark study in 2016 reported a significant reduction in reinjury rate for each month RTS was delayed until 9 months post-surgery; after 9 months, no further risk reduction was observed.[26] Returning to sport as less than 9.5 months after surgery is considered a significant predictor of reinjury,[15] and young athletes have a sevenfold increased rate of sustaining a second ACL injury when they returned to sport before 9 months post-ACLR.[27] Opinions vary considerably among physiotherapists worldwide, with between 22% and 95% allowing RTS before 9 months post-ACLR,[28]  [29] but a recent systematic review recommends "delaying RTS to more than 9 months (especially in non-professional athletes) after index ACL reconstruction, to be better safe than sorry."[30] Other research calls for delays of up to 12 months to 2 years to allow biological healing, maturation and ligamentisation of the new graft.[31][32]

However, the focus is shifting from time alone towards a “combination of time-based and criteria-based RTS decision-making”,[28] with an increased emphasis on functional testing.[33] While clinical guidelines conflict on whether time or objective criteria should take priority,[34][25] prospective research on male athletes showed that adherence to the rehabilitation protocol had a dose-response relationship with RTS rates. When athletes completed their rehabilitation and met the set objective criteria, their odds of returning to pivoting sports significantly increased. Crucially, if the athletes met the objective criteria, time did not influence the risk of a reinjury. This suggests that the focus should rather be on “how” the athlete returns, rather than “when”.[28]

Greater emphasis should be placed on individual patient needs and the repeated assessment of functional competencies throughout rehabilitation, rather than focusing primarily on time. Returning to sport is not a single decision made at the end of rehabilitation, but rather a continuous process. At each stage of this process, objective criteria should guide progression.[28] Rehabilitation programmes should be personalised and consider factors, such as age, motivation, adherence, and rehabilitation complications.[28]

The Influence of Psychological Factors

The psychological impact of an ACL injury can be significant and lasting.[35] Pain-related fear is a key factor in whether athletes return to their preinjury level. Lack of confidence and fear of reinjury are believed to influence function[36] and should be assessed and included in the management plan.[37] Psychosocial factors, including fear-avoidance, self-efficacy, stress, social support, and athletic self-identity, can help explain why some athletes who have otherwise regained full function are unable to return to sport. They are also predictive of outcomes after surgery.[38] Other psychological traits such as resilience, pain catastrophising, locus of control and kinesiophobia may also affect recovery and should be monitored throughout rehabilitation.[39]

Motivation is another important consideration. Research shows 45% of community-level athletes discontinue supervised rehabilitation by 3 months post-ACLR.[26] Athletes may feel ready to compete before meeting all RTS criteria and may stop rehabilitation prematurely. Quality patient education, goal setting, frequent feedback and individualisation of the rehabilitation plan are all recommended to support motivation.[40]

Clinicians should be aware of the impact that psychological factors can have on recovery, quality of life and management preferences, in order to provide person-centred and individualised care.[35]

[41]

Return to Sport

Given the high incidence of reinjury and the evidence of poor outcomes of ACL revision surgery, there is a need for structured RTS protocols. As discussed above, there has been a shift from time-based decision making towards a multidimensional framework that includes physical, functional and psychological readiness. Progression should be guided by specific milestones, such as strength symmetry, neuromuscular control and psychological readiness.[42]

Late-Stage Rehabilitation and Return to Sport

Buckthorpe[43] identified four important variables that should be addressed in the late-stage rehabilitation and return to sport period.

  1. Explosive neuromuscular performance: the capacity to generate force rapidly (rate of force development) is necessary for dynamic joint stability in sport, and this ability is not captured by maximal strength testing alone.[43]
  2. Movement quality: compensatory movement strategies should be identified and addressed before return to sport, as altered mechanics in cutting, landing and change of direction tasks remain a reinjury risk even when limb symmetry indices are good.
  3. Influence of fatigue: athletes must be trained and tested under conditions of fatigue, as neuromuscular control deteriorates with exercise, and sport rarely occurs in a rested state.[43]
  4. Sport-specific retraining: late-stage rehabilitation should progressively reintroduce the demands of the specific sport, including reactive and cognitive elements, instead of the sole reliance on generic strength and conditioning.[43]

Together, these variables shift the focus from whether an athlete is strong enough to return to whether they are truly prepared to perform and stay safe under real sporting conditions.[43]

Framework for Return to Sport

Buckthorpe[43] also suggests a five-stage late-stage rehabilitation and RTS training model, with a criteria-based progression through each stage.[43]

  • Stage 1 focuses on restoring basic neuromuscular function and linear movement, requiring at least 80% limb symmetry index (LSI) for knee flexor and extensor strength.
  • Stage 2 builds multidirectional coordination and explosive strength, with emphasis on good unilateral landing and deceleration control.
  • Stage 3 introduces reactive movement, sport-specific technical skills and physical conditioning, requiring at least 90% LSI and optimal movement quality in pre-planned tasks.
  • Stages 4 and 5 progressively simulate and then replicate the full demands of team training and competitive match play.

Progression through the stages should be criteria-based rather than time-based, and on-field rehabilitation runs alongside gym-based reconditioning throughout the entire process.[43]

Neurocognitive Dimension of Movement Quality

As noted above, Buckthorpe identifies fatigue tolerance and sport-specific retraining—including reactive and cognitive elements—as key variables in late-stage rehabilitation. Standard movement assessments, such as the hop tests, drop jumps, and controlled cutting or change of direction tasks, are performed in pre-planned, predictable scenarios, which do not actually reflect how an ACL injury occurs in sport. ACL injuries usually happen during unanticipated, reactive movements under high cognitive load. After ACLR, athletes tend to rely on increased conscious cortical control of movement, which is effective during a simple or pre-planned task, but can break down under the demands of live sport. This may explain why athletes can pass standard RTS testing yet still sustain a second ACL injury.[43]

Rehabilitation should, therefore, progressively introduce reactive, unanticipated movement tasks, dual-task conditions and sport-realistic scenarios. RTS testing should also assess movement quality under these conditions.[43]

Training Load and Readiness for Sport Demands

The question in the RTS process is not if the athlete is physically capable of performing their sport, but whether they have trained sufficiently to tolerate their sport's demands. An athlete who has been in a gym-based rehabilitation programme for months may pass all the strength and hop criteria, but still have a low training load in terms of fitness and match-specific workload. This may make the athlete vulnerable when they start team training or matches. Late-stage rehabilitation needs to include a structured on-field reconditioning programme that progressively builds training volume and intensity towards the sporting demands to ensure a gradual transition back to full sports participation.[43]

Muscle Function Tests to Inform Return to Sport Decisions

Different tests are used to assess RTS readiness, including muscle function tests, hop tests and patient-reported outcomes. Although these assessments play an important role in clinical decision-making, each has its limitations, and they have been criticised for their limited transferability, standardisation and relevance.

The limb symmetry index (LSI) is widely used, with a ≥ 90% LSI recommended as a clearance criterion to return to sport. However, it has poor predictive value for assessing second ACL injury risk, and it does not account for preinjury strength, bilateral deficits or compensatory movement strategies. It can also overestimate recovery in athletes with bilateral deficits.[43]

Hop tests are also commonly used and provide information on explosiveness and load absorption through the leg. However, they typically focus on distance or repetitions rather than movement quality. This means they potentially miss biomechanical risk factors.

Patient-reported outcomes highlight the psychological aspects of recovery, but cannot be used in isolation to determine if an athlete is ready to return to sport.

Many of these tests provide a snapshot of the athlete's recovery at a specific stage, but RTS is multidimensional. Tests should be used to inform return to sport decision-making and guide rehabilitation progression, rather than as a simple pass or fail clearance point. Clinicians should continuously evaluate their patients across all relevant domains to inform rehabilitation progression and safe return to sport.[44]

Table 1 provides a summary of recommended assessment types for return to sport after ACLR. If you would like to read more, the full article is available here.[44]

Table 1. Summary for recommended assessment types, the suggested tests, variables to consider and the interpretation for each test from Senorski et al.[44] (article under Creative Commons Attribution Licence)
Assessment Type Suggested Tests Variables Interpretation
Muscle strength Isokinetic (60° - 180°/s), isometric, isotonic 1-RM (if no dynamometer)

Test in multiple hip/knee positions

  • Use LSI, relative strength, H/Q strength ratio, endurance, RFD
  • Symmetry alone insufficient
  • ≥ 90% LSI for RTS (limited predictive value)
  • Relative strength ~ 3.0 - 3.5 Nm/kg knee extension and ~ 1.20 - 1.50 Nm/kg knee flexion
  • H/Q strength ratio ~ 60% (angular velocities ≤ 60° per second
Movement quality/biomechanics Two- and three-dimensional analysis of cutting, jumping and other sport-specific movements
  • Assess trunk, hip, knee and ankle angles for compensatory movement strategies
  • Look for stiff landing with knee close to extension, trunk lean in flexion or lateral flexion and knee valgus
  • Altered movement strategies, e.g., excessive trunk flexion or knee valgus can increase the risk for second ACL injury
Neurocognitive measures Tests that simulate sport-specific cognitive load with imposed external focus (e.g., dual tasking, reactive drills, colour response)
  • Performance under cognitive load
  • Mimic sport demands and assess movement patterns under sports-realistic stressors
  • Hop symmetry (≥ 90% LSI) under sport-specific cognitive load
  • Delayed response time in e.g., colour response tasks compared to uninjured side
  • Altered movement strategies, e.g., excessive trunk flexion or knee valgus
Psychological dimensions Assess knee confidence, stress response, life stressors, recovery from stressful events
  • Patient report outcomes such as: ACL-RSI, K-SES, TSK, KOOS-QoL, IKDC-SKF
  • Both high and low psychological scores can lead to second ACL injury risk
  • Track MIC, MCID, PASS for meaningful change
On-field test Deceleration and change of direction drills, such as 5-0-5 agility test and T-test
  • Assess deceleration and change of direction performance and altered movement patterns in sport-like situations
  • Altered movement patterns, e.g., excessive trunk flexion or knee valgus
  • Final clearance for RTS requires integration between several assessment types and RTS should not be decided using only one type
Abbreviations: 1-RM, One-repetition maximum, LSI, Limb symmetry index, RTS, Return to sport; RFD, Rate of force development; H/Q, Hamstring quadriceps strength ratio; ACL-RSI, ACL Return to sport after injury scale; K-SES, Knee self-efficacy scale; TSK - Tampa scale of kinesophobia; KOOS-QoL, Knee injury and osteoarthritis outcome score - quality of life; IKDC-SKF, International knee documentation committee - subjective knee form; MIC, Minimal important change; MCID, Minimal clinical important difference; PASS, Patient accepted symptom state

[45]

Passing RTS criteria has been associated with up to 84% reduction in reinjury risk[26] and a 47% reduction in graft re-rupture rates.[4] However, passing the RTS tests does not eliminate injury risk or reduce the risk of contralateral ACL injury.[4] Moreover, these standard tests are often applied in non-fatigued, pre-planned conditions, which do not reflect the true demands of sport. Clinicians should, therefore, use RTS assessments to identify remaining deficits and guide rehabilitation progression, not as a pass/fail clearance point. It is important to integrate strength, movement quality, fatigue tolerance, sport-specific fitness and psychological readiness to build a complete picture of the athlete's readiness to return.[43][44]

Return to Sport Criteria

Return to sport criteria have evolved, though practice has been slow to follow the evidence.[29][34] The criteria from Barber-Westin and colleagues'[46] systematic review remain widely referenced and provide a useful clinical baseline for physical readiness. They concluded that in order to give clearance to return to sport, an athlete must have:[46]

  • less than 10% deficit in strength of the quadriceps and hamstrings on isokinetic testing at 180°/s and 300°/s
  • less than 15% deficit in lower limb symmetry on single-leg hop testing (single hop, triple hop, crossover hop, and timed hop)[16]
  • less than 3 mm of increased anterior-posterior tibial displacement on Lachman or knee arthrometer testing
  • greater than 60% normalised knee separation distance on a video drop-jump test
  • absence of effusion
  • full knee range of motion
  • normal patellar mobility
  • no or only slight patellar crepitus
  • painless activities without swelling

The International Knee Documentation Committee Subjective Knee Evaluation Form (IKDC-SKF) is another useful tool for assessing knee symptoms, knee function and athletic activity.[47]

Single-legged hop tests assess performance, strength, neuromuscular control, confidence in the limb, and the ability to tolerate sports-specific loads. They also help identify specific deficits to address in the rehabilitation plan.[47] When performed at six months post-ACLR, single-leg hop tests have predictive value for outcomes at 12 months. The 6-metre hop and the crossover hop have been identified as the strongest predictors of self-reported knee function at one year. Performing these tests at 6 months, before sport-specific training resumes, allows sufficient time to address deficits before the athlete progresses towards RTS.[47]

References

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  2. ↑ Wiggins AJ, Grandhi RK, Schneider DK, Stanfield D, Webster KE, Myer GD. Risk of secondary injury in younger athletes after anterior cruciate ligament reconstruction: a systematic review and meta-analysis. The American journal of sports medicine. 2016 Jul;44(7):1861-76.
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  26. ↑ 26.0 26.1 26.2 26.3 Grindem H, Snyder-Mackler L, Moksnes H, Engebretsen L, Risberg MA. Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study. Br J Sports Med 2016;50(13):804-8.
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  41. ↑ Wesley Wang, PT, DPT.The TRUTH about the MENTAL side of ACL rehab. Available from: https://www.youtube.com/watch?v=sOt1HHJYbBQ [last accessed 02/03/2026]
  42. ↑ Wackerle AM, Joreitz R, Sprague A, Senorski EH, Halilaj E, Rabuck SJ, Lesniak BP, Hughes JD, Musahl V, Irrgang JJ, 2024 Panther Expert Group. Freddie Fu Panther Symposium Expert Group 2024: Rehabilitation and return to sport after anterior cruciate ligament reconstruction Part 2: Concepts and emerging technology in return to sport. Knee Surgery, Sports Traumatology, Arthroscopy. 2026 Feb;34(2):531-42.
  43. ↑ 43.00 43.01 43.02 43.03 43.04 43.05 43.06 43.07 43.08 43.09 43.10 43.11 43.12 Buckthorpe M. Optimising the late-stage rehabilitation and return-to-sport training and testing process after ACL reconstruction. Sports Medicine. 2019 Jul 1;49(7):1043-58.
  44. ↑ 44.0 44.1 44.2 44.3 Hamrin Senorski R, Piussi R, Högberg J, Sundberg A, Lindskog J, Prill R, Hamrin Senorski E. Current concepts and future perspective of muscle function tests to inform return to sport decision‐making after ACLR: A narrative review. Journal of Experimental Orthopaedics. 2026 Jan;13(1):e70643.
  45. ↑ Clinical Physio. ACL Rehab Return to Sport Tests | Expert Explains Melbourne ACL 2.0 Return to Play Tests. Available from: https://www.youtube.com/watch?v=x2JEGWxJfZk [last accessed 06/03/2026]
  46. ↑ 46.0 46.1 Barber-Westin SD, Noyes FR. Factors used to determine return to unrestricted sports activities after anterior cruciate ligament reconstruction. Arthroscopy: The Journal of Arthroscopic & Related Surgery 2011;27(12):1697-705.
  47. ↑ 47.0 47.1 47.2 Logerstedt D, Grindem H, Lynch A, Eitzen I, Engebretsen L, Risberg MA, Axe MJ, Snyder-Mackler L. Single-legged hop tests as predictors of self-reported knee function after anterior cruciate ligament reconstruction: the Delaware-Oslo ACL cohort study. The American journal of sports medicine 2012;40(10):2348-56.
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