Author: kickperformancetherapy

Return-to-sport testing after ACL reconstruction should not be a single exam that you pass at the end of rehabilitation. It is better viewed as an ongoing process: measure, expose, adapt, reassess and progressively build toward the demands of your sport.

There are many excellent ACL tests. Strength testing matters. Jump and hop testing matters. Reactive performance, movement quality, psychological readiness and sport-specific testing can all add useful information. But no single test — and no single battery completed on one day — can recreate the accumulated demands of training and competition.

Return to sport is a process, not a test day

One of the most important changes in return-to-sport thinking is the recognition that return is a continuum. An athlete progresses from rehabilitation into participation, then back to sport, and ultimately back toward performance. Testing should help guide that journey rather than act as a one-off gate at the end.

An athlete can perform very well during a controlled testing session and still be underprepared for repeated accelerations, decelerations, changes of direction, fatigue, contact, unpredictable movement and the cumulative load of several training sessions in a week.

What should ACL return-to-sport testing include?

The exact battery should depend on the athlete, their stage of rehabilitation, injury history, available equipment and — most importantly — what they are returning to. A field-sport athlete returning to cutting and reactive play should not necessarily be assessed in the same way as a distance runner.

I generally want information across several domains:

• Knee status: symptoms, swelling, range of motion and tolerance to current training.
• Strength and capacity: particularly quadriceps and hamstring function, but also the wider lower limb where relevant. Side-to-side symmetry is useful, but absolute and bodyweight-relative capacity matter too.
• Explosive performance: the ability to produce force during jumping and other fast actions.
• Reactive qualities: how effectively the athlete can absorb and reproduce force when time on the ground becomes short.
• Landing and force absorption: not simply whether a landing is completed, but how the athlete accepts load.
• Movement quality: acceleration, deceleration, cutting, lateral movement, rotation and the strategies used to complete them.
• Sport-specific capacity: progressively exposing the athlete to the speed, volume, direction changes and uncertainty required by their sport.
• Confidence and readiness: whether the athlete trusts the knee and feels able to express their physical capacity when demands increase.

I have core tests — but I don’t believe in one fixed battery for everyone

There are tests I use consistently because they give valuable benchmarks and allow progress to be tracked over time. But I also use a much broader library of tests throughout rehabilitation.

Which tests I choose — and when I use them — depends on what I am seeing. An athlete with a history of hamstring problems may need a different emphasis from an athlete who struggles to absorb force through the reconstructed knee. A soccer player may need different movement exposures from a runner. An athlete who looks excellent in predictable jumping tasks may need more information from reactive or multidirectional work.

The purpose of testing is not to complete a checklist. It is to ask better questions about the individual athlete.

A passed test can still hide a compensation

Performance outcomes can look reassuring while the strategy used to achieve them tells a different story.

A well-known example is the single-leg hop for distance. Research has shown athletes after ACL reconstruction achieving approximately 97% symmetry in hop distance while symmetry in the work performed by the knee during propulsion was far lower. Athletes could maintain the distance by shifting work toward the hip and ankle and changing trunk and pelvis strategy.

That is clinically important. If we record only the distance, the athlete may appear to have passed. If we watch how the task is completed — or measure the forces and joint strategy — we may reach a different conclusion.

This is why technology is useful but does not replace experienced observation. Force plates, dynamometry and performance measures can quantify things the eye cannot. The coach’s eye can identify strategies that a headline score can miss. The strongest assessment combines both.

Testing should influence training — and training should influence testing

A useful test identifies a priority. Rehabilitation then gives the athlete enough exposure to change that quality. We reassess, progress the demand and see whether the change holds when the task becomes faster, more complex or more fatigued.

That makes return-to-sport testing fluid rather than static. You might assess maximal strength earlier, introduce jump profiling as the athlete develops power, add reactive tasks as ground-contact demands increase, and progress toward cutting, deceleration and unplanned movement as sport becomes closer.

Not every athlete needs every test. The question is whether the testing helps us make a better decision and build a better next phase.

The missing test: weeks of appropriate exposure

This may be the most important part of return to sport that cannot be captured in a single testing session.

Before returning fully, an athlete should have accumulated an appropriate progression of the things their sport actually requires. That may include running volume, sprint exposure, repeated accelerations and decelerations, cutting, jumping, reactive movement, non-contact practice, controlled contact and progressively less restricted training.

These exposures allow us to see how the knee and the athlete respond not just to one successful repetition, but to repeated load over days and weeks. Symptoms, confidence, movement quality and performance can all change as volume and intensity rise.

A test battery is therefore best viewed as a series of snapshots within a much longer film.

So when is an athlete ready?

There is no perfect test that can guarantee a safe or successful return. Return-to-sport decisions involve risk, and the relevant criteria depend on the athlete and the sport.

For me, the goal is to build confidence from multiple pieces of information: sufficient strength and physical capacity; explosive and reactive qualities; effective movement strategies; tolerance to progressive sport exposure; appropriate conditioning; confidence in the knee; and enough sport-specific training to demonstrate that these qualities survive outside the testing environment.

Key research

• Ardern CL, et al. 2016 Consensus statement on return to sport from the First World Congress in Sports Physical Therapy, Bern. Br J Sports Med. 2016.
• van Melick N, et al. Evidence-based clinical practice update: practice guidelines for anterior cruciate ligament rehabilitation based on a systematic review and multidisciplinary consensus. Br J Sports Med. 2016.
• Kotsifaki A, et al. Single leg hop for distance symmetry masks lower limb biomechanics: time to discuss hop distance as decision criterion for return to sport after ACL reconstruction? Br J Sports Med. 2022.
• Kotsifaki A, et al. Symmetry in Triple Hop Distance Hides Asymmetries in Knee Function After ACL Reconstruction in Athletes at Return to Sports. Am J Sports Med. 2022.

Reaching 90% limb symmetry after ACL reconstruction can be useful. It is not the same thing as being ready for sport.

The 90% limb symmetry index (LSI) is still commonly used in ACL rehabilitation. It compares the reconstructed limb with the other limb during a strength or performance test. If the operated side reaches at least 90% of the other side, it is often described as having “passed.”

That information matters. I use strength testing extensively in ACL rehabilitation and believe restoring strength is a minimum requirement for returning to sport. But a percentage can only tell us what was measured. It cannot tell us everything an athlete needs to do when they accelerate, brake, land, cut, react and compete.

What does 90% limb symmetry actually mean?

LSI is usually calculated by dividing the result from the involved limb by the uninvolved limb and multiplying by 100. An athlete producing 90 units on the reconstructed side and 100 on the other side has an LSI of 90%.

The attraction is obvious: it gives clinicians and athletes a simple way to quantify side-to-side recovery. The problem begins when 90% symmetry becomes synonymous with readiness.

Symmetrical does not necessarily mean strong

Imagine two athletes. Athlete A produces 90 units on the reconstructed limb and 100 on the other. Athlete B produces 63 and 70. Both have a 90% LSI.

They are equally symmetrical on paper. They clearly do not have the same physical capacity.

This is one of the limitations of using the uninvolved limb as the only reference. The “good” leg may also lose strength and performance during months of reduced training. Research has shown that LSI can overestimate recovery when the contralateral limb no longer represents the athlete’s previous capacity.

Strength is not one quality

A maximal strength test answers an important question: how much force can you produce? Sport asks several additional questions.

• Maximal strength: how much force can the limb produce?
• Eccentric strength and braking capacity: how effectively can it absorb and control force?
• Rate of force development: how quickly can force be produced?
• Explosive strength and power: can that force be expressed during jumping and high-speed actions?
• Reactive strength: can the athlete rapidly absorb and reproduce force when ground-contact time is short?

This distinction matters because sport rarely gives an athlete several seconds to develop maximal force. Research in professional soccer players after ACL reconstruction found that maximal voluntary strength could be close to preinjury values while rate of force development remained substantially impaired. Other work has identified reactive and vertical-jump deficits in athletes who otherwise looked much better on conventional strength and horizontal hop testing.

Why can an athlete test “strong” and still not feel right?

This is a situation I see frequently. An athlete has worked hard in the gym. Their quadriceps numbers look good. Perhaps their LSI is above 90%. Yet cutting feels awkward, jumping lacks confidence, or the reconstructed side still does not feel like the limb they instinctively want to use.

That does not make the strength testing wrong. It means the test answered one part of a larger question.

The athlete may have adequate maximal force but still be limited in how quickly they access it, how they absorb force, how they coordinate the hip-knee-ankle strategy during landing, or how confidently they load the limb when speed and uncertainty increase. An athlete can also achieve a performance outcome while using a different strategy to get there.

For example, jump distance alone may look symmetrical while vertical-jump testing or biomechanical analysis reveals persistent deficits in knee contribution. This is one reason I prefer to look at how an athlete produces a result as well as the result itself.

Strength is the minimum buy-in — not the finish line

I strongly believe in strength testing after ACL reconstruction. Significant strength deficits can explain a great deal: reduced confidence, altered movement, poor force absorption and compensations elsewhere in the limb. If an athlete is clearly under-strength, there is little value in pretending that more complex testing somehow makes that irrelevant.

But restoring strength should create the physical foundation for the next stage of rehabilitation. It should allow us to ask harder questions.

Can you use that strength quickly? Can you absorb force? Can you repeat it? Can you land without avoiding the reconstructed limb? Can you accelerate and decelerate? Can you change direction? Can you maintain those qualities as speed and fatigue increase? Can you respond when the movement is not pre-planned?

Those questions move us from strength recovery toward sport readiness.

A better way to use the 90% rule

Rather than throwing out LSI, I think it is more useful to put it in its proper place.

• Use side-to-side symmetry to identify meaningful deficits.
• Look at absolute strength and, where appropriate, strength relative to body weight — not symmetry alone.
• Assess more than one expression of force, including explosive and reactive qualities when the athlete’s stage and sport require them.
• Use jump testing to examine both performance and strategy.
• Progress movement from controlled tasks toward acceleration, deceleration, landing, cutting and reactive sport-relevant actions.
• Consider conditioning, confidence, symptoms, training exposure and the demands of the athlete’s actual sport.

A recent analysis in the British Journal of Sports Medicine also questioned relying on LSI cut-offs alone to identify safe return to sport. That does not make symmetry useless. It reinforces that return-to-sport decisions are too complex to be reduced to one percentage.

Key research

• Wellsandt E, Failla MJ, Snyder-Mackler L. Limb Symmetry Indexes Can Overestimate Knee Function After Anterior Cruciate Ligament Injury. J Orthop Sports Phys Ther. 2017.
• Angelozzi M, et al. Rate of force development as an adjunctive outcome measure for return-to-sport decisions after anterior cruciate ligament reconstruction. J Orthop Sports Phys Ther. 2012.
• Hirohata K, et al. Reactive strength index during single-limb vertical continuous jumps after anterior cruciate ligament reconstruction. 2022.
• King E, et al. Single leg vertical jump performance identifies knee function deficits at return to sport after ACL reconstruction in male athletes. 2022.
• Questioning the rules of engagement: a critical analysis of the use of limb symmetry index for safe return to sport after anterior cruciate ligament reconstruction. Br J Sports Med. 2025.

During the Women’s Soccer World Cup currently being held in Australia and New Zealand, the world’s media revisited a topic that many of us physical therapists specialized in this area have become exhausted with.  How to reduce ACL injuries with the female athlete. The reason why this topic has reached world media outlets is because the equivalent of an entire squad of players will missed this year’s tournament due to ACL injury.   This seems an unusually high number, but a high incidence of ACL injuries among female athletes can be found at all levels of the game. Many female athletes have spoken out about the need for more funding into this area, and while this would help avoid the majority of studies being conducted on men and the results being incorporated into female sport. There is a great deal of information we know about preventing ACL injury that is currently being ignored.

What We Already Know……………. The reality is that females are as much as 3.5 times more likely to sustain an ACL injury than male athletes (Arden et al. 1999, Voskcanian et al. 2013). The reasons are many and both modifiable and non-modifiable. The non-modifiable risk factors are factors such as the menstrual cycle and the effect of hormones on ligament stiffness, less muscle mass, anatomical factors such as the female Q-angle (The wider angle of the femur from pelvis to knee) and subtle differences in knee joint anatomy from interarticular notch width to tibia angle.  Studies into the modifiable factors have considered many elements such as gender roles where females are less likely to be multi-sport athletes or engage in ‘athletic play’ as adolescents, therefore theorized to have less body awareness. The main thrust of research shows that we should be focusing on understanding and training the female athlete appropriately. We should be incorporating specific gym and field-based training strategies specific to the female athlete’s needs.   What do we do About it? My personal experience on this topic is that we have an audience that is willing to listen but not ready for change.   I see an ACL injury (particularly in adolescent sport) as a welfare issue. An ACL injury causes significant pain and disability with a return to sport taking 12 months in most cases. Not to mention the mental side of injury with removal of an individual from their hobby and social group. The long-term health of the knee after an ACL injury is also the topic of much research and discussion among experts.The extra bit of detail is that of the athletes who return to multi-directional sport, 20-28% will go on to sustain a second ACL injury (Bodkin et al. 2021.)In short…..the impact is significant and the relative attention to prevention is small. There are a magnitude of protocols that can easily be incorporated into a team’s training that have been shown to significantly decrease the risk of an ACL injury. An accumulation of data across numerous studies shows a potential 70% reduction in risk. (Sugimoto D et al. 2012). The issue is that compliance with these protocols is very limited. I am sure coaches have numerous initiatives they would like to introduce themselves and feel limited by time. We must also accept that few athletes and parents are asking for prevention programs over quality coaching sessions and results on the field. Physical therapists are not innocent either, I have seen many instances where PTs dip their toe in the injury prevention world and prescribe sessions that are time consuming or inadequate to decrease risk of injury. The Future is Bright. We are better off than we were 10 years ago. The female athlete is being taken seriously with improved financial, research and team support. Injury prevention programs will eventually become the norm as it makes sense, as a good prevention program also has the same elements as a performance program.  Athlete testing is currently seeing an evolution with high tech devices that were previously unattainable to any party except at elite sport, are now becoming more affordable and field assessable. There is no longer an excuse for professionals to claim to offer injury prevention or injury rehab and not have access to this level of detail with testing. If you want to do your part, please feel free to forward this email to an athlete or coach, it may make a huge difference.If you know someone who has sustained an ACL injury please put them in touch, I am offering to test the first few athletes who contact me a free force plate testing session to highlight any vulnerabilities as unfortunately you never graduate from ACL rehabilitation, you manage the risk factors for life.