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Your strength numbers look good. Your side-to-side symmetry may even be above 90%. So why does the reconstructed leg still feel weak?

This is a common experience after ACL reconstruction. The answer is often not that the strength test is wrong. It is that “strength” is a much bigger quality than the number produced during one controlled test.

A maximal isometric or slow strength test tells us something important: how much force you can produce under those particular conditions. Sport asks whether you can access, absorb and repeat force quickly — while moving, reacting and becoming fatigued.

A strong test does not always mean a fully restored quadriceps

One of the first things I look at is whether the strength result fits the rest of the athlete.

An athlete can produce a surprisingly good peak isometric force while the reconstructed-side quadriceps still looks visibly smaller. A single maximal value does not directly tell us that muscle size, architecture, endurance and the ability to express force across different speeds and joint positions have all returned.

That distinction matters. Quadriceps atrophy is common after ACL reconstruction, and muscle size is related to strength — but the relationship is not perfect. Neural factors, testing position, familiarization and an athlete’s ability to recruit the available muscle can all influence a maximal test.

The “skinny but strong” quadriceps

Sometimes an athlete reaches a respectable strength number while the quadriceps still appears noticeably underdeveloped compared with the other side. I would not ignore that simply because the dynamometer says the athlete is strong.

Think of the maximal test as one expression of the system. The athlete may be very good at recruiting what they have for a brief effort, but reduced muscle mass may become more relevant when we ask the limb to produce high forces repeatedly, work through different joint angles, absorb eccentric load or maintain performance as fatigue develops.

This is one reason I combine strength testing with the clinical picture: muscle development, training history, movement, jumping and the athlete’s response to progressively harder tasks.

How quickly can you use your strength?

Maximal force and rapid force production are not the same thing.

Rate of force development describes how rapidly force rises. That matters in sport because many actions happen in a fraction of the time available during a maximal strength test. Research after ACL reconstruction has demonstrated that athletes can recover maximal voluntary strength while deficits in rapid force production remain.

This can help explain why an athlete feels strong in the gym but less convincing when sprinting, jumping, changing direction or reacting quickly.

Can you absorb force as well as produce it?

Sport is not only about pushing. It is also about braking.

Landing, decelerating and changing direction require the lower limb to accept and control substantial forces. An athlete may demonstrate good concentric or isometric strength yet still avoid loading the reconstructed knee during these tasks.

That is why I am interested in eccentric capacity and movement strategy. Does the athlete actually use the knee when absorbing force, or do they find a way to shift demand toward the hip, ankle, trunk or opposite limb?

Relative strength matters too

Side-to-side symmetry tells us how similar the limbs are. It does not necessarily tell us whether either limb is strong enough for the athlete’s body mass and sporting demands.

An athlete can be symmetrically weak. Conversely, a reconstructed limb can remain somewhat asymmetrical while both limbs have developed substantial absolute capacity. This is why I prefer to interpret symmetry alongside absolute and, where appropriate, bodyweight-relative strength rather than using one percentage in isolation.

What happens after the first repetition?

Many strength tests capture a short maximal effort. Sport repeatedly asks for force.

If the reconstructed limb loses output more rapidly, movement quality changes under fatigue or the athlete progressively unloads the knee, a single peak value may never reveal the problem. Capacity across repeated jumps, repeated contacts, running and progressively demanding training can therefore provide information that a one-off maximal test cannot.

Sometimes the weakness you feel is a movement problem

Athletes do not experience their knee as a dynamometer number. They experience it while moving.

If you hesitate when loading the reconstructed side, shorten ground contact differently, reduce knee contribution during a jump or rely more heavily on another joint, the limb may feel weak even when maximal strength is respectable.

That sensation can be useful information. Rather than dismissing it because the strength test “passed,” I want to understand where the mismatch appears and what changes when the task becomes faster, more reactive or more sport-like.

So what should we test when strength looks good but the leg doesn’t feel right?

• Maximal strength: confirm that the apparent strength recovery is real and consider absolute and relative values, not symmetry alone.
• Muscle development: consider whether visible or measured quadriceps atrophy remains relevant to the athlete’s overall capacity.
• Rapid force production: assess whether force can be generated quickly rather than only maximally.
• Explosive and reactive performance: examine jumping and shorter ground-contact tasks where appropriate.
• Eccentric and braking capacity: look at how the athlete absorbs force during landing and deceleration.
• Movement strategy: assess whether performance is being achieved through compensations.
• Repeatability and fatigue: determine whether quality and output persist as demands accumulate.
• Sport exposure: establish whether the athlete has actually had enough progressive high-speed and sport-specific work to translate gym capacity into performance.

I have core tests that I use regularly, but I also draw from a wider testing toolbox depending on the athlete, their injury history, sport and what I am seeing during rehabilitation. The goal is not to collect more numbers for the sake of it. It is to identify the quality that is still limiting the athlete.

Strength can explain a lot — but it cannot explain everything

I strongly believe in strength testing. When strength is genuinely deficient, improving it often has a remarkable effect on movement, confidence and performance.

But when an athlete tells me, “My strength tests are good, but the leg still doesn’t feel right,” I don’t think the conversation should end there.

That is often the point at which we need to ask a better question: what can this athlete do with the strength they have?

Key research

• 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.
• Thomas AC, Wojtys EM, Brandon C, Palmieri-Smith RM. Muscle atrophy contributes to quadriceps weakness after anterior cruciate ligament reconstruction. J Sci Med Sport. 2016.
• Kotsifaki A, et al. Research demonstrating that symmetrical hop distance can mask persistent asymmetries in knee function after ACL reconstruction.
• 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.

“When can I start running?” is one of the most common questions after ACL reconstruction. My answer is usually another question: what kind of running are we trying to return you to?

For someone whose goal is recreational distance running, rebuilding steady running volume may be exactly the right progression. For an athlete returning to soccer, basketball, ultimate or another high-speed multidirectional sport, however, a long period of slow jogging should not automatically become the centerpiece of mid-stage rehabilitation.

Running is an important milestone. But the goal is not simply to prove that you can jog. The goal is to progressively prepare you for the forces, speeds, decisions and repeated high-intensity actions your sport actually requires.

Return to running should be criteria-based, not calendar-based

Time since surgery matters because healing biology matters. But reaching a particular postoperative week does not automatically mean the knee is ready to run.

Current clinical guidance recommends combining time with clinical and functional criteria. Suggested markers include full knee extension, close to full flexion, little or no swelling, sufficient quadriceps strength and the ability to tolerate repeated single-leg hopping without pain.

I use core markers like these, but I do not believe one fixed battery suits every athlete. I also draw from a wider range of tests depending on the athlete’s history, sport, graft, current deficits and what I am seeing throughout rehabilitation.

Before running, I want to see the basics

Running should not become a way of bypassing unfinished rehabilitation.

Before progressing, I want confidence that the knee is clinically settled and that the athlete has developed enough strength, control and elastic capacity for the next demand. That can include knee range and swelling, quadriceps and lower-limb strength, calf capacity, controlled single-leg loading, jumping and landing preparation, repeated low-level reactive contacts and movement quality.

The exact markers change with the individual. The important principle is that the athlete earns progression by demonstrating capacity rather than simply reaching a date on the calendar.

Jogging and returning to athletic movement are not the same progression

I have no objection to steady jogging when it serves the athlete’s goals. If running itself is your sport or something you enjoy, rebuilding it progressively makes sense.

For a multidirectional athlete, though, I often prefer to place greater emphasis on drill-based running and movement progressions. These allow us to deconstruct athletic actions and progressively expose the athlete to positions, rhythms and forces that will later underpin faster sport.

Rather than accumulating large amounts of slow running simply because it is easy to prescribe, we can use appropriate drills to develop running mechanics, stiffness and elastic qualities, acceleration positions, force application and eventually greater speed.

This does not mean going from rehabilitation straight into maximal sprinting. It means building a deliberate bridge from controlled movement to higher-intensity movement.

Why I don’t want jogging to dominate the middle of rehab

A long, generic return-to-run progression can add a substantial amount of training volume. For some athletes that volume is useful. For others it competes with the qualities they most need to restore: strength, power, reactive ability, speed and high-quality athletic movement.

If every week becomes longer bouts of low-intensity running on top of gym work and rehabilitation, the athlete can simply become tired without becoming meaningfully better prepared for the actions associated with their sport.

Most non-contact ACL injuries are not sustained during comfortable straight-line jogging. They are more commonly associated with demanding actions such as rapid deceleration, change of direction or landing. That does not make jogging bad. It means jogging alone provides limited preparation for the situations we ultimately care about.

Can drill-based movement begin before continuous running?

In many athletes, elements of athletic movement can be introduced before they are completing meaningful volumes of continuous running — provided the task is appropriate for their stage of healing and current capacity.

Early work may be relatively simple: athletic postures, marching and rhythm drills, controlled foot contacts, low-level landing tasks and rehearsed movement patterns. As the athlete demonstrates success, those tasks can progress in speed, amplitude, force and complexity.

This creates a continuum rather than a sudden transition from “rehab exercises” to “sports training.”

The coach’s eye still matters

Objective testing gives us information that observation alone cannot. But numbers do not tell the whole story.

I also want to see how the athlete moves. Are they accepting load through the reconstructed limb? Do they look comfortable during repeated contacts? Are they creating unnecessary compensations? Does movement deteriorate when speed increases? Can they reproduce a task consistently rather than simply complete one good repetition?

The combination of objective testing, clinical response and experienced movement observation is more useful to me than a single “return-to-run test.”

Physical readiness is only part of readiness

The first running and jumping progressions can also feel psychologically significant after ACL reconstruction. Some athletes are eager to move faster; others remain hesitant despite good physical testing.

That response matters. Progression should create successful exposures that build trust in the limb rather than forcing an athlete through a stage simply because the protocol says it is time.

So when can you start running?

There is no single number that answers that question for everyone. A useful decision considers healing time, symptoms, swelling and range of motion alongside objective strength, jumping and reactive capacity, movement quality and psychological readiness.

And for athletes returning to high-speed sport, I would broaden the question:

When are you ready to start rebuilding athletic movement — and what progression will best prepare you for the sport you actually want to return to?

That is a much more useful rehabilitation question than simply asking when you are allowed to jog.

Key research

• Aspetar. Clinical practice guideline on rehabilitation after anterior cruciate ligament reconstruction. Br J Sports Med. 2023.
• Rambaud AJM, Ardern CL, Thoreux P, Regnaux JP, Edouard P. Criteria for return to running after anterior cruciate ligament reconstruction: a scoping review. Br J Sports Med. 2018.
• Burgi CR, et al. Which criteria are used to clear patients to return to sport after primary ACL reconstruction? A scoping review. Br J Sports Med. 2019.

Being cleared after ACL reconstruction and feeling ready to return to sport are not always the same thing.

You may have restored your range of motion. Your strength numbers may look good. You may have passed hop tests and reached the expected stage of rehabilitation. Yet when you imagine sprinting, cutting, reacting to another player or returning to competition, something still doesn’t feel right.

That feeling deserves attention. Return to sport is physical, psychological and highly individual — and many of the qualities that make an athlete feel capable again should be developed long before the final return-to-sport test.

Cleared does not always mean ready

Return-to-sport clearance is an important milestone, but it cannot perfectly reproduce the demands of sport. Research consistently shows that athletes can still demonstrate physical deficits around the time they are cleared, while psychological readiness is also strongly associated with whether athletes actually return.

This helps explain why an athlete can look good on paper but still feel a significant gap between rehabilitation and competition.

Strength matters — but sport asks you to express it

I strongly believe in strength testing after ACL reconstruction. If the quadriceps, hamstrings or wider lower limb remain significantly under-strength, that can explain many of the compensations and limitations an athlete experiences.

But maximal strength is one physical quality. An athlete may be strong in a controlled test and still lack explosive strength, rate of force development, eccentric braking capacity or reactive strength. They may be able to produce force, but not yet access and organize it at the speed their sport demands.

That is why strength should be considered a foundation rather than the final destination.

These qualities can be developed much earlier

Athletic rehabilitation should not suddenly begin when an athlete is almost ready to return.

From earlier stages of ACL rehabilitation, we can begin rebuilding the qualities that later make sport feel familiar: athletic positions, controlled movement strategies, confidence accepting load through the reconstructed limb and progressively faster force production.

As strength and healing progress, those foundations can evolve into jumping, landing, acceleration, deceleration, multidirectional movement and eventually reactive, sport-relevant tasks. The intensity changes throughout rehabilitation, but the goal of preparing an athlete to move like an athlete can be present from much earlier.

Sport demands more than physical capacity

Sport requires more than strength, power and movement capacity. Athletes also have to process information, make decisions and coordinate an appropriate movement response — often at speed and without time to consciously think about the knee.

These demands are difficult to reproduce with predictable rehabilitation exercises alone. An athlete may perform extremely well when they know exactly what movement is coming next, yet feel very different when they have to respond to another player, a ball, changing space or an unexpected cue.

That is one reason rehabilitation can progress beyond predictable drills. As appropriate, tasks can incorporate external cues, decision-making, reaction, visual information, opponents or teammates, dual-task demands and increasingly unpredictable movement. The aim is not simply to make exercises complicated. It is to reconnect physical capacity with the perception-action demands of sport.

The mental side of returning can feel overwhelming

An athlete can pass every physical test placed in front of them and still not feel psychologically ready.

Fear of reinjury, hesitation, reduced confidence and uncertainty about whether the knee will cope are common after ACL reconstruction. Research shows that psychological readiness is meaningfully associated with return to sport, even when conventional measures of knee function are similar.

This should not be dismissed as something the athlete simply needs to “get over.” It should be explored. What exactly feels threatening? Is it contact? Cutting off the reconstructed side? Landing in traffic? An unpredictable opponent? Fatigue? Returning to the environment in which the injury occurred?

Those answers can help shape rehabilitation. Appropriate graded exposure can allow an athlete to experience successful movement at progressively greater speed, complexity and uncertainty. In some cases, psychological support beyond physical therapy may also be valuable.

Confidence is difficult to test into existence

Confidence often grows through evidence.

Evidence that the leg is strong. Evidence that you can land and absorb force. Evidence that you can sprint. Evidence that you can cut. Evidence that you can react without protecting the knee. And, importantly, evidence accumulated across weeks of progressively harder training rather than one successful testing session.

This is where return-to-sport testing and return-to-sport preparation need to work together. Testing can identify what is missing. Training gives the athlete the exposure to change it. Reassessment tells us whether that change persists as the task becomes faster and less predictable.

If you’ve been cleared but don’t feel ready, what should you look at?

• Absolute strength and symmetry: are you genuinely strong enough, or simply similar from side to side?
• Explosive and reactive qualities: can you produce, absorb and reproduce force quickly?
• Movement strategy: are you confidently using the reconstructed limb or finding subtle ways around it?
• Conditioning: can you maintain movement quality and decision-making as fatigue increases?
• Athletic exposure: have you built enough sprinting, deceleration, cutting, jumping and sport-specific training over time?
• Neurocognitive demands: have you progressed beyond rehearsed tasks into reacting, deciding and moving in response to external information?
• Psychological readiness: what situations still create hesitation or fear, and have they been progressively addressed?

The answer will not be identical for every athlete. That is why I prefer to use testing as part of an ongoing process rather than a universal pass/fail checklist.

Key research

• Webster KE, Hewett TE. What is the Evidence for and Validity of Return-to-Sport Testing after Anterior Cruciate Ligament Reconstruction Surgery? A Systematic Review and Meta-Analysis. Sports Med. 2019.
• Webster KE, Feller JA. Research examining psychological readiness and the ACL Return to Sport after Injury (ACL-RSI) scale.
• Piussi R, et al. Research examining knee confidence, fear of movement and psychological readiness following ACL reconstruction.
• Grooms DR, et al. Research examining neuroplasticity and sensorimotor control following ACL injury and reconstruction.

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.