Is 90% Limb Symmetry Enough After ACL Reconstruction?

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 Research has shown that LSI can overestimate recovery when the contralateral limb no longer represents the athlete’s previous capacity.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.