Metrics-Lab

LSI calculator: limb symmetry index

Enter the value of the affected limb and the healthy limb: the calculator returns the LSI as a percentage and compares it to the 90% threshold most often used for return to sport.
Calculate the LSI
MEASURE TYPE
UNIT
AFFECTED (OPERATED) LIMB
kg
HEALTHY LIMB
kg
LIMB SYMMETRY INDEX (LSI)
--
90% threshold
Enter both values (numbers above 0) to get the LSI.
Decision-support tool, not medical advice. An LSI alone is never enough to decide on return to sport.

What is the LSI?

The LSI (Limb Symmetry Index) compares the result of the affected limb to the healthy limb, as a percentage. It is the central calculation of return-to-sport test batteries after ACL reconstruction.

How to calculate the LSI

For a measure where a higher result is better (peak strength, hop distance), divide the affected limb value by the healthy limb value, then multiply by 100. For a timed measure, where a shorter time is better, invert the ratio: healthy limb divided by affected limb.
• Strength or distance: LSI = (affected ÷ healthy) × 100
• Time: LSI = (healthy ÷ affected) × 100

Which threshold should you use?

In the literature, the most commonly used threshold is 90% on each test, with 85% also described as common. An LSI above 90% is generally considered normal.
These thresholds are conventions, not proof of a safe return to sport: a 2025 study cited by Physiotutors found no significant association between reaching a 90% LSI and a return to sport judged safe after ACL reconstruction. Adapt the threshold to the sport, age and context of the patient.

Limits of the LSI

Passing hop tests does not rule out the risk of re-injury: distance symmetry can hide differences in movement quality.
The LSI can also overestimate knee function. After an ACL rupture the healthy limb can itself be deficient, which inflates the index: the healthy limb is not always a reliable reference, because its pre-injury function is not guaranteed.
The LSI should therefore be part of an overall decision that includes quadriceps strength, psychological readiness, the demands of the target sport and movement quality.

Which formula for which measure?

Type of measure
LSI formula
Most cited threshold
Peak strength (for example quadriceps)
affected ÷ healthy × 100
90%
Hop distance (single hop, triple hop, crossover hop)
affected ÷ healthy × 100
90%
Time (for example timed hop)
healthy ÷ affected × 100
90%
Sources: Physitrack, single leg hop test and return to sport after ACL, Physiotutors, return-to-sport test batteries after ACL reconstruction, Physiotutors, safe return to sport after ACL reconstruction (2025 study), JOSPT 2017 (PubMed), limb symmetry indexes can overestimate knee function after ACL injury.
To go further, explore our physio assessments (BDK, RTS), patient follow-up tool, physio tests and protocols, sample assessment report (PDF), pricing and FAQ.

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