Limb Symmetry Index (LSI) Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
LSI is calculated with the affected limb's value ÷ the healthy limb's value × 100. An LSI of at least 90% is a commonly cited criterion for considering return to sport after ACL reconstruction. For 27 on the affected limb versus 30 on the healthy limb, the LSI is exactly 90%.
Explanation
The Limb Symmetry Index (LSI) compares a measurement taken on the recovering limb to the same measurement taken on the healthy (contralateral) limb, expressing the result as a percentage: 100% would mean perfect symmetry between the two limbs. This calculator is deliberately agnostic to the type of test used — isokinetic strength, single-leg hop distance, number of repetitions at a given load, or any other bilateral test — as long as the two compared values use exactly the same unit and measurement protocol. LSI is particularly used in tracking rehabilitation after anterior cruciate ligament (ACL) reconstruction, where a threshold of at least 90% is commonly cited in sports physiotherapy literature as one of the criteria to meet before considering a return to sport, generally combined with other criteria (absence of pain, functional stability, the athlete's subjective confidence in the knee) rather than used in isolation. An important nuance documented by recent research: the "healthy" limb itself may have lost performance since the injury (general deconditioning, prolonged compensation), which can make the LSI appear artificially high without the affected limb having truly regained its pre-injury level — hence the value of also comparing, when possible, to the athlete's own pre-injury baseline values.
Example: single-leg hop test, 27 on the affected limb versus 30 on the healthy limb
Inputs
Measured value, affected limb: 27. Measured value, healthy limb: 30.
Calculation
LSI = (27 ÷ 30) × 100 = 90%.
Result
This LSI of 90% exactly reaches the threshold commonly cited as a return-to-sport criterion after ACL reconstruction.
Frequently asked questions
Is a 90% LSI enough on its own to return to sport after ACL reconstruction?
No, generally not: LSI is one criterion among others (absence of pain, functional knee stability, complementary strength and hop tests, the athlete's confidence) used together by physiotherapists and sports doctors to assess readiness for return to sport. A high LSI alone, without the other criteria, isn't enough to guarantee a safe return.
Why could the "healthy" limb skew the calculation?
Because after a long rehabilitation period, the uninjured limb may itself have lost strength or performance (general deconditioning, reduced activity during recovery), which artificially narrows the gap between the two values and makes the LSI appear higher than it would be if the healthy limb had kept its pre-injury level. Also comparing against values measured before the injury, when available, helps spot this bias.
Does this calculator apply to injuries other than the ACL?
Yes, the LSI principle applies to any unilateral injury requiring comparison with the contralateral limb (an ankle sprain, a calf or hamstring injury, for example), even though the most documented 90% threshold in the literature specifically concerns ACL reconstruction. For other injuries, the relevant threshold and the type of test to prioritize should be discussed with a rehabilitation professional.
Are there comparable ratios for other joints?
Yes, using the same imbalance-screening principle, see our shoulder rotator strength ratio calculator or our hip adductor/abductor strength ratio calculator — two comparable indicators, but which compare two different MUSCLE GROUPS on the SAME limb, rather than the SAME test on BOTH limbs as LSI does.