Written by Lode Goossens, Sports physiotherapist and Sports scientist (Ghent University) and founder of Hylyght.

1. What is the LSI or Limb Symmetry Index?

The Limb Symmetry Index is a ratio that compares the performance of one limb with a reference limb, expressed as a percentage. It is commonly used to describe interlimb asymmetry in strength, power, hop, balance, and other functional tests.

An LSI of 100% means both sides achieved the same score. A score below 100% means the evaluated limb performed below the reference limb, while a score above 100% means it performed above the reference limb.

The reference limb depends on the testing context. It may be the uninvolved limb in rehabilitation, the non-dominant limb in sports with expected dominance-related adaptations, or the better-performing limb when screening athletes in more symmetrical sports.

2. How is the LSI calculated?

The score is calculated like this: (score of the evaluated limb / reference-limb you're comparing against) x100.

We can for instance compare the left to the right side (left score/right score)x100 and the right to the left side (right score/left score) x100: Left strength value = 144N, right strength value = 155N. Left compared to right = (144/155) × 100 = 92.9% Right compared to left = (155/144) × 100 = 107.6% This describes the same left-right asymmetry from two different reference perspectives. How you apply this depends on the testing context:

Rehab

This is the most straight-forward scenario: there is an involved side and the non-involved side is used as the reference. How far is the involved side from the non-involved? In the example below, the left side is involved and needs to be strengthened some more.

Prevention, with a dominant side

In some sports we compare the dominant limb vs the non-dominant (e.g. in tennis and other overhead sports). We expect the dominant limb to be somewhat stronger than the non-dominant. In the example below, the right side is the dominant side and this is compared to the non-dominant left side.

Prevention, in 'symmetrical' sports

Here we use the other limb as the reference, so we compare left to right and right to left - resulting in the two scores from our example above: Left strength value = 144N, right strength value = 155N. Left compared to right = (144/155) × 100 = 92.9% Right compared to left = (155/144) × 100 = 107.6%

Of course we're mostly interested in the 'weaker' limb and how much weaker this is compared to the other side. In this example, the right hip external rotation strength is 18% below the left-side reference value. This finding can guide further assessment and, where relevant, targeted strength training.

The left side is slightly weaker for Hip Extension and Hip Flexion but this falls within an acceptable range.

The color code is determined by the benchmark, depending on the sport, age, non-dominant vs dominant side but also the milestone in rehab. For instance, a 70% LSI may be acceptable for Return to Run but the same score in a Return to Sport test will not be considered good enough.

3. For which tests is the LSI commonly used?

LSI is commonly used in strength tests, such as isometric or isokinetic knee extension and knee flexion testing. It is also frequently used in functional tests, like single leg hop, triple hop, triple crossover hop, timed hop tests, and the star excursion balance test.

In the Hylyght platform we always present the raw scores as well, because sometimes it's more important to see how a certain score is evolving or what the reference score was before an injury.

4. FAQ

4.1. Why is LSI a useful metric?

The LSI is objective and individualized, as it uses the uninvolved side as a reference rather than population norms. However, the contralateral limb is a practical reference, and not necessarily a representation of pre-injury capacity or sport-ready performance.

4.2. Are there limitations to using the LSI?

Yes. One side might already have been weaker before the injury, reducing the importance of meeting a certain LSI threshold after rehab. The 90% cut-off that is often used is somewhat arbitrary and not universally validated across all tests and populations. In addition, the contralateral limb may lose capacity after injury or during rehabilitation, so a high LSI can sometimes mask bilateral deficits.

These are excellent reasons to test often, and to use a pre-season test as a pre-injury benchmark when an injury does occur.

4.3. What is a good LSI score? Is 90% LSI enough to return to sport?

No. LSI should not be used in isolation. A score close to 100% indicates similar performance between limbs; it does not prove that either limb has sufficient capacity for sport or daily activities.

4.4. Can you have a 100% LSI and still have a deficit?

Yes. If both limbs are deconditioned, they can perform equally while still being below pre-injury level or sport-specific needs.

5. References

• Grindem, H., Snyder-Mackler, L., Moksnes, H., Engebretsen, L. and Risberg, M.A., 2016. Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study. British Journal of Sports Medicine, 50(13), pp.804–808.
• Paterno, M.V., Rauh, M.J., Thomas, S., Hewett, T.E. and Schmitt, L.C., 2022. Return-to-sport criteria after anterior cruciate ligament reconstruction fail to identify the risk of second anterior cruciate ligament injury. Journal of Athletic Training, 57(9–10), pp.937–945.
• Wellsandt, E., Failla, M.J. and Snyder-Mackler, L., 2017. Limb symmetry indexes can overestimate knee function after anterior cruciate ligament injury. Journal of Orthopaedic & Sports Physical Therapy, 47(5), pp.334–338.
• Hylyght

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