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LSI Calculator
for ACL Recovery
The Limb Symmetry Index compares your injured leg to your healthy one. Enter two values and see where you sit against the 90% guideline that physios commonly reference during ACL rehab.
Enter your values
Use the same test, position and effort on both legs. The number only means something if the inputs are comparable.
Enter both values to see your symmetry.
What LSI actually tells you
LSI is a ratio. One number. Your injured leg divided by your healthy leg, multiplied by 100. If the injured side matches the healthy side, LSI is 100%. If it lags behind, LSI drops.
LSI is useful because it normalises away your absolute strength. A 70 kg leg press and a 140 kg leg press are both "100% LSI" if both legs match. The question LSI answers is: has the injured side caught up?
The commonly referenced 90% threshold comes from research on return-to-sport readiness after ACL reconstruction. It is a guideline, not a diagnosis. Clinicians combine LSI with hop tests, movement quality, time from surgery, psychological readiness and the demands of your sport. TODO: add source link for the 90% guideline
How to use the calculator
- Measure both legs on the same test, in the same position, with full effort on each side.
- Enter the injured leg value first, then the healthy one.
- Read your LSI percentage and how it sits against the 90% line.
Common tests clinicians use: leg extension 1RM or 10RM, single-leg leg press, single-leg hop for distance, triple hop, crossover hop, timed 6-metre hop, and isokinetic quadriceps peak torque. Each stresses the knee differently, so a strong LSI on one test doesn't guarantee the same on another. TODO: verify common test battery references
Why your LSI might read lower than you expect
A few patterns that catch people out:
- Different effort between legs. If you push harder on the healthy side (natural reflex), LSI drops. Match effort consciously.
- Different positions. A different seat setting on the leg extension changes the lever arm. Record the setting.
- Different test days. LSI is only meaningful when both legs are measured the same session. A healthy value from three weeks ago isn't current.
- Pain inhibition. If the injured knee is sore that day, you're measuring pain tolerance, not strength.
Why your LSI might read higher than reality
Equally important:
- Compensation. On a double-leg leg press, the healthy side can carry more load than you realise. Single-leg testing exposes this.
- Rep ranges. A 20-rep set with light load isn't the same gap as a 1-rep max. Deep gaps show up more on heavy sets.
- Hop tests only. Hop distance is strongly influenced by confidence and technique, not just strength. People hit 90% hop LSI with meaningful residual quad weakness.
Frequently asked
Is 90% enough to return to sport?
Not on its own. 90% LSI is one factor among several that clinicians consider — alongside hop test batteries, movement quality, time from surgery, psychological readiness and the demands of your sport. Your care team decides, not the number.
Why does my LSI change so much between sessions?
Small session-to-session variation is normal and usually reflects the effort you put in on each leg that day, not real changes in strength. The trend over weeks is what matters, not any single reading.
Can I use this for other injuries?
LSI is used for any single-sided lower-limb injury where you can measure both sides on the same test — meniscus, patella, Achilles, hamstring, ankle. The 90% guideline specifically comes from ACL research; other injuries have their own protocols.
Does this calculator save my inputs?
No. It runs entirely in your browser. Nothing is sent anywhere. Refresh the page to clear the fields.
Track LSI over time in Palio.
Log weight × reps per leg. See your LSI chart for every exercise. Free, with unlimited history.
Get Palio on the App StorePalio is an educational recovery tracker. It does not diagnose, prescribe treatment or clear you for sport. Follow your clinician's instructions.