How the injured limb compares with the uninjured one, as a percentage.
The Limb Symmetry Index (LSI) is a ratio that expresses how the injured or operated limb performs relative to the uninjured limb, usually reported as a percentage. It's most commonly applied to strength testing (e.g. isokinetic quadriceps or hamstring strength) and functional hop tests (single-leg hop for distance, triple hop, crossover hop, timed hop).
On the clinic floor, LSI gives a quick, objective benchmark for return-to-sport or return-to-activity decisions, particularly after ACL reconstruction and other lower-limb injuries. It helps frame the conversation with the patient and coaching staff around readiness, and a low LSI is a concrete, defensible reason to hold someone back from progression rather than relying on time-based criteria alone.
In practice, both limbs are tested using the same protocol — for example, three trials of single-leg hop for distance, or peak torque on an isokinetic dynamometer — and the injured-limb value is divided by the uninjured-limb value and multiplied by 100. A threshold of 90% or above is a widely used target, but it is typically combined with other measures such as strength, psychological readiness (e.g. ACL-RSI), and sport-specific movement quality rather than used in isolation.
The main limitation is that LSI assumes the uninjured limb is a stable, healthy reference, but that limb may itself have deconditioned during rehabilitation, especially after a long layoff, so the ratio can look reassuring even when absolute performance on both sides is poor. LSI can also be misused as a single pass/fail gate for return to sport when it should sit alongside other criteria, and it says nothing about movement quality, landing mechanics, or asymmetries that don't show up in the specific test used.
This guide was auto-drafted and is pending editorial review.