The Limb Symmetry Index is the most cited single number in return-to-sport decision-making. It compresses every metric on a bilateral assessment into one ratio, which is convenient for charting and for thresholds, but the convenience hides three things worth understanding: the weighted average that produces the headline, the per-component cap that prevents a single inflated metric from rescuing a deficient one, and the "lowest metric" line that always renders beside the headline so a green overall cannot hide a red component.
The headline and what's in it
LSI is the ratio of the involved-side value to the uninvolved-side value, expressed as a percentage. 100% is symmetric, below 100% means the involved side is weaker (or less steady, depending on the metric). The platform computes a per-metric LSI for every metric on the bilateral assessment (peak, RTD Early, RTD Late, nRMSE, CV, Yank) and then averages them into the overall headline.
The average is weighted, not equal. Peak carries more weight than RTD; RTD carries more weight than steadiness. The weighting reflects the literature's emphasis: peak symmetry is the most consistently studied predictor of re-injury risk; explosive strength is the second-tier criterion; steadiness is a relatively new entrant. The exact weights are set by the platform; they're tuned to roughly match how clinicians describe their own decision-making in published return-to-sport criteria sets.
LSI
mean of 3v3 trials
- Current
- Goal (100%)
Why components are capped at 100% before averaging
A patient who happens to be less variable on the involved side than the uninvolved side will produce nRMSE, CV, or Yank values that read above 100% on those metrics (the involved side is steadier). Without a cap, that 110% steadiness symmetry would pull the weighted average up and partially offset, say, a 75% peak symmetry. The resulting overall would suggest the patient is closer to recovered than they actually are.
To prevent that, every per-metric component is capped at 100% before contributing to the weighted average. Above-symmetric values are still surfaced on the radar (so you can see that the involved side was steadier, which is sometimes clinically meaningful), but they cannot rescue the headline. A patient at 75% peak symmetry and 110% Yank symmetry has an overall pulled down by peak; the Yank value sits at its raw 110% on the radar but contributes a capped 100% to the average.
This is a small mechanical detail that matters because the alternative reads badly in practice. Clinicians making return-to-sport decisions on a "92% LSI" need to know that number reflects the patient's deficits, not the patient's deficits offset by an irrelevant strength on the involved side.
The "lowest metric" surface
A weighted average can be misleadingly green. 92% overall sounds recovered. But if the same patient has 70% on RTD Early and only crosses 95% on the other five metrics, the patient is not actually at 92% on the thing that matters most for cutting, jumping, or landing. The headline number is technically correct but clinically incomplete.
The LSI panel surfaces the lowest individual per-metric LSI on a line directly underneath the headline, with the same color-tier styling (red below 70%, amber 70–85%, yellow-green 85–95%, green above 95%). On a patient with 92% overall and 70% RTD Early, that line reads "70% RTD Early" in red. You see both numbers side by side: the green headline and the red component pulling against it.
The clinical read on that pattern is usually that the patient is strong but slow to fire. The peak deficit has resolved, but the explosive phase hasn't. Recovery of peak before recovery of RFD is the most common pattern in the post-ACLR literature, where the standardized mean difference for early-phase knee extensor RFD versus the uninvolved limb is about −1.07 and stays suppressed well after peak normalizes (Turpeinen et al. 2020). The line on the panel exists so that pattern doesn't get missed by glance-reading the overall.
The radar shape
The radar plots all six per-metric LSI values on the same scale. The shape tells you what kind of asymmetry the patient has, not just how much. Two patients with the same 85% overall LSI can have very different radars:
- A uniform radar: every metric sits around 85%. The patient has a global strength deficit on the involved side. Rehab focuses on building capacity across the board.
- A peaked radar: peak and RTD windows are recovered (95%+), but the steadiness metrics drag the average down (70% on nRMSE, CV, or Yank). The patient is strong but not controlled. Rehab focuses on motor control work.
- A valley radar: peak is recovered but the RTD windows are sub-85%. The patient is strong but slow. Rehab focuses on explosive training.
The radar is the part of the panel most worth reading first. The headline is what you write in the chart note; the radar is what you build the next phase of rehab against.
Steadiness in LSI specifically
The three steadiness metrics in the average (nRMSE, CV, Yank) are an active area of measurement research, and treating them as ratios of involved-to-uninvolved is not without subtlety. Force steadiness depends on muscle size, motor unit population, and target force, with small muscles producing lower CV at the same percent-MVC than large muscles (Enoka & Farina 2021). For within-patient LSI between the same patient's two legs, those confounders are largely controlled (same person, same target percentage). For cross-patient comparisons of steadiness LSI, more caution is warranted.
The post-ACL literature also suggests that nonlinear analyses of force control detect more and larger asymmetries than the linear coefficient-of-variation methods (Schwartz et al. 2025). The platform reports CV as the linear baseline; Yank captures a complementary axis (smoothness, not variability) that the linear methods miss. See reading the steadiness triple together for the full picture.
What to do next
- Read the radar first, the headline second, the lowest-metric line third. All three in one scan; the radar tells you the shape, the headline tells you the magnitude, the lowest metric tells you the limiting factor.
- For return-to-sport decisions, look at all six per-metric LSIs against your published thresholds, not just the headline against a 90% rule of thumb. The literature's thresholds are typically per-metric, not overall.
- A green overall with a red lowest metric is the most informative pattern on the panel. It's what the panel was designed to surface.
References
- Turpeinen JT, Freitas TT, Rubio-Arias JÁ, et al. Contractile rate of force development after anterior cruciate ligament reconstruction, a comprehensive review and meta-analysis. Scand J Med Sci Sports. 2020;30(9):1572-1585. doi:10.1111/sms.13733
- Schwartz AL, Koohestani M, Sherman DA, et al. Knee extensor and flexor force control after ACL injury and reconstruction, a systematic review and meta-analysis. Med Sci Sports Exerc. 2025;57(2):238. doi:10.1249/MSS.0000000000003574
- Enoka RM, Farina D. Force steadiness, from motor units to voluntary actions. Physiology (Bethesda). 2021;36(2):114-130. doi:10.1152/physiol.00027.2020