Testing Limb Symmetry and Asymmetry After Anterior Cruciate Ligament Injury: 4 Considerations to Increase Its Utility

ABSTRACT Anterior cruciate ligament (ACL) injury occurs frequently in sport and surgical reconstruction is often recommended to restore knee joint stability. To guide rehabilitation and determine return to sport readiness, practitioners have used a long-standing practice of calculating the limb symmetry index (LSI) in various functional, biomechanical, and strength tests to compare the injured limb with the noninjured contralateral limb. However, the evidence in support of the LSI calculation to quantify rehabilitation status and return to sport readiness is mixed. We synthesize scientific literature on the LSI calculation and discuss potential reasons for the mixed evidence and limitations. We present 4 considerations to improve the utility of the LSI calculation including (a): the importance of establishing the right benchmark of recovery such as the preinjury contralateral limb or a sport-specific noninjured control benchmark; (b) strategies to manage the high variation in movement asymmetry calculations and the importance of quantifying the intrasubject variability for the component parts of the LSI; (c) the evidence for assessing the movement strategy alongside performance when using the LSI; and (d) how a sport-specific envelope of function can be used to inform post-ACL injury testing that incorporates the LSI.