LSI Calculation Formula
The Limb Symmetry Index (LSI %) expresses the performance of an injured or post-surgical limb as a percentage of the uninjured contralateral limb:
LSI (%) = (Torque or Work of Involved Limb / Torque or Work of Uninvolved Limb) × 100%
Conversely, the Bilateral Asymmetry Deficit (%) is calculated as 100% - LSI. An LSI of 80% represents a 20% bilateral strength deficit.
Construct Non-Homogeneity: Why LSI Values Are Not Interchangeable Across Tests
Critical Scientific Principle
A similar numerical output (e.g. LSI = 92%) obtained from different testing modalities does NOT mean the tests measure the same physiological construct:
- Isometric Dynamometry LSI: Measures fixed-angle joint torque capacity without velocity (0°/s).
- Dynamic Isokinetic LSI: Measures isolated joint torque & work capacity across accommodating angular speeds.
- Force Plate Jump LSI: Measures multi-joint ground reaction force expression, impulse, and landing strategy.
Achieving an LSI >90% on an isometric dynamometer or force plate jump does not guarantee restored dynamic isokinetic torque capacity through the full range of motion.
Clinical Cut-Off Criteria
- <80% LSI: Significant clinical deficit requiring targeted unilateral overload. High risk of compensation.
- 80–89% LSI: Intermediate recovery. Permissible for non-contact linear reconditioning.
- ≥90–95% LSI: Gold-standard benchmark required prior to full contact sport re-entry and unrestricted competition.