How to Measure Limb Strength Index
The limb symmetry index (LSI) is calculated by dividing the strength measurement of the involved/affected limb by the strength measurement of the uninvolved/contralateral limb and multiplying by 100 to express it as a percentage.
Calculation Method
The fundamental formula for LSI is 1, 2, 3:
LSI = (Involved Limb Strength / Uninvolved Limb Strength) × 100
- Values closer to 100% indicate greater symmetry between limbs 1
- An LSI of 100% represents perfect symmetry 2
- Values below 100% indicate the involved limb is weaker than the uninvolved limb 3
Strength Testing Methods
According to the American Heart Association/American Stroke Association guidelines, several validated methods can be used to obtain the strength measurements needed for LSI calculation 4:
Manual Muscle Testing
- Graded on a 0-5 scale (Medical Research Council scale) 4
- Takes less than 5 minutes per limb 4
- Can be used for both upper and lower extremities 4
Handheld Dynamometry
- Provides quantitative strength measurements 4
- Takes less than 5 minutes to administer 4
- Particularly useful when muscle strength is 3-5 on the MRC scale 5
Isokinetic Dynamometry
- Considered the gold standard for strength assessment 6, 3
- Measures peak torque at specific angular velocities (commonly 60°/s, 180°/s, 300°/s) 3
- Requires specialized equipment but provides highly reliable data 6
- Testing at 60°/s (ISO60) provides the most conservative LSI values 3
Grip and Pinch Dynamometry
- Available in most rehabilitation clinics 4
- Normative data available for comparison 4
- Takes less than 5 minutes 4
Motricity Index
- Consists of manual muscle testing at 3 key upper extremity segments and 3 key lower extremity segments 4
- Yields a score from 0-100 indicating strength of each limb 4
- Takes less than 5 minutes for upper extremities and less than 5 minutes for lower extremities 4
Critical Caveats and Pitfalls
The Uninvolved Limb May Not Be a Valid Reference
Recent evidence demonstrates that using the uninvolved limb as the reference standard can significantly overestimate recovery 7:
- The uninvolved limb may weaken over time due to deconditioning, artificially inflating LSI values 2
- In one study, 23.1% of patients had >10% decrease in contralateral strength between 6-12 months post-ACL reconstruction, leading to falsely elevated LSI values 2
- Patients achieving 90% LSI frequently did not achieve 90% of their estimated preinjury capacity (EPIC) 7
- LSI values were less sensitive than EPIC levels in predicting second injuries 7
Test-Specific Variations
LSI values vary significantly depending on the testing method used 3:
- Hop tests consistently produce higher LSI values than isokinetic strength testing 3
- Single hop and triple hop tests showed significantly higher LSI than isokinetic testing at 60°/s 3
- Isokinetic testing at slow speeds (60°/s) provides the lowest and most conservative LSI values 3
Age-Related Considerations
Younger athletes display greater limb asymmetries in strength-related tasks 8:
- Youth athletes (10-14 years) show higher LSI values than elite athletes in strength tests 8
- The commonly used <10% difference threshold may not be appropriate for youth and adolescent populations 8
- Age-related performance levels must be considered when interpreting LSI values 8
Poor Predictive Value for Safe Return to Sport
LSI cut-offs demonstrate poor ability to predict safe return to sport after ACL reconstruction 1:
- Athletes meeting ≥80% or ≥85% LSI thresholds had significantly lower odds of safe return to sport (OR=0.32 and OR=0.39, respectively) 1
- LSI showed poor discriminatory ability between athletes with and without second ACL injuries (AUC 0.50-0.59) 1
- Neither incremental LSI increases nor deviation from 100% symmetry predicted safe return to sport 1
Recommended Testing Protocol
Based on the evidence, isokinetic dynamometry at 60°/s should be prioritized when LSI is used for clinical decision-making 3:
- Test the uninvolved limb first to establish baseline 9
- Perform strength testing on the involved limb using identical methodology 9
- Calculate LSI using the formula: (Involved/Uninvolved) × 100 1
- Do not rely solely on LSI for return-to-sport or functional recovery decisions 1
- Consider comparing current involved limb strength to preinjury baseline (EPIC) rather than current uninvolved limb strength 7
Alternative Approach: Estimated Preinjury Capacity
A more valid approach compares the involved limb at follow-up to the uninvolved limb measured before injury 7: