Limb Symmetry Index: Chasing Equal Function

Contents

1. Introduction
2. Review of the Research
3. Conclusion
4. About the Author
5. References
6. Comments

Introduction

Limb Symmetry Index (LSI) is a ratio between opposing limbs (e.g. right leg and left leg) that is commonly used within rehabilitation protocols, to compare an affected limb to (what would usually be) its unaffected counterpart (Palmieri-Smith et al., 2015).

It is calculated as follows:
(Affected limb value / un-affected limb value) * 100.

The LSI value is expressed as a percentage – the affected limb value as a percentage of the unaffected limb value. This value could represent numerous qualities such as strength in a particular muscle (e.g. the quadriceps) or performance in a hop test (e.g. single-leg vertical jump).

As it’s an easy-to-calculate, inexpensive and flexible objective measure of limb symmetry that can be used for different tests throughout a rehabilitation protocol, there’s no surprise it’s so commonly used within clinical practice.

The use of LSIs also removes the issue associated with progressing patients through their rehabilitation by being able to complete a certain number of repetitions in a particular exercise, which heavily relies on research in a very specific population.

Although LSIs have many appealing advantages, they don’t come without their limitations. Not only are these limitations crucial to be aware of, but their potential implications require constant consideration and reinforcement within clinical settings.

Therefore, the aims of this Blog Post are to highlight the key considerations from peer-reviewed literature that practitioners should be aware of when using LSI’s in their everyday practice

Review of The Research

It is clear that there is abnormal limb symmetry following procedures such as anterior cruciate ligament reconstruction (ACLR) (5). In uninjured populations, using LSI’s for measures such as knee extension and flexion range of motion, endfeel and effusion have previously demonstrated excellent reliability (3). However, the same study suggests that when LSI’s are used for hip adduction strength, scores can show larger variation.

In patients who have had an ACLR, there is reduced maximum voluntary activation bilaterally in the quadriceps (10, 11). This is further supported by Palmieri-Smith et al., (2008), who suggests that this may also be due to arthrogenic (reflex) muscle inhibition (7). Depending on the measure being assessed and the stage of rehabilitation, LSI ‘pass’ scores can typically vary from 80-95%.

Another study found that even 7-months post ACLR, both male and female participants who scored mean LSIs of 95.4 % across 3 different hop tests, their raw scores (e.g. jump distance) were still much lower when compared to healthy participants (2). This data further supports that there is true bilateral muscle weakness present following ACLR, which is highly neural.

These implications lead us to another study, where pre-operative ACLR participants completed a quadriceps strength test and hop test to gather LSI scores, with the same testing administered 6-months post-op (12). At 6-months post-op:

• 40 participants achieved 90 % LSI scores.
• However, only 20 participants achieved 90 % of their pre-operative scores across all measures.
• Oppositely, 24 of the 40 participants who achieved 90 % LSI 6-months post-op, did not achieve 90 % of their pre-operative scores.

This means that even though participants scored 90 % LSI 6-months post-op, over half still weren’t at 90 % of the level they were at before their operation. So, even though for some clinicians, this criteria may be a pass to progress rehabilitation, patients may not be at an acceptable level. This further supports the notion that there is bilateral quadriceps weakness following ACRL.

It also appears that LSI scores are significantly higher in a Single Hop for Distance and Triple Hop for Distance versus peak knee extension isokinetic torque and a bilateral power leg test (4). This again is likely due to the decline in function of the unaffected limb (9).

Conclusion

This body of research highlights the utmost importance of patients continuing to train their non-affected limb to maintain as much function as possible. This minimises the chance of LSI scores post-op underestimating knee function. It is crucial to encourage and reinforce this with patients, so they are committed to their rehabilitation.

Where possible, pre-operative testing is recommended to set a true benchmark (Davies et al., 2020). However, it’s important to consider when an ACL injury was diagnosed and how long the patient’s level of physical activity has been reduced, as this may have already resulted in a decline in function of the unaffected limb.

This also allows practitioners to take advantage of benefits such as cross-education. Providing a high neural training stimulus to an unaffected limb can also help reduce the loss of neural function in the affected limb (Papandreou et al., 2013).

Limb Symmetry Index is an easy-to-calculate, inexpensive and flexible objective measure of limb symmetry that can be used for different tests throughout a rehabilitation protocol. It is also an effective method. However, for this to be true, The above limitations must be considered to ensure that affected limb function is not overestimated and true equal (or close to) function is achieved.