Why 90% limb symmetry index is not enough

Most rehabilitation professionals use some form of hop testing, but which hop tests should we use and what should our passing criteria be?

Which hop tests should I choose?

While it is important to use double limb hopping as part of the progression in rehabilitation, it is unnecessary for return to sport and discharge testing. In a study by, Myer et al double limb activities did not identify the unilateral deficits found after ACL reconstruction. Here are the unilateral tests supported by research:

Single hop for distance
6m timed hop — I don’t use this one. This is not solely based on published research (although the reliability is the lowest of the hop tests), but what I have observed clinically and through studies we have done. Basically, unless you use timing gates, your stopwatch trigger finger error is pretty close to any right/left asymmetries you would find (except in the cases of severe asymmetry — which the other hop tests would pick up)
Triple hop
Triple crossover hop

The above tests are reliable and modifiable  (Munro & Herrington 2011Reid et al 2007). I have not seen any studies demonstrating the injury prediction value of these hop tests (if you do know of such studies, please let me know) but they do have decent discriminant validity.

 Criteria for Discharge  and/or Return to Sport:

To me, the MOST important question is “What should the return to sport and/or discharge criteria be for hop testing?”

The most common return to sport criteria that I have come across in the literature is 85% and 90% Limb Symmetry Index (LSI). I believe neither of those is stringent enough. Remember, previous injury is the most consistently reported risk factor for future injury and we are currently discharging individuals with modifiable risk factors. So, on that basis alone, our standards should be higher.

But, let’s look at the research.  Reid et al 2007 repeated hop testing on 4 separate time points after ACL reconstruction (16 weeks, a couple times more that week, and at 24 weeks post op)  and  found good longitudinal and concurrent validity for the four hop tests.  However, consider this interesting fact from their research:

At 24 weeks post-op ACL reconstruction, the average overall Limb Symmetry Index was 88.5% and the average Lower Extremity Functional Scale score was 69.3. An athlete with a 69.3 LEFS would have moderate difficulty with the following activities

“Your usual hobbies, recreational or sporting  activities”
“Running on even ground”
“Running on uneven ground”
“Making sharp turns while running fast”

If someone reported this much difficulty with these activities (I realize this is an “average” report but their LSI report is also an “average”), should she return to sport? 


Bottom line: In this study, 89% hop testing LSI equates with moderate difficulty with simple sport activities. Thus, 90% is not enough. Also, just because someone has returned to sport (which is what is typically considered “success”) does not mean that she is not at substantially increased risk of injury.

Finally, Munro & Herrington 2011 found that the average LSI for the four hop tests was 100% (98.38 to 101.61%.) and that 100% of healthy subjects have at least an LSI of 90%. Based on these results, the researchers advocate that the return to sport LSI criteria be increased to 90%.

Given our current re-injury rate, I suggest hop testing LSI should at least be above 95% and recommend it to be above 97%-100%. Do you think we can achieve this in rehabilitation? Do you think we should use 90% for return to sport and 97-100% for discharge?