With increasing incidence of ACL re-injury rates, there has been scrutiny placed on the current practices of surgical intervention and rehabilitation that athletes are subjected to. Currently, the risk for secondary ACL injury is ~20% irrespective of gender (Patel et al 2021). Increasing efforts in the literature have highlighted (quite ironically) that those that pass RTS criteria are actually at an increased likelihood of second ACL injury compared to those that don't (Paterno et al 2021) whilst some authors have concluded that there is a failure amongst current testing batteries to identify who is at risk of a secondary injury (Welling et al 2020) (Webster & Feller 2020). This highlights a gap in our current clinical practice as rehabilitation professionals, we cannot, with a great deal of certainty advise our athletes whether they will re-injure or not. Furthermore, the efficacy of hop testing has been challenged greatly with criticisms placed upon what were once the bread and butter of return to sport testing batteries (Losciale et al 2019). The purpose of this blog will be to outline the role that hop testing plays in the assessment of function post ACLr (and in extension non-operative ACL management) and why it should not be abandoned as part of the rehabilitation professionals assessment toolkit.
THE KEY TO DISCOVERY IS TO KNOW WHAT YOU ARE LOOKING FOR
Throughout the assessment journey post ACLr, obvious deficiencies slowly diminish and it becomes the job of the clinician to challenge the athlete in ways that will ideally unveil skeletons in the athletic closet. This is why traditionally, testing protocols shift from more clinical and physiological based testing to higher demanding tasks later in the rehabilitation journey. Typically for my athletes I will follow a progression of the following:
ROM & Physiology
Maximal Muscular Strength
This is where I feel that the purpose of hop testing is lost amongst clinicians as the more functional a test becomes, more areas of the body contribute to the 'outcome' of the test. That's where the wonders of human movement strategy can cloud deficiency. I see this trend all the time when assessing athletes, those that are functionally at a lower capacity tend to hip dominate through the propulsion and landing phases and have increased ground contact time on multi-hop tests. At this stage it is important to identify the normal contributions to movement during hop testing tasks.
Hop testing tasks are usually split into either vertical or horizontal based demands and range from singular to multi-hop based tasks. During these tests there is a propulsive phase and a landing phase. Vertical hop testing has relatively even contributions amongst the hip, knee and ankle (in healthy active individuals) whilst in horizontal testing there are large disparities between joint contributions in the propulsion & landing phases (Kotsifaki et al 2021).
Figure 1: Joint contributions during vertical & horizontal hopping (Kotsifaki et al 2021)
It's important to note that this has been identified in healthy individuals however the numbers recruited in this study sat at a meagre 20 so the results should be taken with a grain of salt. There may in fact be larger variation in these numbers, however it is a useful baseline for clinicians to utilise. With this in mind, to me the solution to the problem at hand lies not on a tape measure but rather in the clinicians eyes and their smartphone camera.
DECONSTRUCTING THE EVIDENCE: WHAT SHOULD WE BE LOOKING FOR?
With the above data in mind, my personal perspective is we should be analysing how an athlete jumps and hops more-so than analysing their raw data (Davies et al 2019). I'm a firm believer that limb symmetry index (LSI) is one of the best assessment tools that a clinician has in their toolbox (more to come on this in a future blog). As far as raw data goes there can be less comparisons that can be made to literature if we are talking purely from a 'exit criteria' perspective. Every paper that establishes normative data, does so in respect to the population group they are assessing. One of my biggest bug bears in regards to literature papers is when they describe 'healthy individuals'. I understand that lines in the sand need to be drawn but what on earth does 'healthy' mean? Nil medical conditions? Nil prior history of injury? Athletic background? White blood cell count and muscle fibre composition? These definitions are vague and fail to account for the individuality of the human race. No paper is going to have normative data for 15 year old female soccer players who also are cross country runners, lacrosse players and enjoy English but not Mathematics. I use hyperbole but the principle of my message is that whilst 'normative' data can help establish performance metrics across a broad population group (i.e. excellent single leg counter movement jump height in professional male soccer players), they can't identify what are good and poor scores relative to the individual you are assessing. Yes I am aware that in the context of ACL re-injury, athletes that are more plyometric tend to be at a reduced risk than those who are less plyometric (King et al 2021), however at what point do we make the call that an athlete is performing well relative to their own personal ceiling of performance?
Now that rant is over... I can get back to my original point of this section, the qualitative analysis of our hop tests. Whilst completely subjective (unless you have access to a VICON camera setup) we can highlight when an athlete is uncomfortable in utilising their knee to absorb and produce force. Favoured methods that I tend to use are a combination of fast and slow motion camera views, comparing injured and uninjured limbs to gain a 'qualitative limb symmetry index' (QLSI). In regards to specific components of the hops, my personal perspective is that the pre-flight eccentric phase of a vertical & horizontal hop and the degrees of knee flexion an athlete lands into on a horizontal hop will give the practitioner the information they need in regards to whether the athlete has enough eccentric rate of force development in their quads.
UNDERSTANDING SENSITIVITY & SPECIFICITY
As clinicians we are taught extensively the concept of sensitivity and specificity in regards to diagnostic special testing, however the concept doesn't tend to carry over towards our rehabilitation testing. For me, I believe we can look at our rehabilitation testing in a similar light and hop testing is one of the better examples that I use in this space, particularly in relation to specificity. In this respect, I mean that if an athlete performs poorly during hop testing, whether that be in raw scores, LSI or in qualitative movement analysis it is highly likely that they will suffer problems out on the sporting field. If however they perform well at the testing it does not necessarily guarantee an athlete a successful return (whatever the hell successful means). In this light we need to understand that hop testing is highly useful at RULING IN problems rather than RULING OUT risk of injury.
CORRELATION DOES NOT EQUAL CAUSATION
Barbara Kingsolver (I'd never heard of her either I just googled it) once said “Cars with flames painted on the hood might get more speeding tickets. Are the flames making the car go fast? No. Certain things just go together. And when they do, they are correlated. It is the darling of all human errors to assume, without proper testing, that one is the cause of the other.” Whilst never hearing her name in my entire life, she establishes a good point. One which must be considered when evaluating literature that associates passing return to sport criteria with increased risk of secondary ACL injury (Paterno et al 2021) (Welling et al 2020). Personally, my take on this data is that athletes that pass return to sport criteria are more likely to return to sport than those that fail it, which is supported in the literature (Webster & Feller 2020) (Paterno et al 2021) (Welling et al 2020). It is also logical to assume that for those that don't return to sport, there is going to be a lower re-injury rate because they aren't being subjected to the chaotic rigours associated with injury that those who do return to sport are subjected to. By the same token we can conclude that those with a drivers licence are more likely to crash their car than those who do not have a drivers licence, despite the former being more qualified to drive than the latter. With all this in mind, I believe as clinicians we need to approach the evaluation of literature with skepticism and more importantly... Common sense.
ADVANTAGES OF THE HOP TEST
In my personal opinion, the utility of a test in clinical practice is a culmination of a combination of factors. These being:
Equipment & Space
In a practical example, Isokinetic dynamometry is the gold standard of isolated muscle strength testing. This is based upon its reliability, validity and its ability to produce an outcome based on specific muscle groups. However the cost associated with the machinery and the time it takes to put an athlete through the testing protocol limits its practical applicability to infrequent testing. With this in mind there can even be question marks over how useful it is if an athlete say completes a standard '9 / 12 month testing protocol' and then doesn't play a game of sport for another 3-6 months later, who's to say that athlete would register the same scores at that period of time compared to when they did their testing battery? Hop testing, at the other end of the spectrum are easy to perform and provide low costs, low time and low equipment demands whilst maintaining high levels of validity & reliability (Reid et al 2007). Navigating this balancing act is the challenge of the rehabilitation professional where we need to, on any given day and time what might be useful 'screening tools' for the athletes in front of us.
Hop testing has been recently scrutinised at having a poor ability to determine secondary ACL injury. Recent literature has brought to light data that identifies that those that pass return to sport criteria, namely that involving hop testing are at an increased risk of secondary ACL injury compared to those that don't. Hop testing is a reliable, valid and probably a misunderstood tool in the rehabilitation professionals toolkit that should be used as a screening method to rule in biomechanical risk factors associated with injury rather than ruling out future injury. Correlation should not be confused with causation and the likely explanation regarding increased re-injury amongst those with stronger hop testing performance is due to increased return to sport rates compared to those that produce poorer outcomes. Clinicians should always maintain high levels of skepticism to evidence (including this blog) and I encourage all that hop testing is useful in the process of ACL rehabilitation. Whilst further evidence is needed in regards to the efficacy of hop testing we should not throw the baby out with the bath water and keep utilising these methods as part of our assessment procedures.
Patel AD, Bullock GS, Wrigley J, et al Does sex affect second ACL injury risk? A systematic review with meta-analysis British Journal of Sports Medicine 2021;55:873-882.
Paterno, MV, Thomas, S, VanEtten, KT, Schmitt, LC. Confidence, ability to meet return to sport criteria, and second ACL injury risk associations after ACL-reconstruction. J Orthop Res. 2021; 1- 9. https://doi.org/10.1002/jor.25071
Welling, W., Benjaminse, A., Lemmink, K., & Gokeler, A. (2020). Passing return to sports tests after ACL reconstruction is associated with greater likelihood for return to sport but fail to identify second injury risk. The Knee, 27(3), 949–957. https://doi.org/10.1016/j.knee.2020.03.007
Webster, K. E., & Feller, J. A. (2020). Who Passes Return-to-Sport Tests, and Which Tests Are Most Strongly Associated With Return to Play After Anterior Cruciate Ligament Reconstruction? Orthopaedic Journal of Sports Medicine. https://doi.org/10.1177/2325967120969425
Losciale, J. M., Bullock, G., Cromwell, C., Ledbetter, L., Pietrosimone, L., & Sell, T. C. (2020). Hop Testing Lacks Strong Association With Key Outcome Variables After Primary Anterior Cruciate Ligament Reconstruction: A Systematic Review. The American Journal of Sports Medicine, 48(2), 511–522. https://doi.org/10.1177/0363546519838794
Kotsifaki, A., Korakakis, V., Graham-Smith, P., Sideris, V., & Whiteley, R. (2021). Vertical and Horizontal Hop Performance: Contributions of the Hip, Knee, and Ankle. Sports health, 13(2), 128–135. https://doi.org/10.1177/1941738120976363
Davies, W. T., Myer, G. D., & Read, P. J. (2020). Is It Time We Better Understood the Tests We are Using for Return to Sport Decision Making Following ACL Reconstruction? A Critical Review of the Hop Tests. Sports medicine (Auckland, N.Z.), 50(3), 485–495. https://doi.org/10.1007/s40279-019-01221-7
King, E., Richter, C., Daniels, K. A. J., Franklyn-Miller, A., Falvey, E., Myer, G. D., Jackson, M., Moran, R., & Strike, S. (2021). Can Biomechanical Testing After Anterior Cruciate Ligament Reconstruction Identify Athletes at Risk for Subsequent ACL Injury to the Contralateral Uninjured Limb? The American Journal of Sports Medicine, 49(3), 609–619. https://doi.org/10.1177/0363546520985283
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