Welcome to the 6th edition of the Learn.Physio Research Review!

Happy Monday and welcome back to my Research Reviews!
This issue I’m going to review a debated topic amongst experienced clinicians, and it’s the value of performing hop tests in our patients – specifically, what information are we getting out of the hop tests and what does that data tell us?

The first paper I reviewed was really interesting and showed that in a large percentage of healthy, uninjured young athletes could not pass all of the hop tests in a hop test battery that they were given – raising a very important question – “if healthy, unijured athletes cant pass all the hop tests, how can we expect our ACLR patients to do the same?”.

The second paper I reviewed aim to answer the question for the clinician that does not have access to expensive isokinetic testing facilities – “can we use a hop test to infer strong quadricep muscles in our ACLR patients?”.

I hope you enjoy these 2 reviews! As always, I have provided a reference list with links to abstracts and full text links (where possible), so if you’re after more information, check the reference list for further readings!

Introduction

The incidence of ACL injuries and surgical reconstructions has been increasing over the last 20 years; both here in Australia and abroad (Zbrojkiewicz et al 2018, Shaw et al 2017, Abram et al 2020). With that, we are also seeing an increasing trend in 2^nd ACL injuries (both graft and contralateral limb), with up to 30% of under 20-year-old athletes experiencing a 2^nd ACL injury within 2 years upon returning to sport (Webster & Feller 2016, Webster et al 2014 & Morgan et al 2016).

In order to reduce the risk of 2^nd ACL injuries and other types of knee injuries from occurring upon, it is recommended that ACLR patients have their lower limb strength and functional performance assessed prior to returning to competitive sport; with a battery of single-leg hop tests being the most common performance outcome measure. A pass mark for return to play is often set at 90% limb symmetry index – but this certainly has its limitations that I pointed out in issue 1 of my Research Reviews (Patterson et al, 2020).

Despite these recommendations, many young ACLR patients do not achieve at least 90% on many of the strength and hop tests measures - yet have already returned to sport. Therefore, it would make sense to understand what a group of healthy, uninjured controls can do on the same performance tests. There are current normative values on hop test measures published in older high school and college athletes who play basketball and soccer (Myer et al 2014), but none in a younger, paediatric age group.

Thus, the purpose of this paper was to report single leg hop test (SLHT) performance within a group of uninjured paediatric athletes and establish normative limb symmetry index values.

Methods
A group of active, healthy youth athletes aged between 8 and 14yrs of age were enrolled in this study. All data was collected on-site at athletic fields or courts during practice or between games or tournaments.
Inclusion/exclusion criteria were:

• All adolescents had to be without a history of ACLR
• No current lower limb injuries
• Presently participating in organised, competitive sports

• Single leg hop for distance
• Triple hop for distance
• Crossover triple hop for distance
• 6m timed hop test

For the hop test to be valid, the athlete was required to land in a controlled manner without loss of balance or secondary hopping to steady themselves. Participants could have 2-5 practice trials, and the average distance or time of 3 successful trials was used for data analysis.
Limb symmetry index was calculated using non-dominant limb / dominant limb x 100; except for the 6m timed hop tests being dominant limb / non-dominant limb x 100 as better performance on this test was indicated by a lower score. A “pass” mark of 90% LSI was set for all of the SLHT, with a “fail” mark being any score 89% LSI or lower.

Results

340 athletes (average age 10.9 years and 54% males) had complete SLHT data analysed. Soccer (52%) and basketball (22%) were the 2 most common sports played in these athletes.
The mean LSI on each individual hop test indicated nearly symmetrical hop performance:

• Single leg hop for distance: 97.9%
• Triple leg hop for distance: 96.8%
• Triple crossover for distance: 96.8%
• 6m timed hop test: 96.5%

• 95% were able to pass 1 test
• 84% were able to pass 2 tests
• 71% were able to pass 3 tests
• 45% were able to pass all 4 tests

Discussion

The finds of this study on SLHT performance in healthy paediatric patients is quite fascinating and shows that despite 95% of the population able to pass 1 isolated test, less than half were able to pass all 4 SLHTs (45%).
This has big implications for this commonly used hop test battery in ACLR patients; and may force us to have a re-think about what performance outcome measures we use in our ACLR patients (especially the 8-14yrs old’s that were included in this study).

Given that only 45% of these healthy, uninjured participants can achieve a “pass” mark on all 4 SLHTs, it would be unreasonable then to expect that all of our young ACLR patients to “pass” all 4 SLHTs when determining if they are ready to return to competitive sport.

These results are very similar to a recent study of ACLR patients (mean age 17.5yrs, 8months post-op) by Toole et al (2017). They showed a similar trend that LSI was >90% on each individual hop test across, but only 53% of the participants could achieve >90% LSI on all 4 hop tests.

The 4-hop test battery is unique in that each test will measure a different construct in performance; which might make it challenging for young athletes to pass all 4 SLHTs. The single leg hop test is all out power. The triple hop really highlights the plyometric performance of the athlete. The crossover will challenge the athlete in changing directions under 1 leg, and the 6m timed hop test will test plyometric ability as well as speed.

This may explain why many of the young athletes in this study can’t pass all 4 SLHTs – although one could argue that most of them should if they want to play high level cutting, pivoting sports; especially when the argument comes from an increased performance mindset rather than an “injury prevention” mindset.
There were some limitations to this study:

• Hop tests results were collected from athletes at different sporting venues and different playing surfaces, so this variability may affect some of the results.
• As they were also tested at sporting sites, sometimes in between games, some athletes may have been affected by fatigue.
• The authors did not present mean hop test distances or times that could be used as a reference value to compare our young ACLR patients against (unlike Myer et al 2014)

Introduction

Weakness of the quadriceps have been associated with increased risk of the ACL graft and other types of knee injuries (Grindem et al 2016). As mentioned in the previous review, the rate of 2^nd ACL injuries is as high as 30% in the first 2 years upon return to sport, and it make clinical sense to try and identify and address contributing factors to increased risk of 2^nd injuries.

Given that the quadriceps play a pivotal role in the short term and long-term outcomes of the ACLR patient, strength testing of the quadriceps prior to return to sport make sense. Isokinetic strength testing is currently considered “gold standard”, but protocols vary widely regarding mode of contraction, angular velocity, number or reps and ROM.

Like hop tests, comparing the injured limb to the uninjured limb and getting a LSI for quadriceps strength is the most common outcome measure. Although, current best practice recommends that an acceptable standard of quadriceps strength LSI is 90%, many studies set an “acceptable” measure as 85% - yet only 28-60% of patients have an LSI of 85% or more of quadriceps strength at the time of return to sport, and it would be common for many patients to experience quadriceps strength of 85% or less for as many as 2 years post-op.

Although isokinetic strength testing is considered “gold standard”, access to isokinetic strength testing can be quite limited – especially in non-metropolitan areas. As a result, functional testing such as single leg hop testing is often employed as an alternative or even as a surrogate for strength testing. However, it is not clear whether the single leg hop test does in fact reflect quadriceps strength.

Therefore, the purpose of this study was to examine whether there was an association between quadriceps strength LSI and single leg hop for distance LSI at 6- and 12-months post-op ACLR.

Methods

ACLR patients ages between 14 and 45 were recruited for this prospective cohort study. Patients were excluded if they had previous ACLR, multi-ligament injury and major chondral/meniscus damage in the contralateral knee.
All patients had hamstring graft ACLRs and were given guidelines for rehabilitation. The authors acknowledged that rehabilitation no doubt differed greatly between patients but was reflective of the realities of orthopaedic practice here in Australia. The guidelines for rehab were as follows:

• First 3 weeks: aimed at reducing knee swelling and knee pain with ice, compression, elevation and AROM exercises.
• From 3-4 weeks: bike, wall squats, lunges (if tolerated) and hamstring curls.
• From 5 weeks: Gym work such as leg press, half squats, bike, rower, cross trainer, stepper, hamstring curls, bridges, calf raises, core stability and leg extensions (from week 8).
• From 10-16 weeks: hopping, landing drills and jogging once there was no evidence of effusion.
• From 16-26 weeks: patients allowed to return to sport specific drills and activities including kicking, dribbling and COD work.
• From 26-52 weeks: encouraged to increase the intensity of their training.
• Return to sport: no knee effusion, full ROM, stable clinical exam, good quads strength, good single leg step down quality and good sense of confidence in their knee. Patient also to have completed at least 4 weeks of unrestricted training prior to return to pre-injury level of sports.

• Strength test:
  • Isokinetic concentric maximal contraction from full flexion to full extension at 60deg/sec for 3 repetitions.
  • Both legs tested – uninjured limb first, then injured limb.
  • Peak torque for the highest value of the 3 reps was the outcome measure of interest.
• Single leg hop for distance test:
  • Each leg was tested twice, and the maximal hop distance used for data analysis.
  • Hop distance was measured from the toe at take-off to the toe at landing.
  • Hops were not recorded if the person could not control the land and touched the opposite foot down, or double hopped forward to control themselves.

For both outcome measures, patients were grouped at both time points according to “satisfactory” (85% or above LSI) or “unsatisfactory” (below 85%).

Results
69 ACLR patients (mean age 27yrs, 32% female) completed both 6- and 12-month assessments.
At 6 months post-op, only 27.5% had recovered “satisfactory” quadriceps strength in the operated leg, improving to 46% at 12 months.

“Satisfactory” hopping distance recovered considerably faster than quad strength with 66% at 6 months post-op and 90% at 12 months.

Recovery of “satisfactory” quad strength was associated with hop symmetry. Of the patients who had >85% quad strength at 6 months, 95% could hop >85% LSI at 6 months. At 12 months post-op, 100% could do so.

However, “satisfactory” hop distance was NOT associated with “satisfactory” quad strength as only 39% of patients who could hop >85% LSI at 6 months had >85% quads strength at 6 months; rising to just 51% at 12 months.

Discussion

The main finding of this research clearly shows that the ability to get a “satisfactory” single leg hop for distance score does not imply “satisfactory” quadriceps strength – and as a result, single leg hop test should not be used as a surrogate measure for quadriceps strength assessment in ACLR patients.

Furthermore, less than 50% of ACLR patients had >85% of quadriceps strength at 12 months post-op.
The low numbers of ACLR patients passing >85% LSI quad strength at 12 months post-op in this study (46%) is consistent with Beischer et al (20-28% at 12 months post-op), Toole et al (28% at time of RTS) and Herrington et al (2018) who showed in a group of professional soccer players that at the time of their return to sport, over 80% of them failed to achieve >90% LSI on quads strength testing, yet 75% of them could pass single leg hop tests.

Given that most non-professional ACLR patients are trying to return to sport around the 12 months post-op mark, the results from Barford et al’s study has big implications.

Firstly, if we assume that the single leg hop test (or a battery of single leg hop tests) is all we need to do to assess the readiness for the ACLR athlete to return to sport, then we are kidding ourselves and we are over-estimating our patient’s ability to do so.

Given the quadriceps have such an important role in protecting the knee and absorbing forces when the athlete cuts, pivots, and lands; if we don’t measure the quadriceps strength directly, we are sending the athlete back to sport with a higher chance of knee re-injury.

Grindem et al (2016) showed that for every 1% LSI difference below 90% LSI on isokinetic quadriceps strength testsing at 60deg/sec carries an increased knee reinjury risk of 3%. In simple terms, if an athlete has isokinetic strength testing done, and they have an 80% LSI and they choose to go back to sport and accept this risk, they are doing so with a 30% increased chance of reinjuring not only their ACLR graft, but their other knee as well.

The million dollar question is, “if a single leg hop test does not infer a “strong” quadricep, how can you measure quadriceps strength reliably when you don’t have access to “gold standard” isokinetic strength testing?”. Here are 2 cheaper alternatives:

1. Hand-held dynamometry (HHD)
   1. Still not the cheapest option, but cheaper than buying an isokinetic machine; although in metropolitan areas there may be isokinetic testing places.
   2. Note: Isokinetic testing services exist here in Melbourne and Sydney, and cost similar to a physiotherapist consultation.
   3. This HHD testing configuration observed below from from Lesnak et al (2019) is one that I employ and removes the limitation of tester strength present during other test positions.
   4. Participants performed a warm up consisting of three isometric contractions at 50% effort, 75% effort, and 100% effort with 30-60sec of rest between each contraction.
   5. After the warm up, participants completed 3 maximal isometric contractions, holding for approximately 5 seconds, with one minute of rest between each contraction.
   6. Methods were consistent with the isokinetic dynamometer trials, but did not include visual biofeedback.

1. Knee extension test
   1. Cheapest option, but less valid and reliable.
   2. Can be performed with 1RM, 3RM, 6RM or 10RM tests – I personally like 10RM as it gives you a nice round number
   3. Test the uninjured side first on a knee extension machine
   4. Select a heavy weight and get the patient to perform as many reps as possible; aiming for the patient to fail on 10RM
   5. Rest 2mins
   6. With the same weight, aim to perform the same amount of reps on the injured limb
   7. Example, if the athlete can do 10 repetitions at 30kg on the uninjured limb, but can only do 8 repetitions on the injured limb they have a 80% LSI quad strength
   8. This option is far from perfect but gives an overall impression of quadriceps strength and is far better than doing a MMT, which a recent survey of physiotherapists found that 60% of physiotherapists use this method to measure strength and only 28% used isokinetic or 12% use isometric (Greenberg et al 2019)

Conclusion
In summary, from both of these papers presented in this issue of Research Reviews it does appear that hop testing does many limitations, however I think they still do have a role to play in the screening, pre-operative assessments or the uninjured limb, and an indicator of overall lower limb performance following serious lower limb injury. What we do need to be spending more time however on with our ACL injured patients (both ACLR and ACL deficient knees) is quadriceps strength testing before return to training and unrestricted sports activity.

Oh and open chain knee extensions. We need to spend more time with our patients, confidently prescribing open chain knee extensions in their rehab. Do this - and I’ll go out on a limb here by saying this – but if we get our patients do open chain knee extensions, their quad strength symmetry will improve, and so will many of their post-op outcome measures. You can hold me to that.

References:

Article 1

References:
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Shaw L, Finch CF. Trends in Pediatric and Adolescent Anterior Cruciate Ligament Injuries in Victoria, Australia 2005-2015. Int J Environ Res Public Health. 2017 Jun 5;14(6):599. doi: 10.3390/ijerph14060599. PMID: 28587262; PMCID: PMC5486285.

Abram SGF, Price AJ, Judge A, Beard DJ. Anterior cruciate ligament (ACL) reconstruction and meniscal repair rates have both increased in the past 20 years in England: hospital statistics from 1997 to 2017. Br J Sports Med. 2020 Mar;54(5):286-291. doi: 10.1136/bjsports-2018-100195. Epub 2019 Jan 19. PMID: 30661013.

Webster KE, Feller JA. Exploring the High Reinjury Rate in Younger Patients Undergoing Anterior Cruciate Ligament Reconstruction. Am J Sports Med. 2016 Nov;44(11):2827-2832. doi: 10.1177/0363546516651845. Epub 2016 Jul 7. PMID: 27390346.

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Toole AR, Ithurburn MP, Rauh MJ, Hewett TE, Paterno MV, Schmitt LC. Young Athletes Cleared for Sports Participation After Anterior Cruciate Ligament Reconstruction: How Many Actually Meet Recommended Return-to-Sport Criterion Cutoffs? J Orthop Sports Phys Ther. 2017 Nov;47(11):825-833. doi: 10.2519/jospt.2017.7227. Epub 2017 Oct 7. PMID: 28990491.

Losciale JM, Zdeb RM, Ledbetter L, Reiman MP, Sell TC. The Association Between Passing Return-to-Sport Criteria and Second Anterior Cruciate Ligament Injury Risk: A Systematic Review With Meta-analysis. J Orthop Sports Phys Ther. 2019 Feb;49(2):43-54. doi: 10.2519/jospt.2019.8190. Epub 2018 Nov 30. PMID: 30501385.

Losciale JM, Bullock G, Cromwell C, Ledbetter L, Pietrosimone L, Sell TC. Hop Testing Lacks Strong Association With Key Outcome Variables After Primary Anterior Cruciate Ligament Reconstruction: A Systematic Review. Am J Sports Med. 2020 Feb;48(2):511-522. doi: 10.1177/0363546519838794. Epub 2019 May 7. PMID: 31063403.

Webster KE, Hewett TE. What is the Evidence for and Validity of Return-to-Sport Testing after Anterior Cruciate Ligament Reconstruction Surgery? A Systematic Review and Meta-Analysis. Sports Med. 2019 Jun;49(6):917-929. doi: 10.1007/s40279-019-01093-x. PMID: 30905035.

Grindem H, Snyder-Mackler L, Moksnes H, Engebretsen L, Risberg MA. Simple decision rules can reduce reinjury risk by 84% after ACL reconstruction: the Delaware-Oslo ACL cohort study. Br J Sports Med. 2016 Jul;50(13):804-8. doi: 10.1136/bjsports-2016-096031. Epub 2016 May 9. PMID: 27162233; PMCID: PMC4912389.

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Davies WT, Myer GD, Read PJ. 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 Med. 2020 Mar;50(3):485-495. doi: 10.1007/s40279-019-01221-7. PMID: 31745732; PMCID: PMC7018781.

Herrington L, Ghulam H, Comfort P. Quadriceps Strength and Functional Performance After Anterior Cruciate Ligament Reconstruction in Professional Soccer players at Time of Return to Sport. J Strength Cond Res. 2018 Aug 17. doi: 10.1519/JSC.0000000000002749. Epub ahead of print. PMID: 30124562.

Kotsifaki A, Korakakis V, Whiteley R, Van Rossom S, Jonkers I. Measuring only hop distance during single leg hop testing is insufficient to detect deficits in knee function after ACL reconstruction: a systematic review and meta-analysis. Br J Sports Med. 2020 Feb;54(3):139-153. doi: 10.1136/bjsports-2018-099918. Epub 2019 May 29. PMID: 31142471.

Beischer S, Hamrin Senorski E, Thomeé C, Samuelsson K, Thomeé R. Young athletes return too early to knee-strenuous sport, without acceptable knee function after anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc. 2018 Jul;26(7):1966-1974. doi: 10.1007/s00167-017-4747-8. Epub 2017 Oct 14. Erratum in: Knee Surg Sports Traumatol Arthrosc. 2017 Nov 16;: PMID: 29032484; PMCID: PMC6061439.

Toole AR, Ithurburn MP, Rauh MJ, Hewett TE, Paterno MV, Schmitt LC. Young Athletes Cleared for Sports Participation After Anterior Cruciate Ligament Reconstruction: How Many Actually Meet Recommended Return-to-Sport Criterion Cutoffs? J Orthop Sports Phys Ther. 2017 Nov;47(11):825-833. doi: 10.2519/jospt.2017.7227. Epub 2017 Oct 7. PMID: 28990491.

Lesnak, J., Anderson, D., Farmer, B., Katsavelis, D., & Grindstaff, T. L. (2019). VALIDITY OF HAND-HELD DYNAMOMETRY IN MEASURING QUADRICEPS STRENGTH AND RATE OF TORQUE DEVELOPMENT. International journal of sports physical therapy, 14(2), 180–187.

Greenberg EM, Greenberg ET, Albaugh J, Storey E, Ganley TJ. Anterior Cruciate Ligament Reconstruction Rehabilitation Clinical Practice Patterns: A Survey of the PRiSM Society. Orthop J Sports Med. 2019 Apr 23;7(4):2325967119839041. doi: 10.1177/2325967119839041. PMID: 31041331; PMCID: PMC6481008.

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