Edition 11 - CAN THE ACL HEAL?

Welcome to the 11th edition of the Learn.Physio Research Review!

This fortnight we have a mixed bag of articles with the first one looking at the combination of both the Nordic Hamstring strengthening and Copenhagen Adductor strengthening exercise and their effect on dynamic balance – a fascinating study and outcome!

The second article is also an eye-opener showing the capacity for the ACL to fully heal after complete ACL rupture. The best thing about this paper was that, for these lucky people, there was no specific bracing protocol or rehab program that conducted to achieve their outcome – definite food for thought in allowing patients to see if they can heal and recover on their own terms, rather than rushing into reconstructive surgery.

I hope you enjoy these 2 article summaries and get something valuable from them!


High quality research shows the Nordic Hamstring exercise (with or without other forms of exercise) can reduce the risk of future hamstring strains by approximately 50% in a variety of athletes (van Dyk et al 2019). Furthermore, as we discussed in
Issue #9, the incidence of adductor-related groin pain can be reduced by at least 40% when Copenhagen Adductor strengthening is added to training programs compared to those programs that do not include them (Haroy et al 2019).

When looking at improving balance to reduce lower limb injury risk, much of the focus tends to look at neuromuscular training or proprioceptive training. However, to the best of these authors knowledge, no study to date has looked specifically at the treatment effects of both the Nordic and Copenhagen strengthening exercises on the improvement of dynamic balance. Therefore, this randomised controlled trial (RCT) looked to evaluate the effect of both of these exercises on improving dynamic balance compared to a no-intervention (control group) amongst young athletes.


This study was an RCT that compared the effectiveness of the Copenhagen alone, the Nordic alone, and combined Copenhagen and Nordic on improving balance to a no-intervention group in a group of 249 young male athletes aged between 18-27 years of age. The study itself went for 6 weeks during a competitive sporting season (variety of sports).

Inclusion criteria:

  • Male athletes practicing at least 3 days er week
Exclusion criteria:
  • History of medical issue or lower limb injury that required medical attention in the last 6 months
  • Systemic disease, cardiovascular disease, neurological disorders, bone fractures or surgery in the previous year

After exclusion criteria were applied, they were randomly allocated to 4 experimental groups with 200 male athletes (mean age 21.9 years) included in this study (50 males in each of the 4 groups – Copenhagen, Nordic, Copenhagen + Nordic, Control group).


Depending on the allocated intervention group, the Copenhagen Adductor and Nordic Hamstring strengthening exercises were performed 3x per week as per the table below, allowing for modified versions if the person couldn’t perform the advanced level:
NB: The Copenhagen + Nordic group did both exercises and the control group did not perform either of these 2 exercises at all
Outcome measure:

The primary outcome of this study was assessing change in the athlete’s balance performance over the 6-week intervention. This was carried out on a specific device called the Biodex Stability System. Prior to the 6-week intervention, each athlete had their limits of stability (LOS) assessed on the Biodex Stability System, and once again at the end of the 6-week intervention.

When testing the LOS on the Biodex Stability System, the testing platform moves relative to the horizontal surface while the athlete’s foot remains fixed; therefore, challenging the athlete’s ability to return the centre of their mass to within their base of support. This test has been previously identified as an indicator of dynamic control. For further information on the test protocol, email
[email protected] and I’ll provide you with the full text article.


A total of 177 athletes had complete data from this 6-week trial (42 Copenhagen group, 44 Nordic group, 47 in the Copenhagen + Nordic group, and 44 in control group), with no differences between groups on basic demographic data of age, weight and height.
For the primary outcome measure being change in LOS over 6 weeks from pre-intervention to post-intervention, the results were as follows:

Prior to the interventions – no significant differences between any of the 4 groups on LOS measures (dynamic balance)

Post interventions – LOS (dynamic balance) improved significantly in all 3 intervention groups compared to the control group. When looking at the 3 intervention groups, the group that did both Copenhagen and Nordic strengthening exercises had significantly higher LOS (dynamic balance) scores at the end of the 6-week intervention than the other 2 groups that did either Copenhagen strengthening alone or Nordic strengthening alone.



The findings of this RCT were fascinating in that high-load eccentric exercises such as the Copenhagen Adductor strengthening exercise and the Nordic strengthening exercise have the capacity to improve dynamic stability (as measured on a Biodex Stability System). Even better, when you perform both of the 2 exercises, the improvements in dynamic balance (in as little as 6 weeks) are superior to when you perform just 1 of the 2 exercises.

Given that these 2 exercises have both been highly effective in reducing injury burden in adductor-related groin pain (Copenhagen strengthening) and hamstring strain injuries (Nordic strengthening) the potential for them to have a positive effect on other types of lower limb injuries such as ankle ligament and knee ligament injuries is an exciting prospect.

The fact that these 2 exercises also require no equipment (other than a partner to hold the legs) and can be performed anywhere, anytime, is a huge plus for community level clubs that may not have the financial resources for fancy equipment or access to gyms for their players.

The carryover in terms of improved balance of the athletes has tremendous implications for time poor athletes as well. If combining these 2 exercises yields significant improvements in dynamic balance, then it’s fair to say that players wouldn’t necessarily need to perform any specific “balance exercises” also in their weekly preparation.

In saying that though, herein lies one of the limitations to this study.
In a perfect world, it would have been really nice to also have a group who did “balance” exercises only and see what their LOS performance changes were over 6 weeks.

Other limitations that the authors pointed out were that the Biodex Balance Stability unit, despite being an excellent testing device, it is expensive and not readily available, and also doesn’t mimic the actual balance required on the field of play.

The athletes included in this study also had no prior history of lower limb injury that may bias them to having already high levels of dynamic stability. It would be interesting to see if these results are still consistent in a group of athletes (both males and females) with a history of a variety of lower limb injuries in the past.

Despite some minor limitations that most studies will have, this well conducted RCT on young male athletes showed that a strengthening program that includes both Copenhagen Adductor and Nordic Hamstring strengthening exercises, that is carried out 3x per week for at least 6 weeks, has a significant effect on dynamic balance performance.

Given that both of these exercises have been shown to be highly effective in reducing lower limb injury incidence and recurrences of adductor-related groin pain and hamstring strains, it should be considered essential that these exercises are included in the weekly preparation of athletes (both male and female) who play sports such as Soccer/Football and AFL.

The potential for these 2 exercises, and their ability to improve dynamic balance, would be a wonderful thing to explore in future work in this space on the risk reduction in knee injuries and ankle injuries in athletes who perform these exercises regularly also.
If you're enjoying our research review, check out our newly released Hamstring Masterclass featuring Dr Peter Brukner and Dr Ryan Timmins
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ACL injury is very common in young athletic individuals and ACL injuries and reconstruction rates are increasing in Australia, New Zealand and the United Kingdom over the last 15-20 years (Zbrojkiewicz et al 2018, Sutherland et al 2019, Abram et al 2020).

It is believed that the ACL has poor potential for healing and recovering without ACL reconstruction, which has led to it being one of the most common orthopaedic procedures performed worldwide. However, there is a growing body of literature that is showing that the ACL can in fact spontaneously recover; albeit this research shows that bracing and a specific rehab plan has an influence on the outcome.

Given that the ACL can heal in some people, the authors of this paper wanted to find out whether a complete ACL rupture can heal without the use of a specific rehab program or bracing protocol; and if these patients can return to their athletic activity after healing.



The authors retrospectively reviewed 14 patients with acute complete ACL rupture that evolved with spontaneously healing.
Complete ACL rupture was established by positive Lachman and Pivot Shift tests and MRI findings. Although the authors did not have data on the exact number of patients who with acute ruptures who underwent non-operative rehab during the 3-year retrospective analysis, they did perform 462 ACL reconstructions.
The average age at the time of injury was 31 years of age (12 males and 2 females). All patients had at least 1 follow up evaluation at 12months post injury and the average time from injury to last follow-up was 30months post injury with all retrospective data obtained from medical records and MRIs.
In regard to tear location of the ACL, 8 patients had tears in the proximal 1/3 of the ligament and the remaining 6 had mid-portion tears.
The authors reported that all of these 14 patients required ACLR at the time of their assessment but surgery was postponed for a variety of reasons including; labor problems, need to travel, illnesses, planned holidays and patient’s personal decision not to have surgery. All the patients decided to manage their injury without bracing or a specific rehab plan.
Outcome measures included:

  • International Knee Documentation Committee (IKDC) subjective knee evaluation form
    • This documentation considers subjective evaluation, presence of symptoms, passive ROM deficit, ligament evaluation, compartment findings, radiographic findings and functional test
    • The final result is normal, near normal, abnormal or severely abnormal.
  • The Lysholm knee scale was also used that takes into account pain, subjective instability, use of a walking device, claudication when walking, articular blocking, effusion, possibility of climbing stairs and difficulty in kneeling.
  • Tegner activity scale.
  • KT 1000 arthrometer with side-to-side difference being measured in mm with 3+mm being indicative of a complete ACL rupture.


At last follow-up, all patients had a stable knee with end point on Lachman test and negative pivot-shift.
The average KT-1000 test was 1.9mm.

In all patients, MRI at the time of follow-up showed an end-to-end continuous ACL with homogenous signal and disappearance of secondary signs. Only 1 person was available to have this arthroscopically confirmed due to a lateral meniscus tear that was treated with partial menisectomy.
All patients returned to the same or almost same physical activity before the injury. The average Lysholm score was 97 and IKDC evaluation was “normal” in 10 patients and “nearly normal” in 4 patients.


The ability of the ACL to heal is controversial, and historically it has been reported that the ACL does not heal (Hefti et al 1991, Malanga et al 2001 and Woo et al 2002). One biological explanation for this is presented by Yoshida and Fujii (1999) who reported that ACL cells have lower responses to growth factors than human MCL cells, suggesting a decreased reparative capacity of ACL tissue. Tang et al (2009) concluded, although the regular wound-healing mechanism also occurs after ACL injury, accumulation of matrix metalloproteinase activity in the synovial fluids due to all the intra-articular tissues may be at least one of the important reasons why an injured ACL cannot be repaired.
However, this retrospective case series shows that the ACL can in fact heal in some people; but because the authors were not able to gather data on how many people who went down a non-operative treatment path and did not heal compared to those who did heal, we just don’t know what percentage of people do heal, and we don’t confidently know what types of ACL injury lend themselves to more likely heal.
The big positive to come out of this retrospective analysis however was that the ACL has the potential to heal without the need for bracing or specific rehab protocol. Once again, without a prospective randomised control trial, we may not truly know whether the addition of a specific type of brace and/or rehab protocol may yield more spontaneous ACL healing.


Although this retrospective analysis showed the capacity for the ACL to heal, and the majority of patients do resume previous levels of activity and have normal to near normal knee function, what we don’t know just yet is what is the chances of the ACL healing and/or what is the percentage of patients who do spontaneously heal following ACL injury, or is the capacity to heal enhanced with bracing or rehab? More work is required in this space with stronger prospective, randomised control trials.

My two cents worth? If there is going to be any capacity of the ACL to heal on its own terms, biological healing times would suggest that we’d start to see some signs of recovery on MRI around the 3-month mark and naturally see progressive change over the next 9-12 months. Ihara et al (1996) showed that in their MRI case series study of non-op treated patients managed in a brace, that 37 out of 50 patients (74%) demonstrated a well-defined normal-sized straight ACL band at 3 months; and the 3-month mark showing early recovery appears to be a consistent trend in their other studies in this space.

Given that it is going to take at least 3 months to see some MRI and clinical change, maybe we shouldn’t be rushing our patients into surgery prior to 3 months, and instead re-evaluated after 3 months to see if they are one of the “lucky ones”?

Of course, there will be some patients who rehabbing for 3 months would be unacceptable for them (as they will feel that its 3 months “wasted” or their professional sport doesn’t lend itself to waiting 3 months to “wait and see”), and there will be some patients who have concomitant injuries that need early surgical intervention (eg. Fracture or repairable meniscus injury); but most patients we see won’t have those tight time demands or concomitant injuries, so why not give them a fighting chance to see if they can be one of the “lucky ones”?

In the end if they are not one of the lucky ones and they want to proceed to surgery, then the 3 months of rehabbing is essential “pre-op rehab” which will only work in their favour to produce a superior outcome post-op.

And to me, that is a win/win situation to be in.
If you want to learn more about ACL healing
Renowned sports physician Dr Louise Tulloh and experienced orthopaedic surgeon Dr Tim Musgrove weigh in on this controversy. Does it matter? Does this change our management?
Click Here to Watch
Take your learning to the next level with our Non-Operative ACL Masterclass. Learn a balanced approach on non-operative rehabilitation and whether your client qualifies to undergo this treatment approach. 
Non-Operative Masterclass

The below paper is full text reference for your own reading. References cited throughout this article can be found in the reference section of this paper

Research Review 1

Saleh A Al Attar W, Faude O, Husain MA, Soomro N, Sanders RH. Combining the Copenhagen Adduction Exercise and Nordic Hamstring Exercise Improves Dynamic Balance Among Male Athletes: A Randomized Controlled Trial. Sports Health. 2021 Feb 15:1941738121993479. doi: 10.1177/1941738121993479. Epub ahead of print. PMID: 33588644.
van Dyk N, Behan FP, Whiteley R. Including the Nordic hamstring exercise in injury prevention programmes halves the rate of hamstring injuries: a systematic review and meta-analysis of 8459 athletes. Br J Sports Med. 2019;53:1362-1370.
Harøy J, Clarsen B, Wiger EG, et al. The Adductor Strengthening Programme prevents groin problems among male football players: a cluster-randomised controlled trial. Br J Sports Med. 2019;53:150-157
Research Review 2

Costa-Paz M, Ayerza MA, Tanoira I, Astoul J, Muscolo DL. Spontaneous healing in complete ACL ruptures: a clinical and MRI study. Clin Orthop Relat Res. 2012 Apr;470(4):979-85. Doi: 10.1007/s11999-011-1933-8. PMID: 21643922; PMCID: PMC3293953.
Zbrojkiewicz D, Vertullo C, Grayson JE. Increasing rates of anterior cruciate ligament reconstruction in young Australians, 2000-2015. Med J Aust. 2018 May 7;208(8):354-358. Doi: 10.5694/mja17.00974. Epub 2018 Apr 23. PMID: 29669497.
Sutherland K, Clatworthy M, Fulcher M, Chang K, Young SW. Marked increase in the incidence of anterior cruciate ligament reconstructions in young females in New Zealand. ANZ J Surg. 2019 Sep;89(9):1151-1155. Doi: 10.1111/ans.15404. Epub 2019 Aug 26. PMID: 31452329.
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.
For further reading, here are other full text papers on this topic that may be of interest to you:
Ihara H, Kawano T. Influence of Age on Healing Capacity of Acute Tears of the Anterior Cruciate Ligament Based on Magnetic Resonance Imaging Assessment. J Comput Assist Tomogr. 2017 Mar/Apr;41(2):206-211. Doi: 10.1097/RCT.0000000000000515. PMID: 28045756; PMCID: PMC5359784.

Thanks for reading and staying up to date. I look forward to sharing more in the future.

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