Welcome to the 12th edition of the Learn.Physio Research Review!

Welcome to issue 12 of Mick’s Research Reviews!

This week I bring to you a variety of papers looking at hamstring injury and the capacity of the ACL to heal.

The first paper is a beauty that shows how a comprehensive, multi-factorial approach to hamstring injury prevention can significantly reduce the risk of hamstring injuries incidence and recurrences in elite male soccer players.

The second paper builds on what I covered in the last issue on the capacity of the ACL to heal with the identification of 4 different sub-type of ACL rupture, with 2 out of the 4 different types of tears showing good intrinsic ability of the ACL to heal (if given the opportunity to do so!).
Like always, I hope these 2 summaries give you something to think about and help you improve your clinical practice!  


Hamstring injuries are the most common injury in male football players and are associated with a significant time loss and high financial cost to players and clubs. As a result, hamstring injury prevention exercises and programs are of great importance to each athlete and club.

One of the most common and evidence-based hamstring injury prevention strategies is the implementation of the Nordic Hamstring strengthening exercise, with this strategy reducing hamstring injuries by 50-65% (Arnason et al 2008, Petersen et al 2011 and van Dyk et al 2019) and recurrences by up to 85% (Petersen et al 2011).

Despite its reported success in non-professional levels of sports such as soccer, injury rates and re-injury rates have remained consistent in professional European football players.

Given that all injuries, especially hamstring injuries, are complex and multi-factorial, and not one intervention is likely to succeed in preventing hamstring injuries and recurrences in professional sport, the aim of this trial was to investigate the preventative effect of a complex training program based on holistic hamstring health in elite professional soccer players.



Male football teams were recruited for this study from the same club (ACF Fiorentina) who play in the Serie A league in Italy. This study was conducted over 12 seasons, with the last 2 season (2015-16, 2016-17) being the “intervention” period and the first 10 seasons (2005-6 – 2014-15) being the control seasons.

Intervention seasons:
Consisted of a multi-factorial prevention program organised into different interventions with the players through the week. Interventions included:

  • On-field training
  • Strength training in the gym
    • Consisted of both hip dominant and knee dominant strengthening exercises and utilised a mix of free weights, inertial devices, resistance bands etc.
    • Key target of eccentrically strengthening long head of biceps femoris in outer ranges
    • Also correct/minimise potential known risk factors for strains (e. low eccentric strength)
    • Also expose and recreate different injury mechanisms to prepare the athlete for these scenarios
    • Sessions were conducted as a circuit before on-field training
    • Dependent on the number of weekly matches, this circuit was conducted 1-2 sets of 6-10 reps per exercise (10-12 exercises in the circuit completed).
    • An example of how strength training was organised into a weekly training schedule can be found in the table below.
    • Examples of exercise choices can also be found below
  • Individual training to work on the weak points
    • Targeted primarily at players with a history of hamstring strains
    • Carried out in the gym and largely focused on strengthening the posterior chain
  • Physiotherapy treatment
    • Players were reviewed daily by physio and doctor to discuss if, and/or what, manual therapy or physical therapy treatment was required for the athlete.
  • Recovery training sessions
  • Training load management
    • Mix of sessional RPE scores and GPS running data
    • Also, weekly well-being data was collected to assess fatigue, sleep quality and muscle soreness.

NB: All interventions were organised based on good communication between the staff and medical team with the player’s health being the priority of all interventions.

Control seasons:
Players did not perform any structured prevention programs regularly. In 3 of the control seasons, no gym-based strength training was conducted by the players.

Hamstring injury reporting:
Hamstring injuries were included in this analysis if they occurred in training or matches. A hamstring injury was defined as “an injury received during training or competition that caused an absence from future football participation”.
Ultrasound and/or MRI were performed to verify the clinical diagnosis, with all hamstring injuries being registered according to the type of injury.
Injury severity was defined as number of days that elapsed from the date of injury to the day of the players return to full training; and was classified as:
  • Minimal (1-3 days missed)
  • Mild (4-7 days missed)
  • Moderate (8-28 days missed)
  • Severe (>28 days missed).
Recovery was considered when the player was fully recovered to train with the team and available for a match.

During the 9 seasons analysed (3 control seasons were not included as not all information related to training exposure was available), 55 hamstring injuries occurred in 228 players (ave age 27.8yrs of age). 71% of these injuries occurred during match play and the remaining 29% occurring in training.

On average, 30% of the players sustained at least 1 hamstring injury during the control seasons vs 10% in the intervention seasons. In regards to recurrences, there was a 10% recurrence rate in the control seasons, whereas there were no recorded recurrences in the 2 intervention seasons.

Overall, hamstring injury incidence was 3x lower during the 2 intervention seasons than the 7 control seasons; with match injury rate being nearly 3x lower and training injury rates being nearly 4.5x lower in the intervention seasons than control seasons.


The main finding of this study was the incidence of hamstring injuries being approximately 3x lower during the 2 seasons when players were exposed to a regular and consistent multifactorial prevention program that encompassed multiple aspects of the player including regular gym-based strengthening, targeted approaches for high-risk players, load management, monitoring of well-being, regular physiotherapy contact and high levels of open and honest communication.

These findings are very supportive of a multi-factorial approach to reducing the risk of hamstring injuries and recurrences in an elite professional environment, and the medical and high-performance teams of this particular club need to be commended for their completeness and thoroughness in formulating this multi-component plan to reducing hamstring injury burden.

A nice additional finding of this paper was that the authors reported very high compliance and adherence rates to their injury prevention program. Whereas other papers looking at adherences rates, such as Nordics, which has been observed to have very low adherence rates (Bahr et al 2015), the authors believe that their comprehensive, yet flexible and adaptive approach that was ingrained throughout the entire playing group and coaching/support staff lead to the success of their intervention.

For example, never did they use a pre-prescribed set of sets and reps of any given exercise. Each player, each week was taken into account based on a number of different factors before exercise selection was made. This is in contrast to the Nordic Hamstring exercise program which is often carried out in a very prescriptive manner.

There are some key limitations that the authors identified that may make it difficult to generalise this information to other athletes and settings who have high rates of hamstring injuries (eg. AFL).
Firstly, the multiple interventions given throughout the intervention seasons cannot have been standardised. Secondly, it was not a randomised controlled trial (RCT) and the true reason as to why the intervention seasons saw a significantly lower number of hamstring injuries and recurrences cannot be identified.



This present study showed that the incidence of hamstring injuries was reduced 3-fold in an elite, professional soccer environment when a comprehensive injury prevention program was employed across the whole football team for 2 years, as compared to 9 previous seasons when the same measures were not put in place.

The challenge lies for us clinicians working in semi-professional team environments (or professional environments with less open and honest communication between player, coach, medical team and high-performance team) and amateur environments where there will be many contextual factors as to why the athlete and the medical/high performance/coaching staff will not be able to employ these strategies as outlined in this paper.

This is where other papers that have looked at the effectiveness of single interventions such as Nordics and/or High-Speed Running (HSR) have more real-world applicability to the vast majority of us who aren’t working in professional sport.

For example, in semi-professional athletes who also work 20hrs a week (minimum) and don’t have the luxury of regular gym access and GPS systems, as great as it would be to have a multi-factorial approach about their injury prevention, the reality is that they would most likely benefit from focusing on regular implementation of Nordics and regular exposure to HSR as the best way to prevent hamstring strains and recurrences.

Nevertheless, despite some limitations of generalising these findings of an elite environment to less elite environments, we should all be striving to manage our athletes in a similar way with an individualised and targeted treatment strategy based on their needs and with open communication with other key stakeholders (coach, doctor, exercise staff), rather than a “one-size fits all” approach, or a “let’s just treat you with Nordics approach”.
If you're enjoying our research review, check out our newly released Hamstring Masterclass featuring Dr Peter Brukner and Dr Ryan Timmins
Hamstring Masterclass


Historically it has been reported that the ACL has little capacity to heal, with 2 main reasons behind the inability for it to heal:

  1. Inability of blood clots to form during the initial healing that results in a lack of scaffolding between the 2 ruptured end-points
  2. Layer of synovial tissue forming over the ruptured ACL end points impeding tissue healing.

However, these authors report that the healing capacity of the ACL has been largely under-estimated; and the authors have reported previously healing after ACL injury after applying mechanical stress (exercise) to the joint and ligament to promote healing.
The purpose of this study was to assess the relationship between age at the time of injury and assess the progress of morphological recovery on MRI in patients who underwent conservative treatment for acute ACL injury.


Between 2007 and 2014, 105 consecutive ACL injured patients were included in this study. There were 44 males and 58 females (mean age 28 years) ranging from 13-70yrs.
Patients were stratified into 2 groups:

  1. under 20 years of age (n=39)
  2. over 20yrs of age (n=63)
  • Patients excluded if the ACL rupture was older than 20 days, had concomitant PCL injury, or other lower limb injury that made wearing a knee brace impossible. There were no limits on age, and included patients with concomitant MCL injuries.
    3 patients were lost to follow-up during the first 6 months; thus 102 patients were included in this study who were available for follow-up MRI at least 6 months post ACL rupture.
    Initial MRI was performed 6 days after injury, and follow-up MRI was performed after 6 months post-injury.


Patients were encouraged to mobilise FWBAT soon after the injury and were encouraged to commence strengthening exercises early (rehab protocol not stated and not clear of exercises). Patients also encouraged to wear a specially designed knee brace to limit sagittal deviation between the femur and the tibia to encourage the ACL to heal. The brace was worn as soon as ACL injury was confirmed and removed after 3 months. Running was encouraged after 5 months post injury and return to contact sports was allowed after 12 months if they wished.


The MRI scanning protocol included both sagittal and coronal imaging but only sagittal imaging was considered for the purpose of this study. Until May 2011 Images were obtained via a 1.5 T MRI system, thereafter the authors used a more powerful 3.0T MR system.
MRI Images of the acute ACL injury were classified into 4 different types according to the degree of injury:

  • Type I – straight and continuous band
  • Type II – curved and continuous band
  • Type III – displacement
  • Type IV – disrupted or horizontally orientated or unclear
For data analysis:
  • Types I and II were grouped together to form the Type I + II group
  • Types III and IV were grouped together to form the Type III + IV group
Follow-up imaging:

Similarly, images of the treated ACL that were available for MRI follow-up >6 months post injury were classified into 4 grades according to the morphological recovery:
  • Grade I – taut and straight band
  • Grade II – straight band with partial thinning
  • Grade III – thinned
  • Grade IV – unclear
For data analysis:
  • Grades I and II were combined to form Grade I + II
  • Grades III and IV were combined to form Grade III + IV group
Laxity testing:
Ligament testing was assessed via KT-1000 arthrometer at time of injury and at follow-up and pivot shift was assessed at final follow-up (not at initial assessment due to pain).
MRI at 6 months revealed the ligament was retracted and no longer visible in 7 patients and no further MRIs were performed. The 6-month MRI was also considered to be the final MRI in a further 34 patients; (one can only assume that there was no signs of healing ie. Grade III and IV recovery). The 61 remaining patients had their final MRI at approximately 12 months post-injury.
In regards to the initial ACL injury classification on MRI is as follows:
  • 23/102 were Type I
  • 34/102 were Type II
  • 24/102 were Type III
  • 21/102 were Type IV
In regards to classification of ACL morphological recovery on MRI healing at 6 months was:
  • 35/102 were grade I
  • 28/102 were grade II
  • 20/102 were grade III
  • 19/102 were grade IV
At final 12-month post injury MRI assessment, the following classifications were determined:
  • 41/102 were classified as grade I
  • 17/102 were classified as grade II
  • 25/102 were classified as grade III
  • 19/102 were classified as grade IV
In regards to age; younger athletes aged under 20 were significantly less likely to “heal” with only 30% of under 20yrs showing type I+II morphology at follow-up compared to 72% of those aged over 20yrs
Side to side differences measured by arthrometer at the time of injury was 5mm and 1.3mm at final follow-up.
Furthermore, the pivot-shift test was negative in 81% of all cases at final follow-up, and negative in 98% of grade I+II cases at final follow-up.

The authors highlight 3 key points from their present study:
  1. Under 20yr old patients were less likely to heal; however, this age group in the present study also had higher percentage of type III+IV injuries at the time of injury
  2. Patients classified as type III+IV and under 20yrs +type II/III/IV should not be considered for non-operative management
  3. In tears limited to straight and continuous bands at the time of injury (type I+II); grade I+II ACL morphology or “healing” was seen in 84% of cases
The authors believe that this sub-type of ACL injury is an avulsion injury from the femoral attachment where the ACL stays in tact but remains nearby its footprint and/or ACL injuries where the torn ACL fibres form a bundle with minor displacement away from the femoral stump of tissue.
To avoid confusion on whether we’re seeing a partial tear in these types I and II injuries, rather than a full thickness tear, the authors reported that the degree of laxity in type I injuries was similar to types II/III/IV and that type I injuries were in fact complete tears and not partial tears.
There were some limitations to this study that need to be considered:
  • Firstly, this was not a prospective, randomised controlled trial (RCT) – and being a case series paper - the strength of evidence is low on the hierarchy of evidence compared to a RCT.
  • As it was a case series, without a control group who rehabilitated without a brace, we don’t truly know if the brace truly adds anything to the likelihood of the ACL morphologically healing.
  • Given that the coronal sequence of MR images can also detect ACL injury and recovery, it’s a limitation that these sequences were not also viewed and interpreted
  • Arthroscopic exploration of the Grade I and II morphological recovery patients would have added strength to these findings of the true ability for the ACL to heal back to its original state and location

The findings of this study strongly challenge the notion that ACL injuries cannot heal. This study shows that morphology of ACL tissue can certainly show signs of improvements in a sub-type of ACL injured patients in as little as 6 months.

This present study demonstrated that in a sub-type of ACL tears limited to straight and continuous bands (avulsion injuries, proximal tears with little displacement), the ACL has a high intrinsic ability to morphologically and mechanically heal (as assessed via MRI and clinical tests). However, in younger patients with more severe ACL injuries (type III+IV) the morphologically recovery is less likely than older adults aged over 20yrs.

Given that most ACL injuries are treated with ACL reconstruction within the first 6-8 weeks following surgery, it appears that we may be operating too soon and not allowing our patients to see if they can be ones who can heal on their own terms.

What this paper highlights are that not all ACL injuries are the same, and that we as clinicians should be mindful that avulsion tears and proximal tears have a good intrinsic ability to heal and we should be encouraging this sub-group of patients into rehab for at least 3-6 months and seeing how they respond and if they can be one of the lucky ones who heal.

If you’re interested in learning more about this fascinating topic, watch my recent interview with Dr Louise Tulloh and Dr Tim Musgrove
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. 
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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

Suarez-Arrones L, Nakamura FY, Maldonado RA, Torreno N, Di Salvo V, Mendez-Villanueva A. Applying a holistic hamstring injury prevention approach in elite football: 12 seasons, single club study. Scand J Med Sci Sports. 2021 Apr;31(4):861-874. doi: 10.1111/sms.13913. Epub 2021 Jan 22. PMID: 33382128.

Arnason A, Andersen TE, Holme I, Engebretsen L, Bahr R. Prevention of hamstring strains in elite soccer: an intervention study. Scand J Med Sci Sports. 2008;18(1):40-48.

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(21):1362-1370.

Petersen J, Thorborg K, Nielsen MB, Budtz-Jorgensen E, Holmich P. Preventive effect of eccentric training on acute hamstring injuries in men's soccer: a cluster-randomized controlled trial. Am J Sports Med. 2011;39(11):2296-2303.

Bahr R, Thorborg K, Ekstrand J. Evidence-based hamstring injury prevention is not adopted by the majority of Champions League or Norwegian Premier League football teams: the Nordic Hamstring survey. Br J Sports Med. 2015;49(22):1466-1471

Research Review 2

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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