Categories
Injuries

Hamstring Strain Injuries: An Overview (Part One)

Written by: Stephanie Lazarczuk MSc BSc (Hons) GSR/BASRaT Reg FHEA

Introduction

Hamstring strain injuries (HSI) are one of the most frequently occurring injuries in sports involving high-speed running. This blog post is going to cover the key themes and topics related to the current literature base. However, this is not designed to be an extensive source (although it is relatively lengthy) of every piece of evidence that has ever been published, but will try to give you a relatively thorough breakdown with links to other resources which can be explored at your leisure. Nor is this designed to be a purely academic piece so I will be pointing you towards other sources. In this first part of the blog we are going to focus on the aetiology and risk factors associated with HSI in preparation for Part Two which will focus predominantly on exercise selection and programming for healthy and injured populations.

I generally think my views on HSI are quite moderate, and hopefully you will find this piece to be fairly balanced but I’m not shy of an opinion. Of course, you are free to disagree with me and open (sensible) debate is encouraged. For some context, I am a Sport Rehabilitator by trade with a MSc in Strength and Conditioning and a MSc by Research investigating kicking-related injuries in Rugby Union. I have a decade of clinical experience, predominantly working with populations who are prone to HSI. If you would like to hear more, feel free to check out Episode One of the Thrive PES Podcast [SL3] series where I talk to Liam and Matt.

I’m also going to preface this post with a reminder about the fundamentals for recovery and performance, whether in relation to injuries or bouts of exercise: nutrition, sleep and hydration. If any one of these areas are sub-optimal, you are unlikely to be maximising your athlete’s (or your own) potential. It’s beyond the scope of this blog to go into great detail on these topics, and there are people far more learned than I to instruct you on them. However, I strongly recommend that you take some time to engage with the available literature to get a rounded view.

Hamstring anatomy and function

This section just serves as a relatively brief reminder of the anatomy to consider. The hamstrings are located in the posterior thigh and are categorised as being biarticular (biceps femoris long head, semimembranosus and semitendinosus) or monoarticular (biceps femoris short head). The biarticular hamstrings attach proximally to the ischial tuberosity and travel downwards to the pes anserine insertion (semitendinosus), the posterior medial tibial condyle (semimembranosus) and the fibula head with fascial slips to the lateral tibial condyle (biceps femoris long head). The monoarticular biceps femoris short head originates from the lower half of the linea aspera on the posterior femur and blends with the long head as it nears insertion.

The primary function of all hamstrings is to flex the knee and extend the hip in the sagittal plane. However, it is worth noting that the functions of the hamstrings are wider than this. Due to the tibial attachments, it is generally thought that the hamstrings can contribute to control of accessory knee movements by limiting anterior shear of the tibia on the femur. Combined with a role in controlling tibial rotation at the tibiofemoral joint, the hamstrings play a role in preventing injury during single limb stance (e.g. ACL injury). By comparison, during swing phases, it is suggested that the hamstrings act to decelerate the shank (lower leg).

In addition, although generally listed as an adductor, adductor magnus shares an attachment to the ischium and contributes to hip extension alongside the hamstrings. As it sits so closely to the proximal hamstrings, it is important to differentiate the adductor magnus if completing palpation during assessment of athletes/patients.

The tendinous tissue in the hamstrings is expansive and covers approximately two-thirds of the biceps femoris. Both proximally and distally, tendon extends towards the muscle fibres, however due to their lack of direct muscle attachment, these are often termed as ‘free tendon’. In pennate muscle, the aponeurosis (also termed ‘intramuscular tendon’ and used interchangeably) spans the length of the muscle fascicles to provide tendinous attachment and the point where these tissues merge is the musculotendinous junction.

I deliberately haven’t directly referenced each part of this section because the information you find will generally be the same across sources. Anatomy is anatomy and doesn’t tend to change much. However, if you are interested in delving deeper, work by van der Made et al. (see reference) is a good place to start.(1)

Epidemiology and aetiology

HSI consistently remains one of the most frequently occurring injuries in English Rugby Union, football (soccer), Aussie Rules football Men and Women, baseball(2,3) and cricket to name but a few. Despite some sports demonstrating fluctuations in seasonal data, the problem remains. The frequency of injury is somewhat variable so rather than spend time here describing each sport in detail, I’d suggest you pick a couple of sports you’re interested in and see how they compare. Aside from published articles, many sports release seasonal reports on their epidemiology data, so looking for these from governing bodies is a good start (some have been linked in above).

Location, mechanism and severity

Biceps femoris is the most frequently injured of the hamstrings across the literature, but again I would urge you to check the rates in your sports of choice. Biceps femoris injuries are most frequently the result of high-speed running mechanisms, whereas stretch-type mechanisms as seen during dance performance, gymnastics, and kicking, are more likely to affect the medial hamstrings, particularly semimembranosus.(4,5)

It’s worth noting though that the assumption is often that running based injuries occur during linear sprints. Sometimes that is indeed the case (see Usain Bolt during 100m relay or Masahiro Tanaka of NY Yankees sprinting between bases), but you will also see people injured after or during a change of direction (see Thierry Henry looking for the ball and cutting in off his line) and this is important from both the point of view of exposure to such stimuli in risk reduction work and in rehab (more on this in Part Two). HSI don’t always have the ‘sniper effect’ where athletes just drop, sometimes they can look fairly innocuous but see players out of action for several weeks (see GWS Giants’ Toby Greene injured and still run off the pitch at 1:10) and is something to consider when assessing.

The reported time lost from professional team sports ranges from a matter of days to several months (aside: if you can return after a day or two, consider whether it’s a strain or there are other factors at play), and likely differs by mechanism of injury, location and severity of injury. For example, Askling et al. demonstrated that the return to play (RTP) time for stretch-type mechanisms was typically longer than that of sprint-related injuries.(4,5) Consequently, having an understanding of the associated mechanism may offer prognostic information (which your athlete/patient will undoubtedly ask you about).

The classification of muscle injuries has continued to evolve during my time in clinical practice. The Munich Consensus provides a thorough breakdown of the types of injury,(6) while the British Athletics Muscle Injury Classification (known as the BAMIC) is predominantly geared towards describing HSI (although the authors suggest it can be extrapolated to other muscles).(7) The BAMIC classifies injuries based on location (myofascial, myotendinous or intratendinous) and severity (0-4: nothing on MRI through to full thickness tear/rupture). Data collected by British Athletics suggests that intratendinous injuries (3c) tend to have far longer return to full activity times,(8) which is also consistent with data in a study on soccer athletes.(5) However, in the athletics data there was only one injury which was as severe but in a slightly different location (3b) so it is difficult to determine that a 3b and 3c classification have significantly different RTP times – more data required. This is further muddied by work from van der Made et al. which suggests only those athletes with full thickness tears of the intramuscular tendon have significantly longer recovery times,(9) with no clear differentiation of RTP across grades of partial thickness tears which were, presumably, also included in the BA study. All considered, this information may indicate that it is the severity of injury which is the main driver of time lost from activity, and the role of location within the musculotendinous unit may be secondary to this.

Further, a recently published study has identified that complete resolution of intramuscular tears may not be necessary prior to RTP. In the paper, Vermuelen et al. demonstrated that approximately half of their participants still had a partial thickness tear in the intramuscular tendon at the point of RTP, but there were no significant differences in reinjury rates between these participants and those who had recovered full tendon continuity.(10) This suggests that other factors may be drivers of high reinjury rates (in this study: 20% within the first year after RTP, with the majority occurring in the first two months). This is a very current and contentious debate at the moment so keep your eyes peeled for more information on this.

Risk Factors

Risk factors for HSI are typically split into two categories: non-modifiable and modifiable. The non-modifiable risk factors of age and past medical history are inherently problematic due to the very nature of not being able to do a thing about them. However, as will be highlighted in the subsequent breakdown, by altering the modifiable risk factors, it may be possible to mitigate some of the risk associated with age and past medical history.

Non-modifiable

Age

Age is consistently flagged as a risk factor for injury with older athletes experiencing HSI more frequently.(11,12) The precise hazard/odds ratios vary, however the increased risk is apparent. This suggests the need to consider how you choose to program for these athletes.

Past medical history

Past medical history becomes slightly more complicated. It’s clear that having sustained a prior HSI leads to an increased risk of sustaining a subsequent one, particularly if the strain was recent.(11,12) The reasons for this are complex but see below in ‘Prolonged neurological deficits’ for one potential reason. Orchard et al. demonstrated that in over two-decades worth of AFL data, the greatest risk of reinjury is within the first month of RTP but that while diminished beyond this time, the risk is still elevated above those without injury history for almost another three months.(13) Given this increased risk, it’s clear that, again, there needs to be consideration of how these athletes are prepared for their sport as well as what happens to them after they’ve returned to the field.

Further to this, it’s necessary to consider an athlete’s wider medical history. A history of ACL injury has also been demonstrated to increase risk of subsequent HSI, while a combined history of ACL and HSI further increased likelihood.(12,14) The links between the hamstrings and ACL injury have been very briefly described in the anatomy section of this blog and it should be fairly intuitive that the loss of the passive knee restraint has an impact on the active restraints of the knee. Further to this, graft choice for ACL reconstruction also appears worth of consideration. Muscle activation patterns are decreased in semitendinosus after use of its free tendon for graft donation.(15) Semitendinosus has been a common choice for grafts during my clinical practice, and despite some shift back to patellar or quadriceps tendon options, in some countries the semitendinosus graft is still widespread for ACL or ‘Tommy John’ repairs.

It is also likely that a history of gastrocnemius injury leads to increased risk for the hamstrings.(12) Currently, the precise mechanism for this is unclear.

If you have access to your athlete’s full medical record and this is comprehensive, it should be possible for you to highlight these risk factors quickly. Otherwise, it will be down to you as the practitioner to use your interviewing skills to elicit such information in the subjective assessment.

Modifiable

Flexibility, range of motion, muscle length

Flexibility is often the first port of call for those wishing to make inroads on soft tissue injuries. However, there is actually scant evidence for this being a major issue for HSI. Van Dyk et al. demonstrated that knee extension range deficits may be linked with hamstring injury.(11) Weight-bearing dorsiflexion was also marginally associated with an increased risk of injury.(11) If there is to be any focus on altering range of motion, it is likely that these areas may be more beneficial than purely chasing straight leg raise improvements (although if this is particularly poor, this might be worth targeting too based on nothing but logic), but it is worth noting that potential gains are likely very small.

Muscle architecture

The now infamous ‘Quadrant of Doom’ was proposed as a method of visualising data from Timmins et al.,(16) plotting eccentric strength against 2D ultrasound-derived values of fascicle length. The principle demonstrated was that if you were both weak and had shortened fascicles, prospectively there was an increased risk of injury. Conversely, being stronger with longer fascicles offered a greater protective effect, even in older athletes and those with a past history of HSI. If you look at the ‘Quadrant’, you’ll note that fascicle length alone does not appear to be effective at reducing injury and requires the interplay with other factors, in this case strength. This is probably a sound principle to bear in mind throughout – i.e. you need to target multiple factors to put your athlete in the best place.

A common critique of this paper is the use of 2D ultrasound as a method for measuring fascicle length as there is a need for estimation from a trigonometry-based equation.(17) In order to reduce estimation of fascicle lengths in lower limb muscles, work has been completed on extended field of view ultrasound, 3D ultrasound (albeit in other regions) and tensor diffusion imaging from MRI.(17–19) It is worth considering the end user, however. As it is most likely that clinics and teams will have access to 2D ultrasound over other pricier and/or labour-intensive methods (note: this is location dependent, high performance environments are very different to lower playing levels and community clinics), this may offer an appropriate and accessible option with which to provide estimates.

Strength

I’m actually going to start this section with the summary point rather than the explanation: strength alone is a poor predictor of HSI. Attempts have been made to model injury risk, either prospectively or retrospectively, and struggled to identify the players who went on to sustain injuries.(20,21) Again, my earlier point of recognising the interplay between risk factors is probably the key. Only one study has identified any cut off points for injury risk, but this is currently standalone and the authors still recognised the weak associations.(22) Injury prediction as a concept is very much a dark and poorly understood art and is the subject of great debate (see unrolled Twitter thread here and blog here).

Now, this doesn’t mean that we should stop taking strength measures (or any other variables for that matter) because having this data still plays its part. It might not convincingly allow you to determine who is going to get injured, but will provide you with baselines for comparison either during maintenance of physical properties across a season, or during rehabilitation after injury. You will need data at some point, ‘going in blind’ is not particularly an option if you can help it. A lack of data, however, does not mean you can’t implement proven positive strategies.

While asymmetry above 15% in eccentric strength has been shown to be a factor in risk of injury in rugby union players,(23) asymmetry may be a natural development in some sports. For example, in British track and field athletes, 400m runners’ right leg was typically stronger(24) which is in line with recognition that the outside limb (right leg) typically produces greater force during bend running and the direction of running is consistent in this event.(25) In team sports where tasks are always completed in the same direction, e.g. pitching or batting in baseball, chasing symmetry may also not be warranted therefore careful consideration of the sporting demands is needed. By comparison, sports such as rugby generally require athletes to use their left and right limbs more symmetrically, which may explain the potential need for symmetry. Knowing and understanding your sport is vital.

Prolonged neurological deficits

The idea of neurological deficits being a factor in increased risk of hamstring injury has been around for several years,(26) with evidence suggesting decreases in strength and EMG excitation persist even after RTP.(27) More recent work by Buhmann et al. produced similar findings, again in those who had returned to sport, with a reduction in several variables including stretch and tendon reflexes.(28) Given the nature of reflex actions being driven from a spinal cord level, this new finding signposts us to consider the role of the central nervous system in peripheral muscular injuries. How we go about addressing this issue, well…TBC.

Exposure to high-speed running

Do you remember the saying: fail to prepare, prepare to fail? Seemingly, this also applies to the hamstrings. Repeatedly data are showing that an athlete’s exposure to high-speed running can alter their potential risk of injury, particularly when there has been limited exposure to high-speed efforts.(29,30) Effectively, the lack of exposure means the athlete is under-prepared for the task at hand, and it is possible that there has not been sufficient adaptation to this stimulus.

Although (potentially) marginally less problematic than under-preparation, over-exposure to high-speed running likely also increases risk of injury, either through a high density of sessions containing high-speed running or a raised proportion of high-speed metres versus sub-maximal efforts.(29,30) You will also hear people talk about how the journey is as important as the destination, and in relation to high-speed running this generally describes how increases in metres needs to be progressive. Large jumps in distance covered over a short period of time can also increase the risk of injury,(31) but once athletes are accustomed to running distances they are then better able to tolerate additional metres.(32) Consider the programming of high-speed exposure and how you might achieve this in your sport.

Overall, the papers in this area suggest there is a ‘Goldilocks’ principle to high-speed running exposure – not too much, not too little, it needs to be juuust right. Although not running related, the same principle may apply to other mechanisms of injury such as rugby union kicking, where HSI ranked second behind the quadriceps/rectus femoris strains for thigh-based injuries. Current data seems to suggest that both lower and higher levels of kicking exposure may be related to greater propensities for injury.(33) Preparation for the tasks being executed is necessary!

Running biomechanics

The hamstrings are important in high-speed running for a number of reasons from force production, the aforementioned control assistance of the knee in stance, and control of the shank in swing phase. The debate over whether the role of the hamstrings is truly eccentric(34,35) or isometric(36) during swing continues to rage and arguments for both sides have been laid out elsewhere. It is worth spending a bit of time with this debate to understand both sides of the argument, but we await confirmation one way or another.

Commonly cited to support the notion that running technique increases risk of injury is a 2017 paper by Schuermans et al. which assessed biomechanics in soccer athletes. During analysis, it was shown that those athletes who exhibited a greater lateral trunk sway towards the stance limb and a more pronounced anterior pelvic tilt had an increased risk of HSI.(37) However, a critical point is worthy here, as this increase in risk was based on only four index injuries. With such a small number of injuries occurring, it may be premature to categorically state a link between mechanics and injury risk. However, groups are working on this principle (see Johan Lahti’s webinar on his PhD work available here) and it will be interesting to see results as they appear. As a further point, it has been suggested that the sprint mechanics of track and field athletes differ in comparison to team athletes (the cited paper specifically examined rugby union players)(38) and you have to consider the context. While I don’t doubt there are some principles of sprinting that are transferable and might make a team athlete more efficient at high-speed running, the contextual factors (i.e. environment, demands) between the sports are very different. Do we want team athletes to look like sprinters? Personally, I haven’t been convinced yet, but never say never.

Fixture congestion

The number of matches scheduled across any given week is varied by sport. In a normal season, English professional rugby union and AFL typically sees one match per first team each week, while football/soccer may play twice per week or have heavy periods if playing across multiple competitions. By comparison, professional baseball in North America have the most insane schedule I’ve ever seen with potential for 10-14 days or more on the trot (travel included) before a single day break and repeat – for months. If you’re not familiar, I strongly suggest you pull up the schedule for last season, it really is something to behold. The reason I bring this up is because the ability to plan recovery and training becomes more of a challenge the less time you have available. Shorter turn-around times may be associated with a greater frequency of injuries,(39) whether this is due to inadequate recovery from the last match’s efforts or from inadequate preparation/maintenance of physiological adaptations. Further, it is worth noting that there is some conflicting evidence for this theory, particularly in relation to HSI,(12) so this idea should be cautiously applied.

That being said, it will be really interesting to see how the post-COVID-19 lockdown seasons pan out in terms of injury statistics, particularly with increased matches in a smaller window and limited preparation time. For example, as part of the AFL COVID-shortened season, matches were being played every day between July 29th and August 17th as opposed to late in the week/over the weekend, with much reduced turnaround times compared to a regular season – some as short as three days. As an example, I pulled up the Brisbane Lions fixtures (for no reason other than I’d watched them play twice in the week of writing this) and they played four games in 14 days – generally they would only have played two in the same timeframe in a regular season. The injury lists on the AFL website seemed to grow and grow and the final epidemiology report for 2020 will be an interesting read. Dr Joel Mason has regularly been blogging on the injury rates and other statistics following lockdown across the Bundesliga and AFL and are a recommended read for easily digestible information (hamstring specific blog here).

Summary

While it is not currently possible to predict who will sustain a HSI, there are a number of well-recognised risk factors. There is still significant debate around these and whether they are true factors and, arguably more importantly, how they mix together. In order to reduce the risk of HSI occurring, a structured approach is likely required which targets as many of these risk factors as possible, in as efficient manner as possible. This should include both off-field conditioning and exposure to the mechanisms of injury, including high-speed running. In Part Two we will focus on exercise choices and the rationale for inclusion in programmes which can be employed in those with and without HSI.

Steph can be found on Twitter or LinkedIn.



References

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2.              Ahmad CS, Dick RW, Snell E, Kenney ND, Curriero FC, Pollack K, et al. Major and minor league baseball hamstring injuries: Epidemiologic findings from the major league baseball injury surveillance system. Am J Sports Med. 2014;42(6):1464–70.

3.              Okoroha KR, Conte S, Makhni EC, Lizzio VA, Camp CL, Li B, et al. Hamstring Injury Trends in Major and Minor League Baseball: Epidemiological Findings From the Major League Baseball Health and Injury Tracking System. Orthop J Sport Med. 2019;7(7):1–7.

4.              Askling C, Tengvar M, Saartok T, Thorstensson A. Proximal hamstring strains of stretching type in different sports: Injury situations, clinical and magnetic resonance imaging characteristics, and return to sport. Am J Sports Med. 2008;36(9):1799–804.

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10.           Vermeulen R, Almusa E, Buckens S, Six W, Whiteley R, Reurink G, et al. Complete resolution of a hamstring intramuscular tendon injury on MRI is not necessary for a clinically successful return to play. Br J Sports Med. 2020;1–6.

11.           van Dyk N, Farooq A, Bahr R, Witvrouw E. Hamstring and Ankle Flexibility Deficits Are Weak Risk Factors for Hamstring Injury in Professional Soccer Players: A Prospective Cohort Study of 438 Players Including 78 Injuries. Am J Sports Med. 2018;46(9):2203–10.

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13.           Orchard JW, Chaker Jomaa M, Orchard JJ, Rae K, Hoffman DT, Reddin T, et al. Fifteen-week window for recurrent muscle strains in football: A prospective cohort of 3600 muscle strains over 23 years in professional Australian rules football. Br J Sports Med. 2020;1–6.

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15.           Messer DJ, Shield AJ, Williams MD, Timmins RG, Bourne MN. Hamstring muscle activation and morphology are significantly altered 1–6 years after anterior cruciate ligament reconstruction with semitendinosus graft. Knee Surgery, Sport Traumatol Arthrosc [Internet]. 2020;28(3):733–41. Available from: http://dx.doi.org/10.1007/s00167-019-05374-w

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17.           Franchi M V., Fitze DP, Raiteri BJ, Hahn D, Spörri J. Ultrasound-derived Biceps Femoris Long Head Fascicle Length: Extrapolation Pitfalls. Med Sci Sports Exerc. 2020;52(1):233–43.

18.           Behan F, Vermeulen R, Smith T, Arnaiz J, Timmins RG, Opar DA, et al. Poor agreement between ultrasound and novel MRI measures of biceps femoris long head fascicle length. Transl Sport Med [Internet]. 2018;21(00):1–6. Available from: https://linkinghub.elsevier.com/retrieve/pii/S1440244018307072

19.           Barber L, Barrett R, Lichtwark G. Validation of a freehand 3D ultrasound system for morphological measures of the medial gastrocnemius muscle. J Biomech. 2009;42(9):1313–9.

20.           Ruddy JD, Shield AJ, Maniar N, Williams MD, Duhig S, Timmins RG, et al. Predictive Modeling of Hamstring Strain Injuries in Elite Australian Footballers. Med Sci Sports Exerc. 2018;50(5):906–14.

21.           Dauty M, Potiron-Josse M, Rochcongar P. Identification of previous hamstring muscle injury by isokinetic concentric and eccentric torque measurement in elite soccer player. Isokinet Exerc Sci [Internet]. 2003 Sep;11(3):139–44. Available from: http://search.ebscohost.com/login.aspx?direct=true&db=s3h&AN=10717086&site=ehost-live

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23.           Bourne MN, Opar DA, Williams MD, Shield AJ. Eccentric Knee Flexor Strength and Risk of Hamstring Injuries in Rugby Union: A Prospective Study. Am J Sports Med. 2015;43(11):2663–70.

24.           Giakoumis M, Pollock N, Mias E, McAleer S, Kelly S, Brown F, et al. Eccentric hamstring strength in elite track and field athletes on the British Athletics world class performance program. Phys Ther Sport [Internet]. 2020;43:217–23. Available from: https://doi.org/10.1016/j.ptsp.2020.03.008

25.           Churchill SM, Trewartha G, Bezodis IN, Salo AIT. Force production during maximal effort bend sprinting: Theory vs reality. Scand J Med Sci Sport. 2016;26(10):1171–9.

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28.           Buhmann R, Trajano G, Kerr G, Shield A. Voluntary Activation and Reflex Responses following Hamstring Strain Injury. Med Sci Sport Exerc. 2020;(February).

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30.           Ruddy JD, Pollard CW, Timmins RG, Williams MD, Shield AJ, Opar DA. Running exposure is associated with the risk of hamstring strain injury in elite Australian footballers. Br J Sports Med. 2018;52(14):919–28.

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33.           Lazarczuk SL, Love T, Cross MJ, Stokes KA, Williams S, Taylor AE, et al. The epidemiology of kicking injuries in professional Rugby Union: A 15‐season prospective study. Scand J Med Sci Sports. 2020;00:1–9.

34.           Chumanov ES, Heiderscheit BC, Thelen DG. The effect of speed and influence of individual muscles on hamstring mechanics during the swing phase of sprinting. J Biomech. 2007;40(16):3555–62.

35.           Shield A, Murphy S. Preventing hamstring injuries – Part 1: Is there really an eccentric action of the hamstrings in high speed running and does it matter? Sport Perform Sci Reports [Internet]. 2018;1:1–5. Available from: https://pdfs.semanticscholar.org/d26b/dbc40dbbc2d9096dd562e901dc92b16b4171.pdf

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