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In current clinical practice, there is a disparity between the rate of ACL ipsilateral re-injury in elite sport compared to sub-elite sport. Depending on which study you read the rate of re-injury in the elite sportsman currently sits ~5% (King et al 2020), in stark contrast this sits closer to ~25-30% in the sub-elite athlete (Webster & Feller 2021). Despite best effort of clinicians the incidence of primary and secondary ACL injury continues to rise (Schilaty et al 2017) and this is largely due to the multi-factorial nature of injury.
The harder the ability to create a vacuum environment, the more difficult it becomes to reduce the incidence of injury. As a result of this, in my opinion as rehab professionals we must utilise a 'deficits first' approach to rehabilitation with the aim of restoring physiological and biomechanical equilibrium prior to contemplating broader factors associated with injury.
One of the biggest discussions in this space surrounding the medical community is the selection of graft tissue and the impact that the tensile strength of the tissue has in the reduction of injury rates. Depending on which corner of the globe you are situated in, the dominant graft of choice may vary however there appears to be a trend towards the use of autografts compared to allografts in the young athlete and that hamstring autograft techniques appear the dominant graft of choice on a global scale (Sherman et al 2021).
One of the challenges associated with hamstring autografts, particularly in the young athlete compared to other autograft selections is that there is growing evidence of an increased failure rate in comparison to BPTB & Quad tendon grafts. (Schuette et al 2017) (Hurley et al 2021) This has led the surgical community to promote the use of lateral extra-articular tenodesis (LEAT) in high risk patients when electing to utilise the hamstring autograft (Getgood et al 2020).
Now my job and role as a rehabilitation professional is not to attempt to move the needle in regards to graft selection choice by the orthopaedic surgeon. On a case by case basis there are many factors that will be considered by an orthopod that range from tissue integrity of the proposed donor site, lifestyle factors and positional specific demands of an athletes sport (amongst many others). My job as the rehabilitation professional is to identify and eliminate what the possible deficits are that occur as a result of the graft that is selected. In the case of hamstring grafts, there is acceptance that athletes will suffer from persistent deficits in inner range knee flexion strength, rate of force development from the hamstrings and eccentric hamstring strength (Buckthorpe et al 2020).
Traditional methods for assessing hamstring strength in clinical practice include a combination of bodyweight capacity testing, hand held dynamometry & isokinetic dynamometry. Whilst these three methods are effective at quantifying functional, isometric and isotonic strength (with varying degrees of reliability & validity) there are less ways to accurately assess an athletes eccentric hamstring strength, particularly for those in the sub-elite sector where re-injury rates are highest. The purpose for the rest of this blog is to highlight the role of eccentric hamstring strength in the reduction of ACL injury risk, methods that this can be assessed, how the treating clinician can intervene with the athlete and to share my experiences from the field.
HAMSTRING ANATOMY
For the purpose of this article, my reference of hamstrings will be in relation to the function of the Biceps Femoris (BF), Semitendinosus (ST) & Semimembranosus (SM). All three (with the exception of the short head of the Biceps Femoris) originate at the Ischial Tuberosity with the BF inserting onto the head of the Fibula whilst the SM & ST insert into the medial condyle of the Tibia at the Pes Anserinus junction. In isolation, the hamstrings play a prominent role in producing hip extension & knee flexion. When considering the gait cycle the hamstrings activate during the final 25% of swing phase to generate an extension force at the hip and to resist knee extension, whilst producing a posterior translation force on the Tibiofemoral joint during the stance phase (Rodgers 2020).
MECHANISM OF ACL INJURY
Biomechanical risk factors associated with increased risk of ACL injury are fundamentally linked to those that place increased strain on the ACL tissue. These can be categorised into sagittal and frontal plane components, with the former thought to place the highest amounts of stress on the ACL tissue.
Three key factors associated with increased ACL loading in the sagittal plane include (Yu 2007):
Small knee flexion angles
Large posterior ground reaction forces
Large quadricep muscle forces
These three factors are interlinked with each other in more ways than one. For example, small knee flexion angles are believed to reduce protective co-contractions that occur from the hamstring muscles, thus increasing quadricep muscle forces. Larger quadricep muscle forces are believed to elicit larger posterior ground reaction forces on the knee (Yu 2007) and ground reaction forces are believed to be associated with how an athlete strikes the ground when cutting. For the pivoting sportsman, the primary concern when considering change of direction strategy is the ability to evade a defender; this leads to two common strategies that place increased strain on the ACL and increased need for high levels of hamstring torque to provide a protective posterior translation force. These are wide foot placements outside the athletes centre of mass and heel striking into the ground (Fox 2018).
At this point in time as a clinician it would be easy to be confused regarding the role of the quadriceps in inducing an ACL injury on the athlete, after all, we are told that the quads are king in regards to protecting the ACL. This is true, the ability to produce adequate quadricep torque provides the athlete with the ability to utilise their strength in joint angles that are less biomechanically advantaged to the athlete, that being in more degrees of knee flexion. The ability to have greater quadricep strength (particularly eccentric and reactive strength) in longer levers will grant the athlete the ability to generate force whilst also protecting their knee. This is fundamentally why the athletes that you see that are weaker in their quads will tend to hip dominate in their plyometric & knee dominant strength tasks and will land with a stiff knee in smaller degrees of knee flexion.
However this blog is about hamstrings... so I must digress away from the discussion on quadriceps from now on.
Whilst the quads are king, unfortunately you can't maintain a royal family without a queen. I tend to think of the hamstrings as the more rational, financially stable partner of the relationship whilst the quadriceps are the life and vibrancy of the party. It's a yin and yang relationship to optimal health. If you haven't quite gathered by now the main role of the hamstrings in this entire complex nature of continual movement strategies is to effectively decelerate the knee every time an athlete takes a step on the ground.
WHY ECCENTRIC STRENGTH?
Now depending on who you talk to out of the Opar, Bourne, Timmins & Hickey camp compared to that of the Bosch party (sounds like a political debate because it basically is) there is conflicting evidence on the exact way the hamstrings function during end swing phase. Like the good upper middle class voter that I am I sit somewhere on the fence and believe that whilst there is a yielding isometric component to the way the hamstrings decelerate the knee, I believe and it is theorised that the hamstrings primarily eccentrically contract to provide a posterior translation force onto the knee (Buckthorpe et al 2020). With this in mind it is further theorised that ACL (& ACL graft) failure is observed when this process fails to protect the knee from anterior translation and the tensile strength of the ACL tissue fails. Eccentric hamstring strength can be observed as largely a muscle architecturally driven physiological adaptation. There is a positive correlation between an increase in muscle fascicle length & increases in eccentric strength (Buckthorpe et al 2020). These adaptations only occur when the hamstrings are either loaded in an eccentrically biased resistance training protocol, or through exposures of high speed running (Mendiguchia et al 2020), again as a middle ground fence sitter I preferentially like to ensure my athletes are exposed to both. With these factors in mind we can move forward establishing the importance on eccentric hamstring strength for the pivoting sport athlete.
WHAT IMPACT DOES A HAMSTRING GRAFT HAVE ON HAMSTRING FUNCTION?
The impact of harvesting the Semitendinosus (and depending on the tendon thickness the surgeon can achieve, the Gracilis) essentially acts as a G4 muscle-tendon lesion on the athlete (Buckthorpe et al 2020). This extent of injury places complete inhibition on the function of the medial hamstrings with prolonged regeneration times of the ST tendon and in approx 10-50% of athletes, failure to achieve tendon regeneration at all. The result of this ends in dominance through the lateral hamstrings (BF) to produce torque and this can functionally be seen in an athletes tendency to laterally rotate their foot during the gait cycle (as the BF acts as a secondary lateral rotator whilst the medial hamstrings act as a secondary medial rotator) and the inability for athletes to achieve full ranges of knee flexion during gait. These functional deficits should not be under-appreciated by the rehabilitating clinician as an externally rotated foot on foot strike is believed to increase ACL loading (Fox 2018) whilst the inability to maximise inner range knee flexion may result in an athlete producing shallower knee flexion angles when they strike the ground to change direction (my own theory, feel free to shoot me down if you disagree).
At an architectural level, post HG ACLR athletes are seen to have shortened muscle fascicle lengths and increased pennation angles that are typically seen with prior HSI and that is associated with decreased eccentric hamstring strength. Clinically what is established in the literature is that post HG ACLR at return to sport athletes are typically exhibiting up to 20% deficits in knee flexion strength and for those that achieve LSI on concentric knee flexion strength there remains deficits in eccentrically produced hamstring strength. This is believed to be as a result of altered neuromuscular activations that occur during eccentric contractions but not concentric and isometric contractions (Buckthorpe et al 2020).
ASSESSMENTS OF ECCENTRIC STRENGTH?
Measurement and quantification of eccentric hamstring strength has proven particularly difficult to date in our field. Current methods that have been assessed include:
Isokinetic Dynamometry (considered gold standard)
Flywheel leg curl machine
Nordbord Hamstring device (& equivalents)
Handheld Dynamometers
Issues surrounding each of the following can be identified:
Isokinetic dynamometry
IKD devices provide consistently the most reliable data in regards to muscle torque and producing quadricep to hamstring strength ratios, my particular issue in regards to these devices are that they are most accurate when they elicit the muscles maximum force output that is done at the slowest speed setting which fails to replicate the ability for muscle contraction at high speeds that is seen during injury settings. Availability to the clinician may also be limited in a sub elite space.
Flywheel Leg Curl Machines
These devices, whilst producing greater eccentric strength scores in athletes have appeared to suffer from a 'familiarisation' component whereby as the athlete technically improves at the task so does their score, significantly (Claudino et al 2021)... This can greatly reduce the intra-reliability of the testing procedure and thus it is recommended for clinicians to expose athletes to multiple trials prior to recording scores to ensure that a ceiling effect is seen on their ability to technically improve their score thus generating a 'truer' score of eccentric strength.
Nordbord Devices
Much like the flywheel devices, nordic hamstring exercise (NHE) assessment devices also are greatly influenced by familiarisation, similarly there altered muscle recruitment patterns can be seen dependent on the hip flexion-extension angle strategy the athlete chooses (Claudino et al 2021). With the hip in neutral extension there can be seen to be higher ST activity in the early phase of the movement and higher BF activity in the latter stages of the exercise, this is reversed when the hip is flexed at 90 degrees (think a razor curl angle) whereby there is greater BF activity in the early stages of the movement and greater ST activity in the final stages (Claudino et al 2021). Now this may be important for the clinician if they are wanting to bias the activation of a particular muscle, something that I commonly see in clinical practice when assessing athletes on the Nordbord is the inability for an athlete to survive until the final stages of the movement when their hip is kept in the neutral position and this may be due to the inability of the ST to achieve high loads, however we must understand that this will impact the validity of the device to accurately measure eccentric hamstring strength, particularly when there is also extremely poor correlation between NHE strength scores and IKD device scores. These devices however are extremely portable, easy to use, cost effective and once an athlete has developed technical competency and a reasonable strength base, can produce some consistent data and thus I am a fan of their applicability in clinical practice.
Handheld Dynamometers
These devices are most widely available and cost effective in clinical practice, the primary downfall with these are that there is extremely poor intra and inter reliability due to the lack of a fixed moment arm (i.e. clinician is producing force as is the athlete at the same time both affecting the total score output). The inability to produce at least reasonable levels of intra-reliability renders most data completely and utterly useless with some literature suggesting upwards of a 20% swing occurring, these are least favoured in my eyes when assessing hamstring strength.
WHAT ARE THE CURRENT TRENDS IN CLINICAL PRACTICE?
The current trends that I am exposed to in clinical practice are that at least 40-50% of athletes post HG ACLR have persistent strength deficits, particularly when assessed eccentrically for >9-12 months post surgery, despite the intervention of rigorous rehabilitation programs that are implemented in my environment. I can only assume and perhaps it is an unfair assumption that in the wider sub-elite athlete community whereby there is minimal methodology of hamstring strength assessment outside of HHD that many athletes are returned to sport with incredibly asymmetrical hamstring strength and incredibly poor eccentric hamstring strength which can be hypothesised as a primary risk factor for ACL re-rupture. As the old saying goes "if you are not assessing you are guessing" and I would be personally surprised if many athletes are being truely exposed to what their hamstring strength deficits are. Many that I assess that see the deficits are blindsided by it. Perhaps it is because an athlete can seem to compensate their way around hamstring deficits compared to quadricep deficits which are markedly obvious to the naked eye and to the more commonly adopted testing batteries of hop and jump tests. I cannot, at this stage prove my hypothesis however the lovely aspect of having my own blog is that I can express my own beliefs and thoughts as I please.
HOW CAN WE INTERVENE & WHAT IS A REALISTIC EXPECTATION FOR THE ATHLETE
This section can (and will) be an entire blog in itself so I won't spend much time on this. I hope that by this point in time that as a rehab professional you are now more aware (if you weren't already) of the role of the hamstrings in protecting the ACL and the impacts that a HG ACLR has on athletic hamstring function. Reverse engineering this, put simply the intervention that is needed is a high priority on hamstring donor site conditioning that is implemented from an early stage post injury. Particular attention needs to be placed on the implementation of medially bias' hamstring exercises (Buckthorpe et al 2020) as well as inner range knee flexion, eccentric hamstring strengthening and methods of overcoming arthrogenic muscle inhibition (AMI) in the hamstring muscle group.
CONCLUSION
In current clinical practice, hamstring autograft ACLR is the dominant graft of choice amongst the surgical community. It is commonly reported in the literature that hamstring grafts have a higher failure rate than other autograft choices such as quadricep tendon and BPTB grafts, leading to additional procedures such as LEAT in high risk groups. The factors behind ACL graft failure are multi-factorial with a combination of modifiable and non-modifiable factors responsible for high levels of re-rupture rates, particularly amongst sub-elite athletes. As clinicians it is pivotal that we utilise a 'deficits first' approach to rehabilitation, connecting the dots between mechanisms of injury and physiological factors that may increase the likelihood for an athlete to suffer a re-injury. Following HG ACLR athletes suffer from a wide variety of biomechanical, architectural and physiological deficits to their function. Eccentric hamstring strength is theorised to play an important role in protecting the knee from re-injury and this particular quality remains inhibited for prolonged periods post ACLR. Eccentric hamstring strength is tough to accurately quantify and most athletes in the sub-elite environment have poor access to the measurement tools required to gain this data, indicating that many athletes may be returning to sport with undiscovered asymmetries and deficits in eccentric hamstring strength. These reasons may be responsible for the higher rates of HG ACLR failure in comparison to other autograft selections rather than any issues with structural integrity of the graft tissue itself.
Stay tuned for Part 2...
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