Explorations in Hamstring Injury Concepts - Part III: The Idea of Injury Prevention and How It Relates to HSI

Where we left off:

Our last discussion focused in particular on two theories I found interesting regarding MOI as it relates to HSI. Before that, we talked about some of the basic anatomy and function of the hamstrings muscle group. To serve as reference for our discussions today, here are some of the key points from each that are worth noting as we move forward: To serve as reference for our discussions today, here are some of the key points from each that are worth noting as we move forward:

Anatomy

Individually:

• BF -> Large pennation angle with small aponeurosis
• ST -> Long parallel fibers arranged at low pennation angle
• SM -> Largest PCSA and has overlapping proximal and distal tendons

Function

• As a group, the hamstrings are responsible for performing hip E and knee F
• As each muscle attaches at the knee, each serve to optimize knee function, stability, and soft tissue structure health (especially at the ACL)

Individually

• LHBF –> produce high force at high velocities, and, due to large pennation angle can generate forces well concentrically and isometrically
• ST –> influences excursion through speed and larger range of movement. Long parallel fibers and low pennation lend well to its ability to work eccentrically
• SM –> largest force producer of the unit given largest CSA. Overlapping tendons allows for coordination of contractions during movement tasks (i.e. at one segment it allows for concentric contraction while at the other it acts eccentrically). Also works well eccentrically

Biomechanics

MOI:

• High speed running injuries most common, normally occurs at LHBF

I’ve seen two well put together presentations describing how these injuries happen:

• Especially as it relates to proximal hamstring strain injury, this injury tends to occur at the terminal swing phase of the running cycle due to an increase in eccentric or negative forces experienced at the LHBF
• Injury may occur more often during the amortization phase of the running cycle in which the hamstrings are responsible for switching from a mode of eccentric to concentric contraction.

This Week’s Primary Sources of Information:

*As is now customary, each week I like to mention that, given my background as a student, much of the information I am presenting and interpretation of this information comes not from me, but from a few keynote speakers of whom I think do a fantastic job discussing this information. I owe most of what I’m putting out here to these select few people, which is why I think it is important to provide reference for these resources before beginning discussion. I work to preference where my interpretation of this information comes into play so as to not confuse their message with my interpretation. If you have the opportunity, and these topics I’m putting out interest you, I’d highly recommend going and either reading or listening to these. As I always mention, if you choose to investigate, make sure you have some time, a note pad and a pen to optimize your learning experience.*

Primary Sources:

Bittencourt et al. – Complex Systems Approach for Sports Injuries: Moving from Risk factor Identification to Injury Pattern Recognition-Narrative Review and New Concept

• Link: https://bjsm.bmj.com/content/50/21/1309

The SportsMap Network – Mastering Lower Limb Muscle Injuries 2020 Online Conference

• Speaker: Nicol Van Dyk
• Speaker: Natalia Bittencourt
• Link: https://www.facebook.com/watch/live/?v=231352004626110&ref=watch_permalink

Risk Factors and the Creation of a Paradox as it relates to Injury Prevention

Understanding that these injuries are experienced by many, and that they often offer the potential for reinjury, it should come as no surprise that rehabilitative and injury preventive strategies have received an ever-growing amount of attention to mitigate risk. As a result, there have been clearly identified modifiable and non-modifiable risk factors to help guide these treatment and programming methods.

Non-modifiable risk factors are as they sound, things that we cannot control. As it relates to the hamstrings, in a recent systematic review and meta-analysis recalibrating the risk of HSI, Green et al. found primary risk factors for initial and recurrent HSI to include both older age and previous HSI (Green, 2020). More interestingly, they also noted that same season injury was another risk – at a rate 5x greater for development of injury – illustrating the unfortunate fact that the potential for reinjury remains high when dealing with these injuries.

Two other non-modifiable risk factors mentioned in the report were previous calf injury and previous ACL injury, demonstrating that something changes after an initial injury. We’ve gathered previously that the hamstrings are important muscles as contributors to knee function/soft tissue health. As both the knee and the hamstrings seem to function interdependently, it may make sense why an injury not immediately involving the hamstrings offers an impact on them. Exactly how these different injuries may influence hamstring function remains a point to debate, but this does illustrate the importance of optimizing rehab of any injury holistically so as to help reduce injury risk at other joints. It also implies that injuries are multifactorial, as one injury at one part of the body (i.e. the calf) impacts the body elsewhere.

Modifiable risk factors on the other hand are where we as rehabilitative and strength and conditioning specialists find ourselves searching for answers to prevent the problem from initially occurring. These include, but are not limited to (Liu, 2012):

• Kinematics (more specifically sprint mechanics)
• Lack of eccentric strength produced by the hamstrings during bouts of knee extension in high speed running tasks
• Hip musculature weakness (adductors again)
• Shorter muscle fascicles (as force generators)
• Deficits in core strength/stabilization

As we’ve worked to establish what these risk factors are, our hope has been to manipulate/influence them to introduce the potential for adaptations. Such adaptations should then in theory decrease the risk of injury. At its most basic roots, this is what injury prevention is. Establish what risk factors exist and program to change. Clearly we’ve established individualized risk factors as they may relate to HSI, so what are we to make of our ability to prevent injury, and how effective are we as clinicians at actually decreasing risk of injury?

The answer to these questions yet remains unclear. As I’ve now established, a large influencer on my desire to begin writing about the hamstrings stems from the fact that there’s a ton of information on them. Given this large body of information, the thought process would be that if we’ve taken time to educate ourselves, we should have the tools necessary to help prevent some of these injuries from occurring; however, it’s not so black and white. 

Instead, our vast array of knowledge on hamstring strain injuries has essentially led us to a paradox. We know more, but the incidence rate has yet to decline. Why then, if we have a deeper understanding of these injuries and the risk factors associated with them, have we yet to be able to reduce the rate of injury? Although lacking an absolute answer, perhaps a different mode of thinking - deemed the “Complex Systems Theory” of sports related injury - may serve of insight and better function to help reduce the rate at which these injuries occur (If interested in a much quicker summary, I briefly discuss this article on my twitter of which I am shamelessly plugging here). 

The Complex Systems Theory

This Complex Systems Theory of sports injury is best described by Bittencourt et al. in their paper titled “Complex Systems Approach for Sports Injuries: Moving from Risk Factor Identification to Injury Pattern Recognition.” Initially, they argue that human health conditions – especially injury – are complex and multifactorial. As a result, they liken that injuries “arise not from linear interaction between isolated and predictive factors (i.e. any one of the risk factors I’ve just discussed), but rather as a result of a complex “web of determinants” (WOD)”.

This WOD is generated as an interconnected “pattern of relationships” (or interactions) amongst these established risk factors, which serves to create a series of “regularities” (risk profiles). When established, these regularities can simultaneously characterize and constrain the phenomenon that is injury. By developing an understanding of the emerging pattern that arises from the complex WOD, we can move from attempting to individually manipulate risk factors to recognizing patterns as these risk factors relate to one another and program instead with the idea of these relationships in mind. This then should help to create an injury preventive strategy more “holistically” (I’ve used this word a couple times now).

According to Bittencourt and colleagues, clinicians may often attempt to simplify complex problems into basic units, seen through identifying isolated factors frequently assumed as the cause of injury or disease. The problem here is that when individuals do this, they assume a linear association in that a singular circumstance may elicit an outcome; however, this should not be considered to be the case. Especially as it relates to injury, it isn’t so A = B – i.e. we shouldn’t assume a singular circumstance always elicits an outcome.

In sports, a prime example in using this mode of thinking (and is an example used in the paper) involves the identification of dynamic knee valgus (DKV) in basketball as a risk factor for ACL rupture. DKV involves an inward collapse of the knee which consequently incurs an increase in strain on the ACL. Through movement analysis, clinicians may identify this faulty movement pattern in an individual and denote that person as someone who is more “at risk” of an ACL injury. The thought process here is that if an athlete or patient performs a countermovement jump test for example and his/her knee collapses into dynamic knee valgus, that person is lacking proper control of his/her knee during jumping tasks. As a result of increased pressure on the ACL and lack of control at the knee during these dynamic movements, the individual would be thought to be at an increased risk of developing an ACL injury. But this relationship represents only a segment of the total picture.

The point of understanding that there are multiple contributors to this web of determinants is to help allow for the recognition of a pattern to ensue. As it relates to ACL injury, DKV can be considered to be one of the three main elements comprising this web (see picture). The other two would be unanticipated environmental events (UEE – a term that offers implication toward just how difficult it ever actually is to stop an injury from happening) and hip weakness.

These three elements take part in most existing interactions associated with knee injury, which is why they’d be considered the three main elements comprising this web. They are also subjected to influence by other members of the WOD – those that are smaller and thus less likely to elicit injury, but may interact with the three main modes and thus incur injury again in the presence of this inciting event, as seen below.

For DKV, it is most chiefly influenced by other risk factors including fatigue, hip muscle strength, NM control, foot complex anatomical alignment and training load. Each of these factors is further modulated by other factors such as age and sex. Recall that both age and sex are non-modifiable risk factors. We now have the potential for DKV to occur as the result of interactions between both modifiable and nonmodifiable risk factors illustrating how the level of complexity associated with injury continues to go. 

Fatigue is often directly associated with DKV, and may come as a result of training load/frequency of training sessions, and also to psychological factors such as anxiety, attention level and level of arousal. Increased fatigue may negatively alter the quality of movement such that there is now more frequent occurrence of DKV in the athlete, potentially leading to an ACL injury in any one of the instances in which the athlete collapses into more knee valgus. Understanding that DKV is perhaps the end result of a series of interactions of items in this web, perhaps this offers insight into training. We as clinicians may be best served to focus on dissipating load properly when fatigued so as to more optimally include instances that may elicit DKV, rather than focusing on it singularly as it relates to ACL injury. If we’re training at higher levels of load and increasing degree of fatigue, the athlete will need to focus on controlling the knee more often in these instances with hope to improve NM control and strength to the point that they are exhibiting less knee valgus with increases in fatigue.

Relating Back to the Hamstrings

Understanding this concept at its foundation, Natalia Bittencourt (one of the main contributors of the paper and founders of the idea) did a great job explaining this theory especially as it relates to the hamstrings in her talk presented at the SportsMap Networks LE injury conference. She argued that for the hamstrings, much like in other injury, it is important to establish etiology and mechanism of injury and use this understanding of each coupled with potential risk factors to build this WOD and facilitate pattern recognition.

Through our last two discussion, I’ve already created a small WOD from simple anatomy, function and biomechanics that we can use as an example. Some quick hitting items in the web include:

• Hip adductor (AM specifically) strength
• Eccentric strength of ST, SM, and LHBF
• Limited width of LHBF aponeurosis
• Exposure to high speed running and resultant forces placed on hamstring MTU
• Hip ROM asymmetry in the swing phase of running (Natalia actually mentions this in her talk I had just mentioned and is not something that I’ve discussed previously).

So how may some of these factors relate to generate that “inciting injury event” I keep referencing? To offer an example, we need to revisit the clinical example that I had mentioned in my first post and dive into it a bit deeper. In case you’ve forgotten, the scenario is as follows:

“I have a very close friend who recently strained his hamstring playing a game of backyard football against some of his friends. While he was running after the receiver who had just caught the ball, he felt a “pop” in his hamstrings and thus began the cascade of symptoms for him that are often experienced in HSI. While initially painful and irritating to say the least, the problem he’s experienced is that weeks after the injury, when it would appear that he is healthy, he’s re-aggravated it while trying to play again.”

*A few important facts I had omitted previously – on his affected leg, he has a history of two ACL tears. He’s also a fellow PT student, so he chose to rehab using generic Askling protocol exercises and rest for a few weeks before attempting to play again.*

Now let’s run through specific risk factors that may led to both the initial injury and his re-injury:

Initial Injury:

Non-modifiable risk factors:

• Previous ACL injury

Modifiable risk factors:

• Lack of exposure to high speed running secondary to the pandemic
• Limited LE strength secondary to lack of exercise and availability to local gyms secondary to the pandemic

Re-injury:

Non-modifiable risk factors:

• Same “season” (and I put that in quotations because it’s tough to call a few backyard football games a season) hamstring strain injuries
• Previous ACL injury

Modifiable risk factors

• Rehab approach: Askling protocol (Extender, glider, diver) exercises 3 x 4 reps 1 time every 3 days + Rest
• Lack of exposure to high speed running
• Limited LE strength
• Time between injury and return to play: 3 weeks

Using complex systems theory, here is what I’ve deduced may have led to his injury:

He’s had a limited exposure to high speed running and potentially reduced hip adductor strength causing a disadvantage initially extending the hip from a position of hip F. This increases demand and load placed on the hamstrings to function both in control of knee E and to extend the hip. Because he has a decreased amount of eccentric strength (likely in his ST/SM since we’ve established they function best eccentrically), he cannot tolerate lengthening forces as well during terminal swing. His biceps femoris is also weak and thus cannot tolerate the increase in eccentric load it must handle due to decreased strength at ST/SM. The BF now responds poorly to increased load and dumps an excessive amount on both its tendon and aponeurosis in this instance and thus elicits a proximal LHBF MTU injury.

He then hasn’t given himself enough time to rehab, nor has he specifically targeted some of the glaring risk factors that led to the initial injury. He is still weak throughout both LEs and has yet to gradually reintroduce himself to instances of high-speed running. Couple potential lack of exposure, decreased strength, previous HSI and previous orthopedic history significant for two ACL tears and there’s a reason he’s a case study for why these injuries may linger.

Again, this example is subject to my own intuition and opinion, but I think it offers a few different areas to target in terms of treatment. Obviously there are a multitude of factors that we can attempt to modify to reduce his potential for re-injury moving forward. Just focusing on one specifically may not address the whole picture and wouldn’t benefit the patient well. Often times Natalia argues that, especially as it relates to the hamstrings, we may get too caught up in linear relationships especially between eccentric strength and HSI injury, without proper analysis of the multi-dimensional combinations which lead to injury. Without an innate understanding of each factor and how they interact with one another, then we’d fail to optimize both treatment and programming. If we can do this however, we can introduce preventive measures and continue to assess effectiveness in order to better develop understanding of the injury moving forward.

Looking Forward

I’ve now mentioned potential for treatment in each of my first three posts without actually discussing anything treatment related. So how do I transition to make this practical? Well, the answer is where I would like to head in terms of a direction. There are a few different approaches to hamstring strain rehabilitation that I’ve seen mentioned and have found interesting/insightful. These may offer methods to consider as we seek to rehab our athletes. I think transitioning to types of treatment, exercises and even clarifying return to sport guidelines will help to ultimately achieve an original goal of mine, which was to determine and discuss most interesting concepts in treatment as they relate to HSI.

As always, if my writing is a drag, confusing, or wordy, here are some key take home points hopefully to better portray what I was going for today:

• Establishing etiology, MOI and risk factors are important to develop a foundational understanding of HSI
• Risk factors can be separated into two categories, with specific ones affecting the hamstrings listed for each:
• Modifiable: kinematics (more specifically sprint mechanics) - lack of eccentric strength produced by the hamstrings during bouts of knee extension in high speed running tasks - hip musculature weakness - shorter muscle fascicles (as force generators) - Deficits in core stabilization
• Non-Modifiable: Age and previous injury (including at the calf and ACL)
• We’ve created a paradox when it comes to reducing hamstring injury rate: we know more about the disease, but we haven’t been able to reduce the rate of injury. Why is this the case? The question remains unanswered.

A more suitable approach to injury prevention especially as it relates to sports related injury would be to consider the “Complex Systems Theory.” The theory can be explained as follows:

• Injury results from a complex interaction among a web of determinants (WOD). This WOD can be considered as an inclusion of each of the identified risk factors discussed above.
• Interactions amongst the constituent members of the WOD forms patterns for us to analyze that ultimately serve to build regularities (risk profiles). This helps to clarify the otherwise complex and confusing phenomenon that is sports related injury
• If we understand these patterns and how the web interacts, we can better understand mechanism of injury and injury prevention to best program for our athletes and hopefully help reduce the rate of injury. 

Citations:

• Bittencourt, N F N, et al. “Complex Systems Approach for Sports Injuries: Moving from Risk Factor Identification to Injury Pattern Recognition—Narrative Review and New Concept.” British Journal of Sports Medicine, vol. 50, no. 21, 2016, pp. 1309–1314., doi:10.1136/bjsports-2015-095850.
• Green, Brady, et al. “Recalibrating the Risk of Hamstring Strain Injury (HSI): A 2020 Systematic Review and Meta-Analysis of Risk Factors for Index and Recurrent Hamstring Strain Injury in Sport.” British Journal of Sports Medicine, 2020, doi:10.1136/bjsports-2019-100983.
• Liu, Hui, et al. “Injury Rate, Mechanism, and Risk Factors of Hamstring Strain Injuries in Sports: A Review of the Literature.” Journal of Sport and Health Science, vol. 1, no. 2, 2012, pp. 92–101., doi:10.1016/j.jshs.2012.07.003.