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Introduction
Hamstring strain injuries (HSI) are one of the most common time loss injuries in sprint based sports (Bramah et al 2023). These injuries provide great economic and physical burden on sporting clubs and playing careers and despite continued research and interventions in clinical practice, incidence and recurrence rates remain high (Hui et al 2012). Recurrence rates of hamstring injuries have been reported to be ~16-34% with some literature referencing even higher rates when the intramuscular tendon is implicated in severe hamstring injuries (Kerin et al 2023). Debates have existed surrounding the best methods for developing resilience within the hamstring complex for a long period of time. A commonly discussed debate is that popularised between Anthony Shield and Frans Bosch surrounding the relevant roles of Eccentric & Isometric contractions during the gait cycle and their association with HSI. More recently, the role of running mechanics and high speed run exposure has gained scrutiny in it's role as both a protector and cause of HSI (Bramah et al 2023). The purpose of this blog will be to synthesise the authors opinion of the current evidence surrounding HSI and provide a practical model for understanding the anatomy and contributors to HSI whilst also highlighting a rehabilitation and injury resilience model. Given the depth of the topic that I will be attempting to discus below, I have chosen to not delve into the space of injuries requiring surgical repair of either of the free tendon or intramuscular tendon of the hamstrings.
Relevant Anatomy
Basic hamstrings anatomy as described in the 41st Edition of Gray’s Anatomy, is described in the below excerpt.
The hamstrings, or ischiocrural muscles, comprise the muscles of the posterior compartment of the thigh, and include the ST, SM and BF (long head – BFlh; short head – BFsh), which attach proximally to the ischial tuberosity (except the BFsh). The ST, SM and BFlh are biarticular muscles, and act in extending the coxofemoral joint (hip), as well as flexing and rotating (medially and laterally) the knee. Distally, the ST and SM attach to the tibia, while both heads of the BF attach to the fibula. The hamstrings present relevant interindividual variation in length. The hamstrings are innervated by the sciatic nerve, emerging at the level of S1 vertebra, although with interindividual variations. (Gray's Anatomy)
Image 1 - Basic Hamstring Anatomy
What has been historically poorly understood which more recently has had increased scrutiny placed upon it is the discovery of the intramuscular tendon (IMT) and aponeurotic tissue that exist within the hamstring complex and how these connective tissues influence injury. Kerin describes the IMT as a "continuum of the free tendon, which extends within the muscle belly, akin to the central rachis of a feather, with radiating myofibrils" (Kerin et al 2023).
Image 2 - Hamstring complex muscles showing the distribution of connective tissue in each muscle belly
Image 3 - Interactions between BF, ST & SM
Mechanisms of Injury
Two major types of injury mechanism are reported in the literature in relation to HSI
Stretch Type Injury
Sprint Type Injury
Stretch type injuries usually involve some form of combined excessive hip flexion & knee extension moment (Danielsson et al 2020). These can and usually result in violent type injuries and may involve either the proximal or distal musculature. More commonly than not, connective tissue either at the free tendon or IMT level is implicated (Danielsson et al 2020).
Kerin expands further on the mechanisms of stretch related injury particularly related to the sport of rugby union and the important functional demands of tackling, decelerating, kicking & rucking (Kerin et al 2022)
Sprint type injuries have gained far more attention as they are the more common MOI in sprint based sports. Eccentric fascicle actions are theorised as the cause of tissue failure when a HSI occurs (Van Hooren & Bosch 2017). Conjecture exists as to whether this MOI is as a result of eccentric muscle action failure or isometric action failure resulting in an eccentric contraction (Van Hooren & Bosch 2017) (Shield & Murphy 2018).
Contributors to HSI
Historically, discussions around contributors to HSI were largely placed at a physiological level investigating risk factors at a cellular level. This led to a widely popularised theory around fascicle lengths and the 'quadrant of doom'.
Image 4 - Depiction of the 'quadrant of doom'.
Key criticisms of this model have been described by Martin Buchheit as the following:
Since the relationship between fascicle length, muscle strength and strain during active lengthening is probably muscle head- and player-dependent, the use of a single measure (i.e., fascicle length) on a single muscle (e.g., biceps femoris long head, BFlh) to assess the overall injury risk of the hamstring group remains prone to approximations.
The “quadrant of doom” being a two-dimensional representation of hamstring injury risk factors only, others extremely important risk factors such as age and previous injury history can’t be integrated into the ‘picture’; this can bias the risk evaluation.
(Buchheit 2019)
More recently, emphasis has been placed on the role of sprint biomechanics and the surrounding global anatomy in the incidence of HSI. When considering the functional anatomy of the hamstring complex, Bramah et al described the respective roles of the hamstring muscles as the following:
The hamstrings are biarticular muscles spanning both the hip and knee joints with distinct roles at each. Proximally, they attach to the pelvis via the ischial tuberosity, where the BFLH and semitendinosus form a conjoined tendon. In contrast to the semimembranosus and semitendinosus, the BFLH possesses an additional attachment to the sacrotuberous ligament, directly connecting it to the sacroiliac joint (SIJ). This attachment suggests the BFLH may contribute to, and be influenced by, pelvis and SIJ stability. Thus, alterations in pelvis and SIJ kinematics will likely impact the strain distribution through the BFLH.
Distally, the semimembranosus and semitendinosus attach to the medial tibia, merging with the medial collateral ligament, meniscus and pes anserine. The biceps femoris descends distally, forming the biceps femoris short head that inserts on the lateral aspect of the fibula head, with fibres blending with the lateral collateral ligament, iliotibial band and surrounding fascia.
This complex anatomy provides clear connections between the hamstrings and proximal segments (trunk and pelvis), as well as distal segments of the knee and lower limb. Therefore, the hamstrings not only serve dual roles extending the hip and flexing the knee, but likely contribute to rotational and translational stability at the knee.
The direct connections between the BFLH and sacrotuberous ligament also highlight the BFLH’s role in pelvis and SIJ stability. Consequently, mechanics at both proximal and distal segments can impact hamstring function and the applied mechanical strain.
(Bramah et al 2023)
Understanding the above can allow the clinician to respect the influence that different movement strategies and inefficiencies can place on the hamstring muscle group. These have been summarised in the table below:
Image 5 - Biomechanical Risk Factors to HSI
Put simply, the contribution to HSI can be broken into categories of 'applied strain' and 'strain capacity' with sub capacities underneath these umbrella terms where the athletes modifiable and non modifiable risk factors lie
Image 6 - Modifiable & Non Modifiable Risk Factors of HSI
Diagnostic Strategies
Clinical characteristics of HSI of varying degrees are usually related to pain, loss of strength, power & flexibility as well possible visible signs of bruising / internal bleeding. In elite settings, diagnostic imaging to determine severity & tissue type involvement is also indicated with the most updated and widely used classification system based around the British Athletics muscle injury classification (Pollock et al 2014).
Image 7 - British Athletics Classification system
Currently there are no conclusive subjective or objective clinical measures that can be linked directly to injury of the IMT with Kerin describing the following in a recent narrative review:
'Where possible, it would be worthwhile in further analyses of IMT injuries for authors to provide greater detail in descriptions as to the biomechanical position of athletes at the point of injury, in order to determine whether there is a distinct, identifiable pattern of injury'.
'It has been suggested that the use of MRI is imperative in the assessment of hamstring strain injuries, as it is otherwise impossible to distinguish between injuries which do and do not involve the IMT'
(Kerin et al 2023)
Complications surrounding physical disability relating to injuries to the IMT surround a potential lack of sensory supply to the IMT, indicating that possible high grade injuries may slip under a clinicians radar as little morbidity may be experienced.
Rehab Planning
There is extremely high variability in RTP timeframes in HSI and whilst it is theorised that involvement of connective tissue such as the musculotendinous junction (MTJ) & / or the IMT can significantly prolong rehabilitation timeframes and increase the likelihood of injury recurrence, evidence on the matter is mixed (Kerin et al 2023).
Image 8 - Comparisons of Recurrence Rates & RTP timeframes on IMT injuries vs other HSI
Whilst Pollock & the group at British Athletics had originally reported extremely high recurrence rates with injury to the IMT, a further analysis had been made based on rehabilitation principles linked to the BAMIC and whilst time to return to full training was significantly longer following IMT injury than other injury locations. However, the athletes’ recovery time was broadly similar to previous case studies (2c injury: mean = 35 days, 3c injury: 51.5), suggesting that it is the difference in rehabilitation that defines the outcomes, rather than just longer time allowed for healing (Kerin et al 2023) (Pollock et al 2022).
Image 9 - Management principles of HSI in British Athletics
Image 10 - Hamstring Rehabilitation from British Athletics
Practically, for the clinician working with any athlete, it is important to understand the athletes sporting & positional demands as well as their individualised injury risk level based on prior history to devise an individualised rehabilitation plan with the above providing useful guides for practitioners derived from high level track and field athletes. Examples of long and short term rehabilitation templates I utilise are depicted below and can be found in other blogs
Image 11 - Long Term Rehab Plan Template
Image 12 - Short Term Rehab Plan Template
Principles of Tissue Loading
As discussed above, there is plausibility to the argument that once an accurate diagnosis is established and the clinician has a succinct understanding on the extent of the athletes pathology, loading strategies can be prescribed in respect to tissue healing. Popular rehabilitation frameworks have been described by Pollock (Pollock et al 2022); Askling (Askling et al 2014) & Mendiguchia (Mendiguchia et al 2017). Below is my own personal clinical commentary and perspective on tissue loading and rehabilitation of HSI as an adaptation of the prior researchers excellent work.
Image 13 - Adapting tissue remodelling phases to phases of rehabilitation
Creating a framework of rehabilitation related to phases of tissue remodelling can allow the rehabilitation professional to understand and respect elements such as acute inflammatory phases which may differ in length based on the severity of injury as well as then assist in load prescription when in regeneration and remodelling phases by aiming for restoration of type 1 collagen. Isometric & Eccentric contractions appear to provide the greatest adaptation on muscle fascicles and favourable tendon adaptation and thus should be prioritised.
Image 14 - Framework for exercise selection in type 'a' injuries
Image 15 - Framework for exercise selection in type 'b' injuries
Image 16 - Framework for exercise selection in type 'c' injuries
The primary difference between the 3 framework models above are that in the presence of type 'b' and type 'c' injuries, there is greater respect given to the inflammatory phase of tissue recovery whereby initial exercise selection is ideally concentric biased before shifting to isometric loading in short muscle lengths to maintain the muscle-tendon unit (MTU) before progressing to long-length isometrics and supra maximal eccentric contractions in late stage rehabilitation to challenge the MTU in a more dangerous position whilst also challenging the global stability based anatomy of the trunk & pelvis.
Snapshotting the Journey - From Protection to Performance
The below infographics detail a gym based framework of progressing a HSI from early stages of injury to late stage rehab with key focus locally and globally highlighted. Noting individual prescription will highly vary depending on location and tissue type involvement of the specific injury you are dealing with as a clinician.
Protection Phase
Image 17 - Protection Phase Framework for HSI Rehabilitation
Load Introduction Phase
Image 18 - Load Introduction Phase Framework for HSI Rehabilitation
Strength Accumulation Phase +
Image 19 - Strength Accumulation Phase Framework for HSI Rehabilitation
Return to Run Systemisation
The return to run phase of rehabilitation is arguably the most crucial, particularly given the above literature describing components of running as a primary contributor to the onset of HSI. Technical development of an athlete's efficiency in the gait cycle is important in preventing re-injury. Importance for the clinician should be placed on developing specified resilience and optimal mechanics in the specific plane of movement the athlete suffered the original injury. I.e. if the athlete suffered the injury kicking, then kicking must be targetted in the end stages of rehabilitation. If the athlete suffered the injury during max velocity sprinting then scrutiny should be placed on the athletes ability to perform said task.
Technical Focus
Whilst an individualised approach should be taken to technical drilling, a useful and safe place to start for clinicians is to develop an athletes ability to produce a strong front side body position during their max velocity phase and effectively cycle / strike their foot under their centre of mass. This will help to reduce the negative impacts that may occur from the following risk factors:
Overstriding
Back Side Mechanics
Lumbar Hyperextension
Drilling sequences to attack this include the following themes:
Cycle based drills
Dribble based drills
Scissor based drills
Video 1 - Segmented Wall Cycle Drill
Video 2 - Speed Wall Cycle Drill
Video 3 - Shin Dribble
Video 4 - Knee Dribble
Video 5 - Scissor Run (Short to long)
In regards to a framework for general physical reconditioning of running, this can be highlighted below with emphasis on three streams:
Accel Development
Max V. Development
Speed Exposure / Tolerance
Image 20 - Return to Run Systemisation
Importantly what is not mentioned in the above framework is the technical drilling component of running. Similarly this is a broad overview of a way for a rehabilitation professional to take an athlete from controlled to chaotic running and it is important for clinical reasoning to be applied as to the timings and specific stimulus that is applied to your athlete in front of you.
Video 6 - Tempo Run / Distance Dribble
Training Integration Periodisation
It is important to recognise that when an athlete returns to team training, care should be taken when accounting for all the loads experienced in their training week. Below is an example of how this might be applied in a team field sport.
Image 21 - Example of Planning Integrated Training
Important to note in the above plan that this would be contextual namely towards amateur and semi professional athletes that may train on a Tuesday & Thursday with their respective team and then may structure their gym based training on a Monday, Wednesday, Friday.
In a professional set up, particularly given that team based sports may not always rely on a 7 day turn around in season the MD + & - system is the preferred method of periodising a micro cycle with MD+3 best primed for the heaviest loading session of the week through ROM and MD-2 best primed for heavy isometric / reduced ROM stimulus.
The idea of the above template is purely to stimulate the thought process behind microcycle planning which can often be forgotten in private practice rehabilitation settings.
Assessment & Monitoring
Post the inflammatory / protection phase of rehabilitation, assessment of isometric hamstring strength is recommended throughout the Load Introduction phases of rehab onwards. From the Strength Accumulation phase onwards I would tend to introduce eccentric hamstring strength testing into this battery as well. For these tests I use a combination of the VALD Performance Nordbord as well as the VALD Performance Forcedecks to achieve metrics of maximal torque and RFD.
Video 7 - Nordbord 30 degree isometric test
Video 8 - Forcedecks 90/90 Hamstring Isometric
Video 9 - Nordbord Nordic Fall Test
Final Thoughts
There are many questions that are still left unanswered in regard to the HSI issue that plagues athletes in all levels of sport. Despite continued better understanding of the anatomy, physiology, risk factors and prevention mechanisms the incidence of these injuries remains high. The hamstring musculature is a complex network that is placed under high levels of duress in sports requiring high sprint and stretch loads. Having an understanding of the functional anatomy allows a practitioner to establish optimal rehabilitation plans to help ensure that athletes are placed at as low risk of recurrence as possible. Whilst debate continues regarding the role of direct strength training to the hamstrings, indirect strength and motor control training to the lumbo-pelvic region and exposure to high speed running my opinion fits in the space where I believe there is sufficient evidence to argue all are important. My final thought to leave you on, if you have made it this far in the blog is a depiction from an all time classic film that I feel appropriately applies to this topic.
Image 22 - Obi Wan Kenobi
Thank you for reading, for a more in depth discussion on this topic, I produced a one hour presentation for the 'Athletes Authority Performance Department'. You can find the link to join this resource below if this and continued discussion with myself is of interest to you.
References
Bramah, C., Mendiguchia, J., Dos’Santos, T. et al. Exploring the Role of Sprint Biomechanics in Hamstring Strain Injuries: A Current Opinion on Existing Concepts and Evidence. Sports Med (2023). https://doi.org/10.1007/s40279-023-01925-x
Hui Liu, William E. Garrett, Claude T. Moorman, Bing Yu, Injury rate, mechanism, and risk factors of hamstring strain injuries in sports: A review of the literature, Journal of Sport and Health Science, Volume 1, Issue 2, 2012, Pages 92-101, ISSN 2095-2546, https://doi.org/10.1016/j.jshs.2012.07.003.
Kerin, F., O’Flanagan, S., Coyle, J. et al. Intramuscular Tendon Injuries of the Hamstring Muscles: A More Severe Variant? A Narrative Review. Sports Med - Open 9, 75 (2023). https://doi.org/10.1186/s40798-023-00621-4
Danielsson, A., Horvath, A., Senorski, C., Alentorn-Geli, E., Garrett, W. E., Cugat, R., Samuelsson, K., & Hamrin Senorski, E. (2020). The mechanism of hamstring injuries - a systematic review. BMC musculoskeletal disorders, 21(1), 641. https://doi.org/10.1186/s12891-020-03658-8
Kerin, Fearghal & Farrell, Garreth & Tierney, Peter & McCarthy Persson, Ulrik & De Vito, Giuseppe & Delahunt, Eamonn. (2022). Its not all about sprinting: mechanisms of acute hamstring strain injuries in professional male rugby union—a systematic visual video analysis. British Journal of Sports Medicine. 56. bjsports-2021. 10.1136/bjsports-2021-104171.
Van Hooren, B., & Bosch, F. (2017). Is there really an eccentric action of the hamstrings during the swing phase of high-speed running? Part II: Implications for exercise. Journal of sports sciences, 35(23), 2322–2333. https://doi.org/10.1080/02640414.2016.1266019
Shield, A.J., & Murphy, S. (2018). Preventing hamstring injuries-Part 1 : Is there really an eccentric action of the hamstrings in high speed running and does it matter ?
Buchheit M., Avrillon S., Simpson B., Lacome M., Guilhem G. The quadrant of doom and hamstring injuries: sexy but too easy? Sport Performance & Science Reports, 2019, June, #63, V1
Pollock, N., James, S. L., Lee, J. C., & Chakraverty, R. (2014). British athletics muscle injury classification: a new grading system. British journal of sports medicine, 48(18), 1347–1351. https://doi.org/10.1136/bjsports-2013-093302
Pollock, N., Kelly, S., Lee, J., Stone, B., Giakoumis, M., Polglass, G., Brown, J., & MacDonald, B. (2022). A 4-year study of hamstring injury outcomes in elite track and field using the British Athletics rehabilitation approach. British journal of sports medicine, 56(5), 257–263. https://doi.org/10.1136/bjsports-2020-103791
Askling CM, Tengvar M, Tarassova O, et al. Acute hamstring injuries in Swedish elite sprinters and jumpers: a prospective randomised controlled clinical trial comparing two rehabilitation protocols. Br J Sports Med. 2014;48(7):532–9.
Mendiguchia J, Martinez-Ruiz E, Edouard P, et al. A Multifactorial, Criteria-based Progressive Algorithm for Hamstring Injury Treatment. Med Sci Sports Exerc. 2017;49(7):1482–92.