Hamstring strain injuries (HSI) are one of the most frequent time-loss injuries in sprint-based sports. They account for 19% of all injuries in football and are reported to result in an economic burden of approximately 90, 367€ per 1000h (Ekstrand et al., 2022; Pulici et al., 2023). If we look at data from current research, we can see that more than 50% of HSI occur during sprint running, during either maximal velocity running or acceleration scenarios (Figure 1) (Aiello et al., 2023; Gronwald et al., 2022; Kerin et al., 2022).
Why do hamstring injuries occur?
A simplistic way to view hamstring injuries is that these injuries result from the interaction between strain application and strain tolerance. If a muscle can withstand the applied strain during sport specific movements it will remain injury free; however, if the strain exceeds the capacity of the muscle, we sustain either an acute muscle injury, or begin the accumulation of microtrauma leading to eventual fatigue induced tissue failure.
Strictly focusing on mechanics, stress/ strain applied to the hamstrings can be influenced by the mechanical demands imposed by the specific task (sprint running) and the technical qualities/features of the individual. Understanding the task-imposed demands, alongside how technical qualities may influence tissue strain and injury, provides us with important considerations if we are to appropriately condition and rehabilitate individuals following hamstring strain injuries.
Hamstring Biomechanics: Task Imposed Demands
The ability to accelerate and attain high maximal running speeds a critical physical quality for sports performance. However, this also places high mechanical loads on the hamstrings, particularly, as athletes get faster. To accelerate we need to generate a horizontal impulse to propel our centre of mass forwards with the hamstrings contributing up to 15% of this total impulse (Pandy et al., 2021). During the stance phase of sprint acceleration internal muscle forces reach up to 4.2× body weight (BW) as the muscle shortens to actively extend the hip; while during the swing phase, hamstring muscle forces reach up to 8× BW acting to rapidly extend the hip towards the ground.
Once at maximal velocity the hamstrings generate rapid hip extension whilst simultaneously decelerating the angular velocity of the lower leg during late swing. At this phase, peak muscle forces, lengths and lengthening velocities occur. Internal muscle forces are reported to reach up to 9× BW (Dorn et al., 2012) with muscle tendon unit length increasing by up to 13% (Schache et al., 2013). Therefore, the biomechanical demands placed on the hamstrings highlights the importance of developing the ability of the hamstrings to develop high and rapid force at long tissue lengths. Thus, contributing to the popularity of the Nordic Hamstring Exercise as a conditioning strategy for hamstring injury prevention.
However, it’s important to recognise that muscle strain injury may also occur during the stance phase of sprinting, where muscle function differs to that during swing. As we contact the ground there is a rise in the ground reaction force which creates an external moment (turning force) acting to drive the hip into flexion and knee into extension. Providing we are able to counteract this force, the hip joint moves from a position of flexion at contact, to extension at toe off; with the hamstrings actively shortening creating positive work/ power at the hip (Figure 2)(Yu et al., 2008).
However, in some instances we see an inability to resist this external force; with the hip dropping into flexion. This movement can be small and therefore not visually seen by the eye. However, when we use 3D motion capture, we can see this movement creates negative work/power at the hip, suggestive of “lengthening” of the hip extensors (Figure 3). Biomechanically we do not know if this represents eccentric muscle action or lengthening of elastic components such as the tendon, but either way, it suggests active tissue lengthening, and thus, increasing tissue strain. (Note: this is based on data from our lab and not published literature).
The stance phase clearly places different demands on the hamstrings, and therefore likely requires different conditioning strategies to adequately prepare the hamstrings to cope with the task-imposed demands during stance. Considering we are trying to prepare the hamstrings to resist/ overcome the external ground reaction force, utilising exercises that mimic this demand and position seems a pertinent addition. Specific examples of exercises I commonly use include bridge variations, starting with endurance capacity work (video 1), progressing to isometric pushes (video 2) and then to more explosive movements including isometric catch variations (video 3) and perturbated roman chair holds (video 4).
The importance of varying conditioning strategies is also highlighted when we acknowledge the differing anatomical structures and functional roles of the hamstrings during running. For example, the semitendinosus has the largest moment arm at the knee and longest muscle fascicle lengths, with onset of muscle activity occurring during early swing (Higashihara et al., 2018). This suggests the semitendinosus may play a role in assisting the biceps femoris in early deceleration of the shank during swing at larger knee flexion angles. In contrast the semimembranosus has a smaller moment arm at the knee than the semitendinosus but is larger than the biceps femoris, along with a greater physiological cross sectional area (Kellis & Blazevich, 2022); this suggests that it may have a greater capacity for generating high force in small knee flexion angles, such as terminal swing and stance. While it is not fully understood how the coordination between these muscles can influence injury, altered metabolic activity between muscle groups has been associated with injury development suggestive of altered coordination patterns (Schuermans et al., 2016).
Considering the anatomical and functional differences between the hamstring muscles, focusing solely upon one method of tissue conditioning is unlikely to prepare the hamstrings for the demands imposed on this muscle group across the running cycle. Therefore, varying conditioning strategies to ensure a range of muscle action types (isometric, eccentric, concentric), at different joint angles, with both hip and knee dominant movement patterns seems a logical consideration when trying to condition this muscle group for the task-imposed demands of sprint running.
What about individual technical qualities?
While the task-imposed demands are well appreciated, individual specific running kinematics (or technical qualities) are often under-recognised in hamstring strain injuries (my personal opinion). This can be easily seen in scientific research and practice, with countless research papers investigating the association between strength related measures, injury development and rehabilitation, but only a small number of papers investigating mechanical factors which may influence HSIs. This is also reflective of clinical practice; where many of us are confident in the assessment and programming of tissue/physical conditioning, but less so for sprint running mechanics.
Several articles are now starting to emerge that call for more holistic approaches to hamstring injury rehabilitation and management. Emphasising the importance of targeting sprint running mechanics alongside tissue specific conditioning (Buckthorpe et al., 2019; Edouard et al., 2023; Macdonald et al., 2019; Mendiguchia et al., 2012). Yet barriers exist to the practical integration of a mechanical approach to injury management, which is possibly explained by a lack of clarity concerning:
- What to assess.
- How to assess running mechanics.
- How to target mechanics within injury prevention and rehabilitation programs.
What to Assess?
The hamstring muscle group have a complex anatomical structure, with direct connections to the pelvis, femur, and knee. The expansive connections and anatomical composition of the hamstrings mean that not only does it have a role in extending the hip and flexing the knee, but will also be influenced by alterations in trunk and pelvis position, rotational movement and stability at the knee and ankle stiffness / force generating qualities. So, in my opinion, we need to consider segments above and below and the quality of individual movement patterns.
Kinematic features influencing hamstring strain can be broadly separated into three main themes:
1. Lumbopelvic control
2. Backside running mechanics and
3. Over-striding.
Many of these features have been shown to influence the applied strain on the hamstrings during running which we have discussed in detail in our Sports Medicine article linked here (Bramah et al., 2023).
1. Lumbopelvic control
Lumbopelvic control is perhaps one of the most common mechanical features reported to be associated with HSIs, yet also one of the most ambiguously defined. We define poor lumbo-pelvic control as the appearance of exaggerated trunk and pelvis movements across all planes of movement. With anterior pelvic tilt appearing as arching of the lower back and separation of the diaphragm from the pelvis (Figure 4A) and rotational control appearing as large movements of the trunk and pelvis in opposing directions, often combined with exaggerated movement of the arms and shoulders Figure 4B.
These movement patterns have been linked to both future hamstring injury (Kenneally-Dabrowski et al., 2019; Schuermans et al., 2017) and have been shown to increase the strain applied to the hamstrings (Mendiguchia et al., 2024; Nakamura et al., 2016). Recent work of Jurdan Mendiguchia reported 5⁰ of anterior pelvic tilt increased the length of the proximal biceps femoris by more than 1cm, with prior work of Nakamura et al. (2016) reporting anterior pelvic tilt to increase the passive tissue stiffness of the hamstrings. Furthermore, modelling work form Chumanov et al. (2007) found the oblique musculature was capable of reducing hamstring tissue strain. Considering the oblique musculature act to control sagittal and transverse plane rotation of the pelvis, this lends some support to the mechanistic link between trunk and pelvis control and hamstring tissue strain.
1. Backside mechanics
Back-side running mechanics is a concept originally proposed by Ralph Mann referring to the proportion of limb movements occurring behind the midline of the body (Mann & Murphy, 2015). Characterised as excessive extension of the trailing limb, back kicking of the leg during swing and large thigh separation angles at touch-down.
At present, the link between back-side mechanics and HSI is only theoretical based on the findings of modelling studies, with no direct evidence linking backside mechanics to hamstring injury. However, it is important to bear in mind that there is also a lack of research investigating the association between the two, with only two prospective studies conducted which incorporate a measure of back-side running mechanics, but failed to identify differences between injured and uninjured athletes (Haugen et al., 2018; Lahti et al., 2022).It is thought that back-side mechanics can influence hamstring strain via two mechanisms:
- Increasing length of the hip flexors
- Driving compensatory mechanics such as over-stride and forward lean.
As the leg kicks up behind the body the length of the hip flexors increases, which can drive the pelvis into anterior rotation inducing strain on the lead leg hamstring. The already mentioned work of Chumanov et al. (2007) found action of the iliopsoas on the trailing leg, can increase length changes on the lead leg hamstring by more than 25mm.
Regarding compensatory mechanics, greater time spent with the limbs behind the centre of mass reduces the time available to reposition the leg for touchdown. This can lead to secondary consequences including over-stride and forward lean (while also being suboptimal for performance). Considering this can occur at phases of the gait cycle where the hamstrings are exposed to high force demands (such as terminal swing and initial contact) lengthening of the hamstrings induced by mechanics may increase the risk of strain injury.
3. Overstride
Examples of an over-stride running gait can be seen in figure 5A; with the foot landing ahead of the centre of mass, ankle joint located head of the knee and in some instances an increase in forward lean with a heel strike / rearfoot contact. From a mechanical perspective overstriding increases the distance between the ground reaction force vector and the hip joint centre while shifting our centre of pressure posterior on the foot. This can lead to a reduction in ankle joint contribution and an increase in hip contribution to the total work done during stance. Figure 6 provides an example of data collected using 3D motion capture for two athletes running at the same speed; athlete A overstrides, and athlete B does not. What we can see from the figure is that athlete A experiences greater hip work across the stance phase of gait when compared to athlete B. If repeated over several loading bouts it is possible that this may lead to early muscle fatigue and reduced capacity to tolerate further application of strain.
How to assess running mechanics
While it might be accepted amongst some practitioners that mechanics can influence HSIs, a major limitation has been how we can practically assess mechanics. Current methods have been limited to two extremes: 3D motion capture or our own eye. Motion capture technology is expensive and not widely accessible for practitioners, while use of our own coaches eye opens the door for ambiguous interpretations of movement patterns which is even more challenging if we are new to observing movement patterns.
To overcome these practical limitations and provide a middle ground to assessment, we recently developed the Sprint Mechanics Assessment Score (S-MAS) (Bramah et al., 2024). The S-MAS is a 12-item qualitative scoring tool using slow motion footage to evaluate the overall quality of sprint running mechanics. Individuals are scored for the presence or absence of 12 mechanical features (gaining 1 point for the presence of each feature), with the total score quantifying their overall sprint movement quality; a score of 0 can be viewed as having “optimal” sprint running mechanics and 12 considered “sub-optimal”.
The score takes approximately 3 minutes to rate a player, with our current data suggesting scores of 4 – 5 are considered average values amongst team sports. For practitioners who may not feel confident or are uncertain about evaluating sprint mechanics, the S-MAS provides a quick and simple method of screening, with the composite nature of the score building practitioner confidence as to whether sprint mechanics are considered a factor which might be worth considering within intervention programmes. The score can also be separated into sub-components of overstride, lumbo-pelvic control and back-side mechanics. So, if you have an athlete who scores high and you think may benefit from improving motor patterns, you can use the score items to decide where you need to focus your interventions towards.
Full details of the S-MAS are available here: The Sprint Mechanics Assessment Score
Targeting Running Mechanics in Rehabilitation
The final limitation for many of us is how can we influence mechanics in practice? Many of us will often think that targeting physical qualities will lead to improved motor patterns (e.g., abdominal muscle strengthening leading to improved trunk control during running). But this is not the case. Several studies have shown that physical factors do not strongly relate to movement qualities, with only small correlations between the two (Baggaley et al., 2015; Rodriguez et al., 2020). Furthermore, it has been shown that gym based conditioning strategies focusing solely on physical qualities, do not transfer to improvements in kinematic features during running (Mettler et al., 2019; Trowell et al., 2020; Vannatta et al., 2021). Therefore, there is a need to think wider about how we address movement patterns in practice.
Motor development theories suggest movement patterns immerge from the dynamic interaction between individual factors, task demands and the environment. Whilst physical factors are clearly a prerequisite for movement, we need to challenge an athlete’s ability to integrate physical qualities into task specific movement patterns. A common framework I use in practice figure 7) starts by braking the task into smaller parts (part-practice) where the task requires the athlete to coordinate movement patterns similar to those required in the whole task but in a slightly more controlled task and environment. Progressing to tasks which are less controlled, more unstable and in chaotic environments. However, a key to all movement training is that the athlete must first understand what they are trying to achieve, and second, be motivated to improve. Without this any attempt to retrain movement will likely be futile.
Using lumbo-pelvic control as a specific example, once physical qualities have been developed, we may first challenge the athlete to control fast dynamic drills with trunk perturbations before bringing this in to run specific movements. Restricting arm movements during drills using medballs or dowels can be a great way to facilitate/ challenge trunk control to begin with; gradually progressing from gym-based movement control to recreating the same movement patterns into running progressions. This is just one example of many that can be used to help facilitate a change in technical qualities. The important thing here is to facilitate thought as to how we might challenge motor abilities, and gradually transfer this to more task specific movement. While some may question whether it is indeed possible to change motor qualities, evidence is slowly emerging to show that well designed interventions are indeed capable of inducing changes to sprint running mechanics (Iwasaki et al., 2023; Mendiguchia et al., 2022), alongside other multi-directional movements, such as cutting (Dos’Santos et al., 2019).
Summary
With hamstring injury rates continuing to rise and more than 50% occurring during sprint running, there is a clear need for more holistic approaches to injury management. The task-imposed demands placed on the hamstrings during running highlight a need for optimal conditioning strategies and exposure to running loads to adequately prepare the hamstrings for sprint-based running. Considering the hamstrings have a complex anatomy and likely different roles across the gait cycle, conditioning strategies should reflect this variability. To date, the technical aspects of sprint running mechanics have been an overlooked factor in both research and practice. This is likely explained by the practical challenges of assessing mechanics and retraining in-field. However as new evidence emerges to show the ability to assess mechanics in-field and capability of retraining mechanics, there is an opportunity for practitioners to integrate a mechanical approach to injury rehabilitation as part of multi-factorial injury risk management processes.
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