Hamstring injuries in football, an unsolved puzzle
As example of what I just said, the research carried out by Ekstrand et al. (2016, 2022) showed an increase in hamstring injuries from the 2001/02 season to the 2021/22 (Image 1A and 1B). However, in the following image (Image 2) we can see how the number of published scientific articles has increased considerably too. In this case, I have written “Hamstring injury” in Pubmed (580 published articles in 2021).
Image 1A. Increase in hamstring injuries from 2001/02 to 2013/14 season. (Ekstrand, et al., 2016)
Image 1B. Proportion of all reported injuries that were diagnosed as hamstring injuries (top) and the proportion of all injury absence days caused by hamstring injuries (bottom) (Ekstrand et al., 2022)
Image 2.
Table 1. Origin, insertion and functions of hamstrings muscles. (Rodgers & Raja, 2020)
Image 3. Hamstring muscle complex
Image 4. ST, gracilis and sartorius common insertion (LaPrade, et al., 2015)
I can’t name all the connections from each article in this post, but I recommend taking a look at the bibliography for anyone interested in hamstring anatomy.
Image 5. Figure and description reproduced from De Maeseneer et al.(2014)
Image 6. Figures and descriptions reproduced from Panayi (2010)
Image 7. Tibial external rotation
Image 8. Large hip flexion with knee extension in football actions
Image 9. The running gait cycle (Danielsson, et al., 2020)
Although almost all research assumes that the hamstrings perform an eccentric action during high intensity running, a few years ago Van Hooren and Bosch (2017) challenged this claim with a rather interesting theory.
According to their theory, the contractile element of the muscle (Muscular fascicles) would not perform an eccentric but isometric action, while the series elastic element (Tendons, aponeurosis and fascial and connective tissue) would elongate and then recoil in preparation for ground contact. In this case, the injury would be caused by the muscular inability to maintain the isometric action when external forces are too high, so the muscle would elongate in an eccentric contraction that could lead to injury. Furthermore, a loss of coordination in the pelvic area is also proposed as an injury mechanism, since it can increase the distance between muscle attachments, also causing an eccentric muscle action.
To explain the muscular elongation of the hamstrings during the gait cycle, they introduce the concept of “muscle slack” (Image 10), which is defined as the delay between the contraction of the muscle fibers and the beginning of the stretching of the series elastic elements. (Van Hooren & Bosch, 2016). As an example, we could compare it to how an elastic band works when we pull it. When we start to pull, it offers minimal or no resistance until it stretches. Once it has enough tension, it begins to generate force and resistance. The same would happen in the hamstring muscles, which first would be in a relaxed position until they receive the neuromuscular signal that activates muscle contraction. This would take out the muscle slack to align the muscle tendon unit to the point where the force is transmitted to the elastic elements in series.
Image 10. Images and descriptions reproduced from Van Hooren & Bosch (2016)
Strength have been measured with quite ambiguous results. Although hamstring muscles strength seems to play a crucial role in injury prevention and deficits are associated with hamstring strain injuries (Green, et al., 2020), evaluation methods differ a lot between each other, as well as the results and associations with hamstring injuries. Some authors point to strength imbalances as a risk factor (Heer, et al., 2019), while others suggest that only eccentric (Breno, et al., 2020) or concentric (Shalaj, et al., 2020) strength influences injury risk.
Image 12. BF peak strain during sprinting and flexibility score (Passive Straight Leg Raise). The largest peaks during sprint are related to lower range of motions in the test. (Wan, et al., 2017)
Actually, research has already shown more powerful and less flexible football players to be at greater risk of sustaining a hamstring injury (Henderson, et al., 2010), which further supports the relationship between flexibility and injury risk.
Other important factors are those related to motor control. For example, increases in hip flexor activity increase the stretch experienced by the contralateral biceps femoris during the late swing phase (Shield & Bourne, 2018). Although more research is needed in this topic, lack of neuromuscular control of lumboabdominal muscles has been associated to increased injury risk too.
A correct functioning of the gluteal muscles is essential to protect the hamstrings (Edouard, et al., 2018). For instance, weakness or wrong activation pattern of the major glute relative to hamstrings and low back muscles in tasks as the Prone Hip Extension (Image 13), are also associated with hamstring injury risk (Schuermans, et al., 2017).
Image 13. Prone hip extension test. (Schuermans, et al., 2017)
Gluteus medium and minimum will also have a protective function, not only on the hamstrings, but also on knees, low back and lumbopelvic-hip complex (Buckthorpe, et al., 2019).
Previous injured athletes also show different kinematics during running compared to non-previously injured. Daly et al. (2016) showed an increase in anterior pelvic tilt and hip flexion during late swing and a greater knee medial rotation during early stance in athletes who had suffered hamstrings injuries in the past. Moreover, in a review from last year, Danielsson et al. showed two studies which reported that running with a forward trunk lean can increase hamstring injury risk.
Competition demands and the player´s desire to play again, can cause us to rush too much, but previous research has shown that sudden big changes in training load are related to increased injury risk (Gabbet, 2016). Wrong “Return to Play” planification could be one of the reasons for the high recurrence rate in hamstring injuries. Further, high-speed running exposure has been also associated with increased risk (Green, et al., 2020). However, players who are used to train high intensity actions show a lower injury rate, so sprint training could also provide a protective effect against injuries.
There is a biomechanical approach related to what I just said from which we can take in many interesting ideas about injury rehabilitation and sports performance: Myofascial chains. This approach assumes that the muscles of the human body do not function as independent units but as part of a tensegrity-like, body-wide network with fascial structures acting as linking components (Wilke, et al., 2016). Up to 11 muscle chains or myofascial meridians have been identified.
The superficial back line (Image 14) connects the hamstrings with several muscles of the posterior part of the body in a long line which runs from the frontal bone in the head to the plantar fascia (Wilke, et al., 2016). But the important fact about this connection is the force transmission between components, as movement or tension in one part will affect the others (Otoni do Carmo, et al., 2013).
Image 14. Myofascial meridians (Superficial back line is highlighted) (Wilke, et al., 2016)
Wilke and Tenberg (2020) investigated how the movement of a muscle affects fascial tissue and its transmission. They evaluated the muscular and fascial displacement of the SM muscle during passive movement of the ankle. They found a strong correlation between fascial movement and muscle displacement and also confirmed the force transmission between the gastrocnemius and the hamstrings.
Ankle mobility is an important factor in lower limb injuries prevention and, if we think about the conclusions of the research above, mobility restrictions of this joint will increase the muscular tension of the posterior chain, something that doesn’t suit well the hamstrings.
Force transmission not only occurs in adjacent muscles, but throughout the entire muscle chain. For instance, Cruz-Montecinos et al. (2015) reported a high correlation between pelvis motion and the displacement of the deep fascia of the medial gastrocnemius (Image 15), and even the influence of hamstrings flexibility on the thoracic posture during maximum trunk flexions has been recorded (Miñarro & Alacid, 2010). Although it is sometimes necessary to train certain parts of the chain in isolation to correct deficits, our work will not be complete until we achieve efficient movement of the entire muscle chain.
Image 15. Displacement of medial gastrocnemius fascia during pelvic motion. (Cruz-Montecinos, et al., 2015)
Very nice piece Raúl, interesting to hear and see your vision on hamstring injuries, founded by some good evidence. Looking forward to hearing more and discussing some of the elements 😉 .