Hamstring injuries in sport are highly pervasive, often representing the most common injury site across a range of sports from rugby to sprinting to American football. One sport in which hamstring injuries have been well examined is that of soccer; during the 2016/2017 English Premier League season, 27% of all injuries suffered were hamstring injuries. This lead to the loss of over 20,000 training days, with the wages of the injured players exceeding £131 million. Alongside this massive financial burden is the issue of future performance decrements; having suffered a prior hamstring injury, players are more likely to suffer a further hamstring injury, an injury at another site, and a reduction in future performance.
The state of research on hamstring injuries
Given the problems associated with hamstring injuries, researchers and sports practitioners have declared war on them, seeking to better understand the underlying mechanisms, and provide interventions aimed at reducing their occurrence. One of the world’s leading research groups in this field are based just up the road from me, at the Queensland University of Technology. This group has lead the way in developing the Nordic Hamstring Exercise, which research shows is effective at reducing the risk of hamstring injury. The mechanisms underpinning this reduction in injury risk are increases in both eccentric strength and muscle fascicle length following an eccentric hamstring loading block, with lower levels of hamstring eccentric strength and shorter muscle fascicles associated with an increased risk of hamstring injury.
However, eccentric loading exercises such as the Nordic Hamstring Exercise aren’t proving as effective in real life as they do in the lab. One of the reasons for this is the poor uptake of, and adherence to, the exercise at the highest levels, in part because of the increased muscle soreness associated with unaccustomed eccentric loading. So, because athletes are less likely to undertake eccentric loading exercises targeting the hamstrings, they aren’t seeing the potential injury prevention benefits.
Genetics, hamstring, and finding optimal training
All of this leads me to my most recent paper, which was published in August in the journal Medical Hypotheses with my doctoral supervisor John Kiely as co-author. The aim of the paper was to explore whether genetic information could be useful in both predicting hamstring injuries, and in providing guidance on the optimal training methods to reduce the overall risk.
Because genetic variation has previously been shown to influence the magnitude of adaptations following mechanical loading, there is the potential that knowledge of genotype may better inform training program design. I was part of a group that demonstrated this from a general strength training perspective back in 2016. This has been quite sparsely studied in terms of strength increases following eccentric loading, but one gene that could potentially have some influence is ACTN3. A small change in this gene has the potential to slightly alter the relative proportions of muscle fibre type, which is turn may alter the hypertrophic response to loading. Similarly, one of the adaptations seen following a period of eccentric loading is an increase in muscle fascicle length; here, a small change in a gene called TTN, which encodes for titan, a structural protein found in muscle, can affect muscle fascicle length, suggesting that this SNP has the potential to impact the increases in fascicle length expected following eccentric hamstring loading.
Another area where our genes may play a role is how quickly we can recover from eccentric loading. This has been somewhat better studied, with an exceptional review on the subject published in 2016. Again, ACTN3 has the potential to modify the muscle damage response to eccentric loading, as do genes such as MLCK and CK-MM. Alongside specific muscle damage, genes such as IL6 can alter the inflammatory response to eccentric loading. As such, it might be possible, following further research, to use genetic information to place eccentric loading at the optimal point in the training week. Individuals who are likely to exhibit greater amounts of muscle damage or inflammation following loading should undertake it further away from competitions (in the case of team-sport athletes with long seasons), or at the end of the training week (in the case of short season athletes with longer training blocks), reducing the negative influence of post-exercise soreness on subsequent performance.
Similarly, from a session design standpoint, those expected to have greater muscle damage may respond better to lower session intensities and volumes, at least in the initial training phrase. Finally, nutritional support could be provided to those expected to experience greater inflammation in order to enhance recovery.
Genetics, hamstring, and injury prediction
A second area where genetic information could potentially help is that of injury prediction. A number of genetic variants, most notably in the genes COL1A1 and COL5A1, have been implicated in increased risk of injury in ligaments and tendons, with some others linked to muscular injuries.
From a hamstring specific perspective, we were only able to find one paper exploring the genetic influence; here, researchers recruited 107 elite male soccer players, and followed them over 5 seasons, recording the rate of hamstring injury. They found variants in five genes were strongly associated with the risk of hamstring injury, as was age and previous hamstring injury. Using this information, they built a model which was able to predict the occurrence of prior hamstring injury with around 75% accuracy. However, when they used the model to predict hamstring injury in the following season, in performed no better than chance. This is likely due to the highly multifactorial nature of injuries, and it means that, for now, we can’t use genetic information to predict hamstring injuries.
The way forward
In summary, the goal of my recent paper was to propose a hypothesis, where we speculated a way through which genetic information may enhance hamstring injury prevention techniques. Ideally, over the coming years more research will be done this area, shining further light on how each individual responds to loading, and better informing coaches and athletes as to a more efficient way to train. In doing so, hopefully there will be a better uptake of the types of exercises reliably shown through a number of studies to reduce hamstring injury risk, thus protecting athletes from such a potentially performance limiting injury that can be highly avoidable.