As a former Olympic sprinter, I suffered from my fair share of hamstring injuries across my career. And I’m not alone; these types of injuries are pervasive across any sport that requires some form of running, representing the most common non-contact injury type in soccer, rugby union, cricket, American Football, and athletics.
Hamstring injuries are bad news. They are costly to clubs, players, and athletes; in athletics, if you can’t race, you don’t get paid. For sporting clubs, injured players represent an employee who is being paid, but produces no output, and research shows that, typically, the fewer injuries a team suffers, the better their performance across a season. More insidiously, a previous hamstring injury is a risk factor for a future injury, both to the hamstrings themselves, and other structures such as the knee ligaments, and a single hamstring injury may have long-term performance consequences.
As a result, there has been increased interest over the last decade or so in better understanding why hamstring injuries occur. At present, it appears that the majority of hamstring injuries occur while the hamstring is both contracting but elongating — termed an eccentric contraction. Eccentric contractions are uniquely damaging to the muscle as they cause damage to the ends of the individual units that make up our muscle fibers. As such, lower levels of eccentric strength have been shown to be a major risk factor for hamstring injuries, as have shorter muscle fascicles.
Increasing the eccentric strength of the hamstring muscle group through eccentric hamstring exercises such as the Nordic Hamstring Exercise has been shown through randomized controlled trials to reduce the prevalence of hamstring injury, and, as a result, they are often put into elite athletes training programs. However, adherence to these exercises is often low, as athletes and players report increased feelings of soreness following their use, which is why we haven’t seen quite as much of a reduction in hamstring injuries as we would like.
Alongside this “war” on hamstring injuries, there has been an increased interest in understanding how a genetic variant may increase someone’s risk of injury, as well as alter their response to injury prevention training. Whilst our DNA is around 99.9% similar, there are small variations between us that can make us slightly different. One type of variation is Single Nucleotide Polymorphisms (SNPs), which is where one of the small building blocks of DNA is switched for another, which can alter the protein made by the gene. In a recent article, which forms part of my doctoral thesis, myself and my academic supervisor John Kiely aimed to explore which SNPs may alter hamstring injury risk and prevention.
When it comes to SNPs associated with muscle injury, there is a surprising dearth of research in this field. A number of SNPs have been associated with ligament and tendon injuries (although not equivocally), with others thought to be linked with muscle injury in soccer players. However, because sporting injuries are complex and multifactorial, we need to test how well these SNPs predict future injuries, as opposed to explaining previous ones.
Recently, a group of authors did just this, following 107 elite soccer players across 5 competitive seasons. They found that five SNPs, along with age, were associated with hamstring injury during those five seasons. However, when they attempted to use this information to predict future injury in a holdout data set, it only performed as well as chance. This demonstrates the difficulty in using genetic information to predict an injury, with more research being required to drive this field forward.
There is better evidence that genetic information may assist in designing an optimal injury prevention programme. A number of genetic variants have been linked to increased improvements in strength following training, and, as lower levels of strength increase injury risk, this can be important. In 2016, we were part of a study that used a 15 SNP panel to improve training outcomes in a group of university-level athletes; as such, we believe that such information might be useful in identifying those athletes expected to see smaller improvements in strength following injury prevention training, and altering the training stimulus to better enhance their training outcomes. Additionally, a number of SNPs have been linked to increased muscle damage and/or inflammation following eccentric loading, and so knowledge of an athletes genotype may assist in understanding their required post-exercise recovery times.
As a result, while it appears that certain genetic variants may increase the risk of a hamstring injury, at present it is not possible to use this information to predict future injury with any accuracy. However, given the potential role of genetic variants on the response to eccentric loading, which is a key component of hamstring injury prevention training, the use of genetic information in the future may hold promise. To get to this point, we need to better understand which SNPs alter both training adaptation and the post-exercise muscle damage response. Then, we need a number of trials that explore whether personalized hamstring injury prevention training — dictated in part by genetic information — outperforms standard advice. In doing so, we should hopefully protect athletes from injuries that often strike at the most inopportune times.
These findings are described in the article entitled Hamstring injury prevention: A role for genetic information?, recently published in the journal Medical Hypotheses. This work was conducted by Craig Pickering from the University of Central Lancashire and DNAFit and John Kiely from the University of Central Lancashire.
Conflicts of Interest
Craig Pickering is an employee of DNAFit LifeSciences, a genetic testing company. He received no financial incentives for the preparation of the original manuscript, which was prepared as part of his doctoral studies, nor for this article.