Hamstring Injuries - Can We Screen and Prevent?
Hamstring injuries are far more common in elite sports than anybody would like, they can be notoriously frustrating and problematic for both the athletes and clinicians alike. The old analogy of prevention is better than cure is the centre of attention now more than ever. van Mechelen et al (1992) described a 4-part model of injury prevention which highlights the appropriate measures to take when suggesting prevention protocols. Finch (2006) however identified a fundamental flaw in van Mechelen et alapproach. Finch states that it does not consider the need for research into implementation issues, once prevention measures have been proven effective. For the most part, Finch’s 2006 model follows suit, but with some key additions. Stage one suggests that clinicians survey the injury. There are a number of concerning statistics regarding the incidence rates of Hamstring injuries in running sports such as American Football, Football (Soccer), Australian Rules Football just to name a few. Hawkins, Hulse, Wilkinson et al (2000) carried out an audit of injuries in English Professional Football. The audit was carried out over 2 seasons and found that 12% of all injuries sustained at the professional level were that of the hamstrings. They also suggested an incidence rate averaging 5 injuries per club per season. Meaning 15 games and 90 days of competitive football were missed over that time frame. These statistics make Hamstring Injuries the most prevalent injury in English Professional Football. What is of more concern is the fact that Hamstring injuries in football are also on the rise, despite our increasing interest in treatment and prevention. Ekstrand, Walden, Hagglund (2015) carried out a 13-year longitudinal study investigating the incidence rates on hamstring injuries in professional football (soccer). The study involved 36 clubs across 12 European countries. They concluded that hamstring injuries are on the rise, with an average of 2.3% year on year increase in incidence. What was also of interest was that they demonstrated a 4.0% increase in hamstring injuries occurring in training. Similar findings to Football (Soccer) were found in Australian Rules with Orchard and Seward (2002) stating hamstring injuries attributing to 15% of all injuries in the Australian Football League. They also found that Australian Rules football has a higher incidence of hamstring strains than the English Football League and average 6 injuries per club, per season. With an average of 21 missed games per season. American football again is found to have a high incidence of hamstring injuries, Elliot et al (2011) carried out a 10 year analysis of Hamstring Injury occurrence in professional NFL American footballers between 1989 and 1999. Over the ten-year period the incidence of injury ranged from 132-210 injuries per season, with 52.7% of reported injuries occurring in training. There were a total of 1716 reported injuries from 1129 different players meaning there was a 16.5% re-injury rate. These epidemiology studies highlight the importance and magnitude of the problem in question.
The second stage of the model states that to prevent injuries, we must understand the causes, aetiology and mechanism of injury. As already stated in the study by Elliot et al, the majority of hamstring injuries sustained are usually during sprinting. Woods et al (2004) reiterated this but went one step further to also suggest the majority involves the long head of the Bicep Femoris (BF). The results of Askling, Tengvar, Sartook et al (2007) second this finding. In a small sample size of 18 elite sprinters, 100% of the primary injuries were all located at the Long head of the bicep femoris. It is generally accepted that this type of Hamstring injury is likely to occur during sprinting, however, the phase of the sprint/running cycle at which injury is more likely to occur is still a controversial topic. Wiemann and Tidow (1995) state In sprinting athletes muscles have 2 functions, they must accelerate the body and propel it in a horizontal direction to the finish line, but in doing so they must counteract the force of gravity acting in a vertical direction. Mechanically, regardless of the duration of muscle activation, both tasks can be achieved only when the athlete has contact with the ground, namely during the support phase, of about 80ms. Orchard (2012), argues that hamstrings are likely to get injured during the early stance phase. This thought process is not new, and was suggested during early investigations (Mann, R, 1980,1981). Mann suggested because of the presence of high hip and knee joint reaction forces secondary to the ground reaction force (GRF). This would be in support of Wiemann and Tidow (1995). It is argued that the stance phase is when tissues must absorb the vast majority of forces, and that GRF’s during sprinting are more than 300% of body weight (Orchard, 2012). However, this has based his thought process on a ‘smoking gun’ video of a case of calf strain. Orchard, Alcott, James et al (2002) show what they feel is the exact moment of disruption of gastrocnemius fibres during late stance/push-off. The evidence put forward cannot be adapted to be used for hamstring injuries; it is short of validity and reliability due to the lack of supporting evidence and its small sample size (1). Orchard himself states that there is no direct evidence for this in hamstring injuries, as there has been no research assessing “in vivo” forces acting on the hamstring. Orchard goes on to concede the importance of Hamstring activity in the late swing phase of the gait cycle, but argues that just because failure to complete their job in that phase actually leads to the tear or injury, it just subjects them to increase likelihood of injury and that GRF's are the main culprit for the tear. Many researchers disagree that the early stance phase is the cause of injury, and rather the late swing phase. Biomechanical studies show hamstrings undergo an active lengthening contraction during the second half of the swing phase, reaching peak stretch just prior to foot-strike. (Thelen, Chumanov, Hoerth et al, 2005 and Schache, Dorn and Blanch et al, 2012). Schuermans et al (2014) expand on this and state that during the swing phase, the semitendinosus (ST) and the BF engage in max eccentric contraction. They act as synergists, alternating in a complex neuromuscular pattern, where the BF is predominantly active in the mid to late point of the swing phase, it is at the late swing phase, where the majority of injuries occur, that the ST muscle takes over. This suggests, that in the in running injuries where the BF is involved, it may be due to a lack of control between the bi-articular hamstrings, or potentially that the BF muscle is exposed to increased workload during the terminal swing stage. This over activity has been shown to be true when using functional MRI (fMRI) to assess metabolic activity in athletes who have sustained a previous injury but are now asymptomatic. Schuermans et al (2016) found that in the group who had sustained previous injury the metabolic rates of the BF and ST when exercised to exertion were more symmetrical, suggesting that there is an increased activity in the BF and a decrease in the activity ST. Suggesting increased metabolism of the BF may be a cause of injury. This supports Orchard when he suggests that there is potentially a lack of control during this phase. However, it may also be an effect of the injury that has not received correct rehabilitation.
Understanding the mechanisms of injury is vital to stage the later stages of Finch’s TRIPP model. Stage three regards developing preventative measures, based on a firm understanding of the mechanics. It involves the identification of potential solutions to the injury problem and development of appropriate preventive measures. One of the ways to potentially do this is via screening protocols. One of the most popular screenings used today in elite sports is the Functional Movement Screen (FMS). FMS plays a pivotal role in several premier league football clubs, as well as NFL and MLS teams to give the medical staff information on individual correctives. But the question is, should it? FMS takes into consideration previous injury as a number one cause of injury, however states that asymmetries are the secondary cause. Bushmann et al (2016) looked at FMS specifically in a large sample of soldiers aged 18-57, and found that soldiers who scored less than or equal to 14 were more likely to be injured than those who scored over 14, however when taking into consideration other factors such as previous injury and age, that are known external predictors of injury, they concluded using FMS to screen for the injury risk was not to be recommended in this population. Bahr (2016) suggests that mass screenings are ineffective more on a sensitivity and specificity level, this due to the large overlap in high-low risk scores leading to difficulty in establishing a cut-off point. Suggesting FMS should play a limited role in screening for Injuries in the sports. It has been suggested that more targeted and specific screens are more effective and should play a bigger role (i.e. Strength benchmarks). Croisier et al (2008) looked at isokinetic strength testing in Footballers (Soccer) from the US. Of the 687 players only 462 players were reachable for a complete follow up, of those, 35 hamstring injuries were recorded. The rate of muscle injury was significantly increased in subjects with untreated strength imbalances in comparison with players showing no imbalance in preseason. Yeung et al (2009) Identified that the risk of hamstring injuries increases with a decrease in concentric Hamstring:Quadriceps ratio. A ratio of less than 0.60 was found to increase injury risk by 17 times. However, Bennell et al (1998) subjected a relatively small sample of 102 male Australian Rules footballers; it included both amateur and professional athletes. Hamstring to quadriceps ratios of less than 0.60 were also common in these players, but despite this they concluded the risk of hamstring injury could not be predicted by findings in this study. It could be argued that these results are flawed due to the participants’ level of play. In the amateur game, results are often flawed due to participants’ alternate non-relative activities (i.e work) as well as compliance. Freckleton (2013) has quite possibly the most promising screening protocol. The study was carried out on 482 Australian Rules Footballers, a total of 28 hamstring injuries were recorded. 16 right, 12 left. Players who sustained an injury to the right hamstring had a significantly lower mean right Single Leg Hamstring Bridge (SLHB) score. This however wasn't true for the left-sided hamstring injury, the injured group was more likely to be left leg dominant, older athletes and there was a trend towards a history of left hamstring injury. More evidence needs to be gathered on this prior to confidently show its role in injury prevention
Stage 4 of Finch’s model involves the Ideal Conditions evaluation of what is raised in the previous three stages. The majority of studies mentioned are carried out in situations that are most convenient for researcher, not necessarily the real world environment. Having said that, Finch believes this is difficult to overcome, and can still significantly contribute to relevant knowledge. A Perfect example of this is Nordic Hamstrings. Petersen et al (2011) demonstrated the importance of these exercises. The intervention group performed additional eccentric loading exercises, in this case Nordic Hamstring, this was compared to the control group who did not. The study showed a significant decrease in overall, new and recurrent acute hamstring injuries in the intervention group. van der Horst et al (2015) also found similar findings and supported the evidence found by Petersen et al (2011). There was a significant decrease in the incidence in hamstring injuries, 0.25 per 1000 hours in the intervention group compared to 0.8 per 1000 hours in the control group. Interestingly, there was no difference observed in the severity of injury sustained between the two groups. Andersen et al (2011) and van der Horst et al (2015) involved a total of 1521 players; the participants were split into 753 players and 768 players in the nordic group and control group respectively. The nordic groups sustained a total of 25 injuries compared to that of the control groups who sustained 77 injuries, meaning the nordic groups obtained a 67.5% decrease in incidence rates of hamstring injuries. Individually, the sample size is of decent size, however combined, the results show good evidence for the use of Nordic’s as an intervention for hamstring injuries. Bourne et al (2016) potentially discovered the reasoning for the effectiveness of Nordics on injury prevention. When looking at fMRI they found there was an increased metabolic activity of the ST, suggesting this would address the potential pre-cursors identified by Schuermans et al (2016). Having said that, there are limitations to the fMRI. Using T2 weighted images as a response to exercises is “highly dynamic” and is multifactorial. Metabolic capacity and vascular dynamics of the active tissue can often affect outcomes. Again, these studies have been implemented in controlled “lab” environments but have been shown to have significant impact on prevention of Hamstring injuries across several sports.
Stage 5 of Finch’s model describes intervention context to inform implementation strategies and how to implement this into real world situations. One intervention that has been seemingly successful in incorporating elements of the lab interventions to on field is the FIFA 11+. Soligard, Nilstad and Steffen et al (2010) investigated the effects of the FIFA 11+ program in female players for regional clubs associated with the Norwegian Football Association. The study had a relatively high compliance rate with 77% of all training sessions and matches with a large sample size (1540). The investigations found that the risk of overall and acute injuries was drastically reduced by more than a third in players who had a high compliance. This study can only be attributed to the female population. However, Silvers-Granelli et al (2015) discovered that the FIFA 11+ program has successfully implemented in male Football (soccer) also. It statistically decreased injury rates and time loss due to injury in their cohort. The control group consisting of 34 teams, 850 athletes, who received no intervention, sustained a total 665 injuries over the course of the 2012 NCAA season. This was significantly decreased in the FIFA 11+ intervention group that consisted of 675 players from 27 teams. The intervention group only sustained 285 injuries over the same time frame. However, this intervention can only be applied to Football and evidence suggests it is effective in injury prevention in this sport. As of yet, there has been no other intervention of its kind put in place amongst other sports. However, similar things are being attempted in Rugby.
The evidence is seemingly more positive for prevention protocols than that of the proposed screening methods mentioned above. For the most part, mass screening methods all seem to lack evidence and reliability for reducing injuries, and as such should play a limited role in injury prevention protocols. Having said that, myself and the team at CT-DRU are currently working on an Injury Screening Protocol based on a combination of movements and orthopaedic tests in order to assess correlation between Pelvic Girdle Dysfunction and Non-Contact Injuries sustained 5 months post screen. This will hopefully be going through publication phase in the forthcoming months.
There is also promising data looking at individually screening strength benchmarks. Evidence is mixed when investigating the Hamstring:Quadriceps ratios, however having said that, the positive data proposed by Yeung et al (2009) seems more convincing than that of Bennell et al (1998) due to the mixed level of participants and sample size. Freckleton’s study of the SLHB is promising, however, needs a larger sample size and should be carried out across more sports. This should at least be considered to play a role in the pre-season assessments due to its simplicity. Prevention is seemingly becoming more effective, despite our lack of ability to identify those at risk, outside of know internal risk factors (Age, Previous Injury etc.). Nordic exercises and eccentric loading techniques have overwhelming evidence for their effectiveness. Given the ongoing debate on the most likely mechanism of injury for hamstring injury, it could be argued that the exercise is carried out at the incorrect angle as it is not carried out at terminal knee extension where EMG studies show the hamstring to be most active.
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