High speed running activities have repeatedly been shown to be the most common cause of HSI’s in a variety of populations and sports (Brukner et al, 2014). They generally occur in the second half of the swing phase when the hamstrings are at their greatest length and are generating maximum tension. In this phase the hamstrings are contracting eccentrically to control knee extension in preparation for foot strike. In this position, the long head of the biceps femoris is under the greatest musculotendinous stretch – and is therefore most susceptible to injury.
The mechanism behind HSI occurrence during high speed running tasks is due to a failure of the tissues to tolerate the forces applied during the task. Some researchers argue this is due to an accumulative “weak link”, where repetitive active lengthening of the sarcomeres due to eccentric muscle actions, generate chronic micro damage in the muscle until the HSI occurs. But more commonly the literature refers to a ‘catastrophic’ type event in which the excessive force applied to a hamstring during high-speed running results in a HSI as the hamstring has inadequate strength to cope with the action.
HSI’s are one of the greatest challenges for physios working with athletes as they are known to be the most common injury in sports that involve high-speed running. Within football, HSI’s have been found to account for 12% of all injuries reported by topflight European football teams and a squad of 25 players will incur approximately seven HSIs per season (Ekstrand et al, 2011).
Furthermore, recurrence levels are also very high, ranging from 12% to 63%, with the first month after return to play being the highest risk time for recurrence. The risk is thought to remain elevated for at least 12 months post injury (Brukner et al, 2014). Therefore, it is vital that physiotherapists working within football and sport know how to effectively treat and manage HSI’s and crucially, we must know how to best prevent reoccurrence as recurrent HSI’s are generally more severe and result in significantly more lost time in comparison to the initial injury (Orchard and Best, 2002).
Risk Factors
The three risk factors which have the most evidence and are consistently linked to the generation of new and recurrent HSI’s are:
1. A previous HSI (strongest predictor for a future injury) – can lead to various other risk factors such as muscle imbalances and decreased strength.
2. Reduced Eccentric hamstring strength – athletes who produce a relative eccentric strength of < 3.45 N per kg are 5x more likely to experience a HSI (Opar et al, 2015).
3. Reduced fascicle length in the hamstrings – biceps femoris long head fascicles which are less than 10.5cms are four times more likely to suffer a HSI. Furthermore, for every 0.5 cm increase in fascicle length, the risk of future HSI is reduced by 74% (Timmins et al, 2016).
Treatment Stages:
The acute stage focuses on early optimal loading of the muscles through painfree isometrics. This is then followed by progressive strengthening of the hamstrings, which may start with concentric exercises to avoid placing the damaged muscle in a vulnerable lengthened position but will gradually progress to utilising eccentric exercises.
As the athlete progresses, eccentric movements will be vital as we begin to strengthen the hamstrings whilst they are in a lengthened state to imitate the mechanism of injury. This is where Nordic’s will be a vital component to the athlete’s rehab. It’s also vital to incorporate a structured running program to progressively return the athlete to full sprinting. Once preinjury levels of sprint speed and strength have been achieved, the athlete can enter the return to play phase.
This phase will require the athlete to build back their tolerance to competitive performance and should involve a criteria-based approach to identify when they are safe to fully return. As recurrence of HSI’s is so high in the first few weeks post return to play, physios and coaches must ensure that an athletes training load is closely monitored, and objective measures are frequently taken to identify any deficits or regressions in performance. This is also a vital stage where the utilisation of Nordic curls is paramount and should be incorporated into the athletes usual training load.
Nordic Hamstring Exercise (NHE’s):
So, what is the Nordic hamstring exercise? The Nordic hamstring curl involves the athlete kneeling on a pad and lowering their body under control whilst the ankles are held in place. The athlete should attempt to resist the forward‐falling motion using their hamstrings to maximise loading in the eccentric phase. They should keep their hips fixed in a slightly flexed position throughout the whole range of motion, and attempt to brake the forward fall for as long as possible using their hamstrings. The athlete will then use their hands to stop the fall, and push themselves back into the starting position minimising loading in the concentric phase. The movement should be slow and controlled and the athlete should only put their hands out in front of them when they can no longer rely on their legs.
How does it work:
The exercise improves the hamstring muscles eccentric strength and enhances the muscles ability to produce and withstand forces during running and other activities. This occurs because eccentric training elicits greater adaptive responses when compared to concentric training regarding both muscle strength and architecture. This is a result of the different mechanisms used to generate force, with eccentric actions occurring due to active lengthening of the fascicles, and concentric actions due to active shortening. The slow eccentric nature of Nordics provides a stimulus, whereby the myosin heads are already attached to actin and forced to detach by the lengthening of the cross bridges. As the exercise is maximal in nature and results in the hamstring muscles being overloaded past their capacity for maximal force production, a high level of muscle damage is incurred and subsequent delayed onset muscle soreness is likely to be reported.
Research:
The most commonly used protocol for Nordics was developed by Mjolsnes et al in 2004, this study used a 10‐week training program for 21 participants, randomised into either the traditional hamstring curl control group or the Nordic hamstring exercise group. This Nordic exercise protocol provides a gradual increase in sets and reps as the weeks progress. The study protocol was designed to minimize muscle soreness and was reported to be well tolerated by the participants, with a 96% compliance rate and no injuries reported during the intervention.
Post intervention, the Nordics group showed an 11% increase in eccentric hamstring torque, as well as a 7% increase in isometric hamstring strength at 90°, 60° and 30° of knee flexion. These improvements were statistically significant compared to the control group. This highlighted that the Nordic hamstring exercise was effective in improving eccentric and isometric strength, which the authors theorised to be advantageous to prevent and rehabilitate hamstring strains. However, as the players were not followed up after the ten weeks, this study could not show if the protocol actually prevented HSI’s from developing throughout the season.
This takes us on to a study by Peterson et al in 2011 who explored the injury prevention effect of Nordics. This study recruited over 900 footballers who were allocated into an intervention group or a control group. Players in the intervention group conducted a similar 10-week progressive eccentric training program as the one described by Mjolsnes, followed by a weekly seasonal program, whereas players in the control group followed their usual training program.
The findings showed that 52 acute HSI’s occurred in the control group compared with 15 HSI’s in the intervention group. The training program was found to reduce the injury rate of new injuries by more than 60% and was highly effective in reducing the rate of recurrent injuries by approximately 85%. Again, team compliance to the intervention was very good, with an average of 91% of the intended training sessions being performed during the 10-week preseason period. However, compliance of the intervention after the first 10 weeks was not recorded so we do not know how well the program was adhered to after the initial intervention period.
Building on these findings, a systematic review and meta-analysis (Van Dyk et al, 2019), including 8 and a half thousand athletes explored if the Nordic hamstring exercise prevented hamstring injuries when it was included as part of an injury prevention intervention. The results indicated a statistically significant and clinically meaningful reduction of 51% of hamstring injuries for all athletes competing at different levels of competition. Additionally, the overall effectiveness of the Nordic exercise remained unchanged despite a large amount of heterogeneity between the studies in terms of age, gender, and type of sport.
However, this systematic review also highlighted a paper by Bahr et al (2015) which demonstrated that despite these beneficial effects and the implementation of the Nordic exercise into football governing body protocols such as the FIFA 11+, the rate of hamstring injuries had not decreased over the past decade in male elite football. To explore the reasons for this Bahr et al invited 50 professional football teams to take part in a survey over 3 seasons. This survey looked at the implementation, effectiveness, and maintenance of the Nordic hamstring programme within each club. Of the 150 club-seasons covered by the study, the programme was completed in full in only 11% of the clubs with 83% of clubs being classified as non-compliant.
These high rates of noncompliance were thought to be due to the high volumes utilised in the Nordic protocol with athletes being required to perform 700 Nordics across 10 weeks. This combination of high volume and the high occurrence of DOMS was thought to be a key reason why in real life situations the 10-week intervention has such low uptake. To improve implementation of this beneficial exercise, research focus had to be redirected towards dose–response relationships, as well as compliance with the prescribed exercise, to try to improve the prevention programmes efficacy.
This leads us onto a systematic review by Cuthbert et al in 2020. This research looked in detail at the physiological effects of the Nordic exercise in 13 studies to determine if lower dosages of Nordic training could still result in HSI injury reduction by effectively increasing hamstring strength and enhancing biceps femoris long head fascicle length. The findings suggest that both high and low volume prescription can produce large-to-very large improvements in both eccentric strength and muscle fascicle length. However, a threshold of 6 weeks as a minimum intervention duration was detected.
Repetitions within sets reached between 8 – 12 towards the end of many of the interventions. Which the authors noted was counterintuitive as traditional strength training recommends 6 or less repetitions at around 85% of one rep max to optimally build strength. With the Nordic exercise being supramaximal and conceptually ‘above 1RM’, the assumption is that fewer reps are needed to create the same stimulus.
The two studies resulting in the greatest increase in eccentric hamstring strength differed greatly in their volume prescription, however, both included periods of ≥ 4 weeks, where volume was not increased. With no increase in volume, the intensity of the exercise may have increased as a result of an increasing breakpoint angle i.e., the individual gets closer to the ground before falling which in turn increases the torque as force is being applied over a greater moment. This increase in intensity, much with traditional strength training, is likely to be the reason for the effectiveness of these two programmes.
The systematic review also attempted to explore compliance rates to assess if any RCT’s had found similarly low compliance to the real-life findings in Bahr et als survey. However, most studies did not report compliance, so this aspect could not be explored. Additionally, the majority of studies failed to report activity levels of control groups and did not use a blind assessor or report concealment of group allocation which all may result in enhanced levels of bias within the studies.
This review demonstrated that a reduction in overall training volume of Nordics need not have a negative impact on the exercise’s effectiveness; and the focus needs to be on allowing the intensity of the exercise to increase. Although a minimal dosage is yet to be determined, the recommendations based on this evidence are to reduce the training volume and progressively increase intensity in order to reduce DOMS and increase compliance.
So, if we delve deeper into the low volume study (Presland et al, 2018) which demonstrated the greatest strength gains in the systematic review, we can have a closer look at their protocol. Their high volume protocol results in athletes performing 440reps by the end of week 6. In comparison, the low volume intervention begins in the same fashion with 2weeks of 40 reps per week, but then reduces the training volume, utilising 2 sets of 4 reps once per week, culminating in 128reps at the end of 6 weeks. The findings demonstrate that high volume Nordic training is no more effective than a low volume protocol for lengthening biceps femoris long head fascicles and increasing eccentric strength.
Petersons study also demonstrated that the time course of architectural adaptations resulting from training or detraining is important. The results of this study suggest that significant increases in fascicle length can be achieved in as little as 14 days of Nordic training. But after a detraining period of only two weeks fascicle length is reduced by around 16% in both groups. This suggests that the removal of an eccentric training stimulus can quickly result in a reversal of fascicle length adaptations which has implications for the application and frequency of nordics to reduce the risk of HSI’s. Both the high and low volume training groups significantly increased eccentric hamstring strength following the 6week intervention. However, whether such low volume prescriptions of Nordics do result in actual reductions in HSI risk requires further investigation via an RCT.
So what does all of this mean for our athletes? To help prevent recurrent HSI’s from occurring, a Nordic protocol of at least 6weeks can be put in place during acute rehabilitation to enhance bilateral eccentric hamstring strength and lengthen the fascicles of the long head of his biceps femoris reducing the risk of HSI injury. Furthermore, a Nordic protocol should be continued after return to play, to prevent detraining and the subsequent shortening of the fascicles which would increase injury risk once again.
As we know that eccentric training can cause severe DOMS which can negatively impact training and performance, it may be best advised to utilise the low volume protocol from Preslend et al and incorporate 2 sets of 4 Nordics once per week. These could be performed day 1 or day 2 post match day to enable sufficient recovery time in preparation for the next game whilst ensuring risk of HSI injury is reduced. Utilising a lower volume may also enhance the athletes adherence to the program as it is less time consuming and will provide less of an interference with sporting and other daily life activities.
Traditionally, the Nordic hamstring exercise is performed with a partner to hold down the ankles, however, it’s important that the exercise can also be performed individually to enable athletes to utilise the exercise when they are not around others. This can be achieved by using a fixed barbell, or other structures in the gym to hold the feet in place. It can even be performed at home using a sofa or bed. Additionally, athletes can use a Nord board, however, this is a very expensive piece of equipment and is only really used at elite sports teams to provide physios with a fast and accurate system to monitor hamstring strength and symmetry.
The Nordic exercise can be extremely challenging, and many athletes will initially be unable to perform a full Nordic Curl. To assist with this, athletes can use resistance bands or swiss balls to create easier variations of the exercise which will allow the athlete to build strength and progress to full movement. On the other hand, for athletes who find a bodyweight rep too easy, instead of increasing training volume, we can increase training intensity by having the athlete hold a load whilst performing the exercise. It’s important to consider that the Nordic hamstring exercise should not be used in isolation and should be incorporated into a wider holistic hamstring rehab and injury prevention programme to account for both strength, architectural adaptations, and the ability to withstand the high velocity actions that are required within sporting activities. One essential element of HSI prevention programmes is the individualisation of the programme. Athletes will typically be screened at the start of each season allowing physios to create individualised rehab which target specific factors relevant to the players, while also serving as benchmark criteria for return to play.
Another key aspect is ensuring that that athlete’s rehab is specific to their sport. Therefore, alongside eccentric strengthening it is vital that rehab includes various drills which simulate high-speed and high-force tasks as well as complex movement patterns. A primary focus of end stage rehab should be on High-Speed Running due to its specificity and biomechanical symmetries to the mechanism of injury. Practising athletic movements will challenge coordination, timing and neuromuscular control, and will lead to an increased ability to apply strength functionally. A further important element of managing athlete injuries is to recognise that they may also be emotionally and psychologically affected. Therefore, the athlete’s readiness to return to play should also account for psychological factors which may become a barrier such as fear of reinjury.
To summarise, the Nordic Hamstring exercise is an evidence-based treatment approach which can promote recovery from HSI’s and prevent reinjury. It’s been shown to increase eccentric hamstring strength and enhance Biceps Femoris Fascicle Length which are the two most crucial factors to reduce HSI risk. Although it has been shown to reduce the occurrence of HSI’s by 51%, high volumes can result in DOMS which can negatively affect adherence.
Therefore, as physio’s we should aim to utilise low but consistent volumes of Nordics as part of a holistic rehab plan which also accounts for the patients sporting requirements. We should also advise the continuation of Nordics as part of an overall injury prevention program once the athlete has returned to full fitness.
References:
Bahr, R., Thorborg, K. and Ekstrand, J. (2015). Evidence-based Hamstring Injury Prevention Is Not Adopted by the Majority of Champions League or Norwegian Premier League Football teams: the Nordic Hamstring Survey. British Journal of Sports Medicine, [online] 49(22), pp.1466–1471.
British Medical Association (2019). The BMA Guide to Sports Injuries : the Essential step-by-step Guide to prevention, diagnosis, and Treatment. London: Dorling Kindersley Limited.
Brockett, C.L., Morgan, D.L. and Proske, U. (2001). Human Hamstring Muscles Adapt to Eccentric Exercise by Changing Optimum Length. Medicine and Science in Sports and Exercise, 33, pp.783–790.
Brukner, P. and Khan, K. (2017). Clinical Sports Medicine. 5th ed. Australia: Mcgraw-Hill.
Brukner, P., Nealon, A., Morgan, C., Burgess, D. and Dunn, A. (2014). Recurrent Hamstring Muscle injury: Applying the Limited Evidence in the Professional Football Setting with a seven-point Programme. British Journal of Sports Medicine, 48(11), pp.929–938.
Buckthorpe, M., Wright, S., Bruce-Low, S. and Nanni, G. (2019). Recommendations for Hamstring Injury Prevention in Elite football: Translating Research into Practice. British Journal of Sports Medicine, [online] 53(7), pp.449–456.
Cuthbert, M., Ripley, N., McMahon, J.J., Evans, M., Haff, G.G. and Comfort, P. (2020). The Effect of Nordic Hamstring Exercise Intervention Volume on Eccentric Strength and Muscle Architecture Adaptations: a Systematic Review and Meta-analyses. Sports Medicine, 50, pp.83–99.
Ekstrand, J., Hägglund, M. and Waldén, M. (2011). Epidemiology of Muscle Injuries in Professional Football (Soccer). The American Journal of Sports Medicine, 39(6), pp.1226–1232.
Mjolsnes, R., Arnason, A., osthagen, T., Raastad, T. and Bahr, R. (2004). A 10-week Randomized Trial Comparing Eccentric vs. Concentric Hamstring Strength Training in well-trained Soccer Players. Scandinavian Journal of Medicine and Science in Sports, 14(5), pp.311–317.
Opar, D.A., Williams, M.D., Timmins, R.G., Hickey, J., Duhig, S.J. and Shield, A.J. (2015). Eccentric Hamstring Strength and Hamstring Injury Risk in Australian Footballers. Medicine & Science in Sports & Exercise, 47(4), pp.857–865.
Orchard, J. and Best, T.M. (2002). The Management of Muscle Strain Injuries: an Early Return versus the Risk of Recurrence. Clinical Journal of Sport Medicine, 12(1), pp.3–5.
Petersen, J., Thorborg, K., Nielsen, M.B., Budtz-Jørgensen, E. and Hölmich, P. (2011). Preventive Effect of Eccentric Training on Acute Hamstring Injuries in men’s soccer: a cluster-randomized Controlled Trial. The American Journal of Sports Medicine, [online] 39(11), pp.2296–303.
Presland, J.D., Timmins, R.G., Bourne, M.N., Williams, M.D. and Opar, D.A. (2018). The Effect of Nordic Hamstring Exercise Training Volume on Biceps Femoris Long Head Architectural Adaptation. Scandinavian Journal of Medicine & Science in Sports, 28(7), pp.1775–1783.
Timmins, R.G., Ruddy, J.D., Presland, J., Maniar, N., Shield, A.J., Williams, M.D. and Opar, D.A. (2016). Architectural Changes of the Biceps Femoris Long Head after Concentric or Eccentric Training. Medicine & Science in Sports & Exercise, 48(3), pp.499–508.
van Dyk, N., Behan, F.P. and Whiteley, R. (2019). Including the Nordic Hamstring Exercise in Injury Prevention Programmes Halves the Rate of Hamstring injuries: a Systematic Review and meta-analysis of 8459 Athletes. British Journal of Sports Medicine, [online] 53(21), pp.1362–1370.
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