Hamstring Injuries and the Nordic Hamstring Curl? Does the research add up?
The hamstring muscle group is one of the most common sites of injury in field sport athletes. Hamstring injuries have been reported to account for 14% of all injuries in Track and Field Athletics , 12-17% of all injuries in Soccer [2, 3], 16-23% of all injuries in Australian Rules Football [4, 5, 6] and 21% of all injuries in Gaelic Football . Given the recurring nature of hamstring injuries (studies show that the re-injury rate with hamstring muscle strains is as high as 22-34%) a lot of research has been performed aiming to provide effective pre-habilitation and re-habilitation exercises and exercise programmes to combat the alarming rate of hamstring injury [8, 9].
Many researchers have suggested that currently there is no definitive consensus for rehabilitation of the hamstring muscles after a strain [10, 11, 12] and that classic rehabilitation treatment regularly neglects the final therapeutic phase, that being accurately mimicking the mechanism of injury. Despite this lack of consensus within the literature, it is commonly agreed that eccentric-focused exercises play a vital role in hamstring rehabilitation and re-strengthening. There is a lot of research on the use and effectiveness of Nordic Hamstring Exercise (NHE) [13-18] but the majority of these studies compare NHE to no specific hamstring training at all [14, 16, 17], to flexibility training  or to stretching , and so the relevance of these results should be questioned. After all, what have these studies actually proven? That something is better than nothing? (and by nothing, I mean flexibility training and stretching in this regard – hardly well-supported methods of preventing against injury). Furthermore, these studies do not compare the NHE to other eccentric-focused hamstring exercises, such as RDLs, seated and lying machine hamstring curl, slide-board curls, swiss ball curls, razor curls and glute-hamstring raises, to name a few.
One such study  which did compare the NHE to another hamstring exercise, that being the Hamstring Curl (HC) exercise performed on a lying hamstring curl machine, was the study by (Arnason et al., 2004) titled; ‘A 10-week randomised trial comparing eccentric vs concentric hamstring strength training in well trained soccer players’. This is one of the more prominent and commonly-referenced studies on this topic to date and has emerged as one of the ‘go-to’ pieces of evidence when exercise practitioners quote the NHE as the superior hamstring exercise in terms of injury prevention. However, for those of you that read past the abstract, you will see that the researchers instructed participants in the HC group to ‘’return the load using as little effort as possible to minimize loading in the eccentric phase’’, yet assessed the effectiveness of both exercises via pre and post-intervention maximal eccentric and isometric tests. Again, what this study essentially proves is that an eccentric-focused hamstring exercise is superior to no eccentric focused hamstring exercise – doing something is superior to doing nothing (in terms of eccentric strength training)! Therefore, with a thorough dissection of the literature, it becomes clear that the NHE has not been proven as superior to other exercises in its ability to reduce the rate and risk of hamstring injuries, rather it has been proven to be effective when compared to stretching, flexibility exercises, concentric-focused hamstring exercises and to nothing at all. It could be argued there is a gap in the literature here and that previous researchers who suggested there was no definitive consensus for rehabilitation of the hamstring muscles after a strain [10, 11, 12] may be correct. Studies need to examine and compare the effects of multiple different eccentric-focused hamstring exercises to truly determine which exercise is most effective. Just because something is effective, does not mean it is the MOST effective! Are we selling our athletes short here by accepting the existing (and limited) research on this area, as opposed to questioning what is already there and seeking a higher standard? Perhaps. Perhaps we need to revisit how a hamstring injury happens and let this guide us towards exercises that can most accurately replicate the mechanism of injury.
Research shows us that the most common mechanism of hamstring injury is a strain or tear as the knee extends eccentrically in the late swing phase of a maximal velocity sprint [19, 20, 21, 22]. In fact, studies report that 53-68% of hamstring injuries occur during sprinting . The most common location of injury is the distal end of the long head of the biceps femoris [23, 24, 25]. Essentially, when we sprint at or near top speed, a strain or tear to the lower and lateral end of our hamstring muscle group is the most common hamstring injury. The muscle action occurring as this injury happens, is a high-force, high-speed eccentric contraction. Therefore, end stage rehab should look to mimic the mechanism of injury, that being high-velocity eccentric loading where the hamstring muscle group transitions from a state of relative relaxation to a state of high tension. When we consider the high volumes of sprinting and high-speed meters covered by athletes playing field sports such as GAA, rugby, soccer and athletics, the importance of effective and accurate rehabilitation programmes become evident! It is imperative that our rehabilitation processes create a robust athlete capable of handling such high volumes of sprinting when they return to their sport. After all, a muscle injury is an indication that the injured muscle group was neither capable nor strong enough to deal with the load or situation it was presented with.
So, with the mechanism of injury in mind, let us revisit the muscular action typically performed in the nordic hamstring exercise. During a nordic curl, the hamstrings contract eccentrically, however this contraction is performed at a very low velocity in comparison to the eccentric contraction performed during a sprint. In terms of muscle specificity, the NHE actually stresses and works the semimembranosus and semitendinosus muscles of the hamstring group more so than the long head biceps femoris (which is the most commonly injured of the hamstring muscle group). Furthermore, whilst there is a high degree of muscular stretch happening during a hamstring tear, there is little to no stretch occurring during the NHE. In fact, many athletes will be limited in the level to which they can perform a NHE, with a lack of strength preventing them from working in a range any lower than 45 degrees of knee flexion – a range which can be accessed and strengthened with alternative exercises. In addition, given the difficulty of the NHE, it cannot be performed unilaterally, so there is always the possibility that compensation occurs, and the non-injured leg performs more work than the injured leg.
When we put all of this together, we realise that the NHE does not primarily activate the specific hamstring muscle we want to target, it is not performed at a high velocity and there is very little stretch occurring to the hamstring muscles. This criteria leads us to conclude that it does not accurately replicate the mechanism of injury seen with a hamstring strain or tear, and given it has not been proven against any other eccentric-focused hamstring exercise, the assumption and common opinion that the NHE is the holy grail of hamstring injury prevention must be highly questioned.
So, what are the alternatives? Well, in order to replicate the mechanism of injury and also the mechanism of muscular action occurring during a sprint, exercise selection becomes crucial. A recent study by (Bourne, Williams, Opar et al., 2017) investigated the impact of exercise selection on hamstring muscle activation and found some interesting results . Their research found that, during eccentric contractions, hip extension exercises more selectively activate the lateral hamstrings (biceps femoris) while knee flexion-oriented exercises preferentially recruit the medial hamstrings (semimembranosus and semitendinosus). With several hip extension exercises to choose from (bilateral RDL, unilateral RDL, straight-leg bridge and 45 degree hip extension exercises), we then must assess which one has the most potential for progression, whilst mimicking the muscular action we desire.
The RDL has the obvious benefit of providing the athlete the ability to progressively overload the hamstrings, in addition to creating a high degree of stretch in the muscle, thus allowing the athlete to build ‘strength at length’. With the potential of increased weight with an RDL, comes the potential of increased force and an increased strength stimulus provided to the hamstrings. Furthermore, in the initial stages of rehabilitation, this exercise can be performed unilaterally to address muscular imbalances if necessary. Once a base of strength is reached, the exercise can easily be adapted to further replicate the muscular action performed whilst sprinting. This can be done by transitioning to an ‘RDL Drop Catch’ – where the weight is dropped from a hang position and caught just below the knee, at the bottom position of a standard repetition. This form of RDL brings high forces, high velocity, a high degree of eccentric stretch and forces the hamstrings to transition from a state of relative relaxation (hang position) to a state of high tension (during the catch), closely replicating the muscular action of a sprint.
In terms of knee dominant hamstring exercises, the seated machine hamstring curl is an exercise that is not prominent in research but one which also possesses a potential for progressive overload of the hamstrings. This potential for overload is something that similar knee-dominant hamstring exercises such as slide-board curls and swiss-ball curls do not really possess. Yes, you can find ways to overload those exercises, but it may not be as easy, and you certainly won’t be in as stable a position as that of the seated machine curl. All three of these exercises allow the athlete to access and control the end range of knee extension (further than the typical 45 degrees controlled by many in a nordic curl). From there, strength in the lengthened range can be built. Aside from its stability, range of motion and potential for overload, the seated machine hamstring curl also has the potential to be progressed to more accurately mimic the mechanism of muscle action seen during a sprint and during a typical hamstring strain or tear. It can be performed single-leg, at higher eccentric speeds and even overloaded eccentrically by getting a partner (or using both legs) to perform the concentric whilst using a heavier weight on the eccentric phase. All of these possibilities are not as easily mimicked in a swiss ball or slide-board curl, thus bringing me to suggest that the seated hamstring curl is the superior option of knee-dominant hamstring exercises.
So, after all of that, should we just abandon the NHE entirely? Not entirely. While there are a number of reasons to opt for alternative exercises, the NHE certainly does have a role within a team environment. There are two main reasons for this. Firstly, the NHE requires no equipment (just a partner) and so it is very practical and easy to implement with large numbers for obvious reasons. Secondly, although the research has not adequately proved it as better than other eccentric-focused hamstring exercises, it has been proved as better than ‘nothing’ (and by nothing, I mean stretching, flexibility exercises, concentric-focused hamstring exercises, and indeed nothing at all) as a means of protecting against hamstring exercises. In comparison to these other forms of hamstring injury prevention, the NHE works. It may not work ‘the best’, but it still works. And it is still effective in the absence of superior methods. Furthermore, where there is potential to pair it with another hamstring-specific exercise, it may be more effective. When working in an amateur team sport environment with large numbers, perhaps it is the only practical choice. However, when higher standards and expectations are demanded, when player compliance to resistance training is high and when the necessary equipment is available (all of which are very reasonable expectations within a high-performance sporting setting) then the reasoning above should be used to inform exercise selection!
Finally, in addition to exercise selection in the weight-room, exercise selection on the playing pitch, in the form of running mechanics drills, is of equal importance to the success of a rehabilitation process. Transferring the strength gains from resistance exercises to more 'functional strength' through running-based drills will lead to an ability to produce force in situations where the hamstring muscles are commonly used and injured. Running drills which require the athlete to replicate maximum velocity running mechanics should be implemented before returning to high-speed running. Drills which emphasize the late swing phase of maximum velocity running will prove even more accurate in replicating the muscular action of the hamstrings during sprinting, and should be progressed to throughout the programme. Once the athlete returns to full training, the volume of high-speed running is another important aspect which needs to be monitored and gradually re-introduced, avoiding large and rapid increases in total weekly volume. So when we consider all of the various aspects of rehabilitation, it becomes evident that the recovery process from hamstring injuries should involve more than the infamous nordic curl!
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