Hamstring injury is the most common injury seen in sports that involve locomotion at high velocity, in fact it is the most common site of injury in sprinters and runners (Chumanov, Heiderscheit and Thelen, 2007; Yu, et al., 2008; Lempainen, et al., 2009). More specifically, Hamstring injuries account for 50% of all muscle injuries in sprinters (Arge, 1985) and 6-26% of all injuries in sports that involve maximal speed running such as football, rugby, cricket, track sprinting and Australian football (Bennel and Crossley, 1996; Orchard, et al., 2002; Woods, et al., 2004; Brooks, et al., 2005; Orchard, Seward and Orchard, 2013).
Injury inevitably leads to time lost from training and competition (Seward, et al., 1993; Brooks, et al., 2006) and may even affect future performance therefore prevention is high on a Strength and Conditioning Coach’s list of priorities during the programme design process. In order to successfully prevent or at least reduce the risk of injury in athletes who take part in sports involving sprinting, it is wise to first understand the mechanics of the Hamstrings during high velocity running as well as the mechanisms of injury.
The focus of this assignment is the prevention of Hamstring injury rather than the rehabilitation of, therefore it is important to establish how they occur in the first instance so that the Strength & Conditioning Coach can design an effective prevention strategy.
Research into running biomechanics has found that, during high velocity locomotion, the Semimembranosus, Semitendinosus and Biceps Femoris are almost always active; contracting concentrically to pull the leg posteriorly through the stance phase, and working eccentrically to slow the forward moving limb at the end of the swing (Thelen, et al., 2005; Yu, et al., 2008; Chumanov, Heiderscheit and Thelen, 2011), with the Biceps Femoris having the largest role of the three muscles (Chumanov, et al., 2012). In more detail, the hamstrings are required to contract whilst lengthening to decelerate the extending knee joint and flexing hip during the terminal swing phase at which point they reach their maximum length. The muscle group is then responsible primarily for hip extension and some knee flexion with peak torques experienced at ground contact (Novacheck, 1998). It has been suggested by Verrall, et al., (2001) that it is during this rapid change from eccentric to concentric function that the muscle is more susceptible to injury during maximal speed running, particularly at terminal swing phase as this is when the muscle-tendon units are at their longest length (Yu, et al., 2008; Chumanov, Heiderscheit and Thelen, 2011).
Mechanisms of injury
Muscle weakness appears to be linked with Hamstring strain injuries (Heiser, 1984; Croisier, et al., 2002). More specifically, research has shown how the muscle fibres can tear when the eccentric forces acting on the Hamstrings exceed the maximum tolerable level of strain (Heiderscheit, et al., 2005; Thelen, et al., 2005; Schache, et al., 2009). Further investigations into the biomechanics of running have also found that an eccentric contraction is a condition significant enough for a strain injury (Schache, et al., 2009).
This information would appear true considering that most injuries seem to occur in sports involving eccentric contractions as opposed to those that are primarily concentric based such as swimming (Johnson, 2003). With this in mind, there is evidence to suggest that there is a connection between a reduced concentric strength imbalance, also known as the Hamstring:Quadriceps ratio, and increased injury rate (Croisier, et al., 2003). In fact, a study conducted by Yeung, Suen and Yeung (2009) demonstrated that a Hamstring:Quadriceps ratio of less than 0.6 can increase the risk of Hamstring injury by 17 times in sprinters. This information in reinforced by an earlier study by Orchard, et al. (1997) who found a significant increase in risk of Hamstring injury in professional football players when the ratio was less than 0.61. A meta-analysis by Freckleton and Pizzari (2013) also supports how an increase in quadriceps peak torque is a risk factor. On the other hand, Opar, Williams and Shield (2012) explain how this suggestion is inconclusive as a number of authors have also found no association between asymmetry and injury risk following a review of a large number of studies.
There is evidence to suggest that muscle fatigue is linked with such injury (Worrell, 1994; Mair, et al., 1996). Fatigue has been shown to reduce eccentric Hamstring torque (Small, et al., 2010) whilst increase knee extension and decrease hip flexion during running (Pinniger, Steele and Groeller, 2000) thus reducing running efficiency. It is not just the acute effect of fatigue that appears to be a potential cause as the accumulation of microscopic damage may also contribute to the increased risk (Morgan, 1990).
Older studies have suggested how poor hamstring flexibility may increase the risk of strain injury (Ekstrand and Gillquist, 1983; Jonhagen, Nemeth and Eriksson, 1994; Witvrouw, et al., 2003). This is likely linked with poor lumbo-pelvic-hip control (Mendiguchia, Alentorn-Geli and Brugheli, 2012), for example, an excessive anterior pelvic tilt will reduce the efficiency of an athletes running mechanics and increase the strain on the hamstring complex during terminal swing phase.
Whilst there is data suggesting that an insufficient warm up may result in injury, there appears to be very little research into hamstring injuries specifically, although a suggestion of poor or lack of warm up contributing to hamstring injury is given by Safran, et al., (1988).
Potentially, any one of these factors may be the cause of Hamstring injury depending on numerous variables, but what is clear, is that previous injury appears to have a high influence (Bennell, et al., 1988; Verrall, et al., 2001; Gabbe, et al., 2006). In fact, previous hamstring injury may pose the greatest risk (Mendiguchia, Alentorn-Geli and Brugheli, 2012) with reoccurrence rates as high as 13.9-63.3% across multiple sports (De Visser, et al., 2012). Of course, prevention will always be greater than a cure, therefore it is recommended that the Strength & Conditioning professional not only utilises some form of screening method to predict injury, but also aims to prevent injury in the first instance through effective programme design and the correct coaching of such preventative measures.
The development and utilisation of screening methods may provide an essential element to the prevention of lower limb injuries (Dallinga, Benjaminse and Lemmink, 2012), allowing the Strength & Conditioning Coach to adjust a training programme accordingly and design an effective preventive strategy.
Numerous studies have been conducted looking at various tools that can be used to predict Hamstring injury. These tools include flexibility assessments such as the sit and reach test, active knee extension, passive straight leg raise and Thomas test (Gabbe, et al., 2005) to which it is suggested that decreased hip flexor flexibility, as measured by the Thomas test, may identify athletes with a greater risk of hamstring injury. Such result may identify poor lumbo-pelvic-hip control and greater strain on the hamstrings as a result of an excessive anterior pelvic tilt, as discussed in previous research (Ekstrand and Gillquist, 1983; Jonhagen, Nemeth and Eriksson, 1994; Witvrouw, et al., 2003). Other studies have examined Hamstring length, hip range of motion and a Nordic Hamstring strength test.
Flexibility has been shown to be a good predictor, however despite significant data, small sample sizes and observed injury rates in the current studies make research regarding the various screening tools inconclusive (Engebretsen, et al., 2010; Dallinga, Benjaminse and Lemmink, 2012). It is therefore advised that such tests are used however are not solely relied upon, rather the Strength & Conditioning Coach should be aware of such signs however provide the athlete with a preventive programme year round, regardless of the predictions given from screening tools.
The use of exercises to develop muscular strength plays a large roll in the prevention of injury, typically through the balancing of strength between agonist and antagonistic muscles and thus improving joint stability (Holcomb, et al., 2007). Both concentric and eccentric hamstring strength training has been shown to restore the normal hamstring:quadriceps ratio (Croisier, et al., 2002; Askling, Karlsson and Thorstensson, 2003) and therefore decrease the risk of hamstring injury (Croiser, et al., 2008). It appears that particular attention should be placed on strengthening the eccentric function of the hamstrings as athletes who have returned from a hamstring injury display eccentric weakness of their injured limb when compared to their uninjured leg (Opar, et al., 2013), even when pain-free.
A number of studies have looked at the various types of strength training protocols available to Strength and Conditioning Coaches with an emphasis on improving the Hamstring:Quadricep ratio. Holcomb, et al., (2007) showed how incorporating Hamstring specific strength exercises into a programme such as leg curls, stiff legged deadlifts, good mornings and trunk hyperextensions significantly improved the functional Hamstring:Quadricep ratio after only 6 weeks. This finding was coupled with an increase in the conventional Hamstring:Quadricep ratio however this result was not significant although still above the minimum requirement of 0.6 as detailed by Yeung, Suen and Yeung (2009). The lack of significance may have come down to the fact that Holcomb, et al., (2007) incorporated such exercises into their programme along with those that would also develop the strength of the quadriceps such as resisted sled walks. This point is emphasised by the fact that an earlier study performed by Mjolsnes, et al., (2004) focused on training the Hamstrings only and witnessed a significant increase in Hamstring strength compared with Quadriceps, thus resulting in a significant increase in Hamstring:Quadriceps ratio.
Strengthening of the Hamstrings should not be the only focus of a training programme put together by the Strength & Conditioning professional as there is research to suggest that increased activity of the Gluteus Maximus and External Oblique have been shown to reduce peak Bicep Femoris strain (Chumanov, et al., 2006; Chumanov, Heidershcheit and Thelen, 2007) resulting in a reduced risk of injury. The development of the Gluteus Maximus will not only reduce some of the strain or work done by the Hamstring during locomotion, but also improve lumbo-pelvic-hip control, also assisted by the External Oblique by placing the pelvis in a more neutral position and help prevent an excessive anterior pelvic tilt discussed by Mendiguchia, Alentorn-Geli and Brugheli, (2012). It is advised that increase of hip flexor flexibility is also considered in order to improve pelvic positioning and retested frequently using the Thomas test suggested by Gabbe, et al., (2005). Training programmes that address postural control have been shown to be superior to those that don’t when it comes to injury reoccurrence (Sherry and Best, 2004).
If an athlete’s flexibility does appear to be an issue, then self-myofacial release techniques such as foam rolling could be recommended by the Strength & Conditioning coach. In a study performed by Kuruma et al (2013), myofascial release techniques significantly increased both active and passive range of motion, however, reaction time reduced significantly. Conversely, MacDonald et al (2013) concluded that foam rolling significantly increased range of motion without decreasing muscle performance, activation or force. Regardless of which conclusion is more accurate, it would make sense for the Strength & Conditioning professional to instruct the athlete to complete such method away from training sessions due to the simplistic nature of such, thus supervision not being required. This will allow more time for other injury prevention work to be carried out during training sessions whilst having no negative affect on reaction time.
The Strength and Conditioning professional should look to plan and instruct an effective neuromuscular warm up before the athlete takes part in any event or physical activity. Previously, stretching alone was deemed sufficient however more recent studies have suggested that this will provide no injury prevention benefit (Herman, et al., 2012). This warm up should be constructed in a way that improves the mobility of joints whilst enhancing the neuromuscular efficiency. Isometric warm up exercises, hamstring flexibility and eccentric strengthening exercises may be protective against hamstring injury (Petersen and Holmich, 2005), however studies by Soligard, et al., (2008) And Steffen, et al., (2008) both incorporated 5 repetitions of Nordic Hamstring curls into their warm ups however the number of repetitions performed may not have been enough to prevent injury (Herman, et al., 2012).
It is the Strength & Conditioning Coach’s responsibility to design and deliver an effective programme to not only improve the athlete’s performance but also prevent injury. This training programme must include a number of variables that can be adjusted over a period of time to induce new stimulus and therefore prevent stagnation in performance whilst also allowing for recovery and adaptation. The basic variables included within a training programme are the selected exercises, the intensity at which that exercise is performed, the repetition volume of that given exercise as well as the frequency of training during a given microcycle or week.
It is important to consider the type of exercise that is best used in order to achieve a specific goal. When looking to design a programme with the main purpose of Hamstring injury prevention, it appears to be evident that a good starting point will be to select an exercise that develops the eccentric function of the muscle group and therefore help reduce any eccentric weakness. McAllister, et al. (2013) performed a study where by they found that all 3 muscles of the hamstring group showed greater activity during the eccentric phase of the Stiff leg/Romanian Deadlift compared with 4 other exercises that were analysed in their study. This information is extremely useful given that eccentric strengthening exercises should be used to prevent hamstring injury. It should be noted, however, that the Stiff Leg/Romanian Deadlift appears to target the Semitendinosus more so than the Biceps Femoris (Zebis, et al., 2013) as does the Kettlebell swing when looking at surface Electromyography data in the same study therefore would be sensible to include other exercises into the training programme which do involve greater Biceps Femoris activity given that it is this muscle that appears to play the largest role in high speed running (Chumanov, et al., 2012).
Electromyography research by Zebis, et al. (2013) suggests how it may be worth including supine leg curls, like that of a single leg swiss ball leg curl, into the training cycle as this exercise has been found to show greater emphasis of the Biceps Femoris compared with the kettle bell swing and Romanian Deadlift/Stiff Legged Deadlift. The Strength & Conditioning coach may wish to ask the athlete to perform the swiss ball leg curls with an emphasis on the eccentric portion or lowering phase of the exercise. Alternatively, a leg curl machine may be used if available, allowing the athlete to place emphasis and load during either phase of the repetition. One method of loading the hamstring eccentrically using a leg curl machine would be to carry out the concentric movement using both limbs whilst slowly controlling the resistance eccentrically using only one. A consideration for during the design of the training programme would be to ensure that there is a supine leg curl variation placed amongst other hamstring exercises to ensure that there is a balanced development between individual Hamstring muscles.
Another eccentric exercise that can be utilised is the Nordic Hamstring curl, otherwise known as the Hamstring Lowers. This exercise has been shown to reduce injury rates by up to 70% following a ten week Nordic protocol where by repetitions where steadily increased from ten during a single session in the first week, to thirty repetitions three times per week in the last (Petersen, et al., 2011) without inducing any significant Delayed Onset of Muscle Soreness when compared with protocols of a higher volume. This repetition range is still considerably greater than that used in the warm up protocols suggested by Soligard, et al., (2008) and Steffen, et al., (2008).
When looking at concentric activity of the Hamstrings, McAllister, et al., (2013) also found that the entire hamstring group displayed greater activity during the concentric phase of the Glute Ham Raise compared with other exercises in their study, although other exercises such as the Good Morning do involve a large amount of Hamstring activity. Despite the title, the Glute Ham Raise is not the most efficient exercise at developing the Gluteus Maximus in order to improve lumbo-pelvix-hip control. Exercises that load the hip horizontally such as the Barbell Hipt Thrust and Cable Pull Through are much more effective at increasing Gluteus Maximus activity.
Plyometric activities are also useful as these develop the myotatic reflex of the muscle, creating fast explosive movements concentrically followed by a forceful deceleration of the limb upon ground contact. Whilst extremely sport specific and of a much higher velocity than traditional strength training methods, Single leg sagittal plane hurdle hops have been found to produce the greatest gluteal and hamstring activity compared with four other commonly used plyometric exercises (Struminger, et al., 2013).
In order to provide a stimulus for improvement in muscle strength, it is recommended that exercises that can be loaded externally are done so to a high intensity. More specifically between 80-95% of the individuals one repetition maximum. This intensity will allow the individual to complete repeated efforts of between 2-10 repeitions provided that there is sufficient rest periods between sets to allow for the recovery of the energy system in use. Whilst the intensity is to be kept high, it is generally understood that periods of very high intensity must be followed by a deload period in order to allow for recovery and adaptation. This deload can be structured through the reduction in training volume an/or intensity.
Training frequency must also be considered. This will largely depend on whether the athlete is currently within a competitive stage as if this is the case, sufficient recovery is essential to ensure that no soreness, microscopic damage or physiological fatigue is present to not only ensure that the athlete performs but also reduce the risk of injury (Morgan, 1990; Worrell, 1994; Mair, et al., 1996). During the competitive season, it may be a case of reducing Hamstring specific strength training down to once per week.
It would be advised that the Strength and Conditioning coach should prescribe the athlete with a flexibility routine with the main aim of improving hip range of motion. This routine, combined with an eccentric and concentric strength training programme will help reduce an excessive anterior pelvic tilt whilst strengthen the muscles responsible for hip extension thus leading to an improved lumbo-pelvic-hip control. The progress of such flexibility routine can be monitored through frequent assessment using the Thomas test identified by Gabbe, et al., (2005). Once the Strength and Conditioning professional has taught such a routine, the athlete may perform this along with myofascial release techniques away from training sessions so as to not negatively affect performance (Kuruma et al., 2013). The non-technical nature of such activity will mean that the athlete can complete such a routine unsupervised and thus the Strength and Conditioning coach can spend more time screening the athlete along with instructing and re-emphasising technical improvement during the strength training programme.
The Hamstrings are one of the most common injury sites in athletes who take part in sports that involve maximum velocity running. The likely cause for injury is unclear and certainly up for debate, however there appears to be a number of factors that can contribute. These include an imbalance between the Hamstrings and Quadriceps, eccentric weakness, lack of flexibility, insufficient warm up prior to physical activity and poor lumbo-pelvic-hip control. There is emphasis placed on the eccentric function of the hamstring as a potential area of injury risk as most injuries seem to occur at terminal swing phase at which point the muscle-tendon unit of the hamstring is at its longest. One common factor amongst a lot of the current research is that previous hamstring injury is a large predictor of injury risk, accounting for up to 63.3% of injury rates, identifying the importance of injury prevention in the first instance.
Whilst there does not appear to be any screening tools that are extremely effective at predicting hamstring injury, it may be wise for the Strength & Conditioning professional to monitor the athlete’s flexibility around the hip joint, whilst ensuring that they place emphasis on the prevention of hamstring injury throughout an on going training programme.
The research strongly suggests the importance of eccentric strength training, through the utilisation of exercises such as the Romanian Deadlift, Supine Leg Curls and Sagital Plane Hurdle Hops. It may also be useful to include the Glute Ham Raise as this movement involves greater concentric activity than other exercises detailed in the research. One other useful tool is the Nordic Hamstring protocol as this tool has been found to reduce injury rates by up to 70%. The Strength and Conditioning professional is also advised to take other precautions when designing an preventive strategy through the inclusion of an effective neuromuscular warm before the athlete begins any physical activity, and the improvement of hip range of motion, particularly of the hip flexors.
In conclusion, prevention will always be superior to finding a cure. This is highlighted in the findings of the current literature with potential mechanisms of injury being debatable and the lack of significant screening tools available to predict injury. It is therefore the Strength and Conditioning Coaches responsibility to provide the athlete with a training programme that has been designed to prevent Hamstring injury year round to avoid time lost from training, competition, financial loss and also future reductions in performance.
Author: Karl Page
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