If you’re an athlete reading this and have been involved in a serious training program for a length of time, it is likely that you have suffered a hamstring injury. Similarly, if you’re a coach involved in a speed-power sport, it is likely that you have had many athletes suffer a hamstring injury. Hamstrings are a sprinters “bogey” muscle – the one that seems to get injured the most. During my career as a sprinter, I suffered some bad hamstring injuries. In 2008, I tore it quite badly in February, and didn’t run again until April, missing a total of 7 weeks of running, which isn’t ideal in an Olympic year. The injury only settled down following a corticosteroid injection into the area, which made running tolerable, if not pain-free. I managed seven weeks of sprint training before my season opener, in which I promptly re-tore the same hamstring. This was problematic, as it was only five weeks until the National Olympic trials, and I still had to run the qualifying time of 10.21 seconds. Fortunately, I had a world-class physiotherapist who worked with me twice a day, along with a brilliant team of support staff. I made the team – just. I remember standing on the start line for my comeback race, knowing that I had to run to try and get the qualifying time but also being aware that my hamstring was really, really sore!
What are hamstrings?
We should probably begin by taking a look at what the hamstrings are. The hamstrings are the muscle group that runs down the back of your thigh. The hamstring muscle group is comprised of three actual muscles; biceps femoris (that has a long head and a short head), semimembranosus, and semitendinosus. The muscles group is bi-articular, which means that it works on two different joints; the hip and the knee. Specifically, the hamstrings work to extend the hip, and also flex the knee. This makes them important in sprinting because they control the lower leg movement throughout most of the stride cycle, and also assist the gluteal muscles in creating a powerful hip extension. The hamstring muscles also have a few other functions. They act as a secondary knee stabiliser and also play a role in controlling rotation of the whole leg (Koulouris et al. 2007). There is some evidence to show that the hamstrings also act as a shock absorber during foot contact in sprinting (Malliaropoulas et al 2012).
Like all muscles, the hamstrings can work both eccentrically and concentrically. Concentric muscle action is where the muscle shortens whilst it contracts, and an eccentric action is where the muscle lengthens whilst increasing tension. Eccentric actions occur with both a larger force and higher velocity than concentric actions, and so are more likely to cause injury.
During the sprint cycle, as the thigh reaches its maximum flexion angle in front of the body, the lower leg begins to escape forwards. This is crucial, as it allows the athlete to cover greater distance per stride, increasing their stride length. Whilst this lower leg is moving forwards, however, the hamstrings are working hard to control the movement. The hamstrings first slow the lower leg as it moves forward, bringing it to a stop. They do this eccentrically, because the hamstrings are lengthening throughout this movement. Once the lower leg has stopped moving forwards, it is then rapidly accelerated towards the ground. This is achieved mostly through hip extension, which requires the hamstrings to concentrically contract. The more powerful this movement, the shorter the ground contact time, which plays a role in improving stride frequency. Once the foot makes contact with the floor, the hamstrings continue to work in combination with the gluteal muscles, actively pulling the body over the foot (Mann 2011). These actions demonstrate the major contributions of the hamstring muscles during the sprint cycle.
Why do we injure our hamstrings?
Recalling what I have just mentioned, we can explore why hamstring injuries occur. Firstly, the muscles are bi-articular, and so work on two different joints. This increases the amount of movement that the muscles undergo, increasing the injury risk. Secondly, they undergo an eccentric action, which in sprinting occurs at high force. Muscles tear when they cannot handle the force that is being placed upon them. If there is an underlying issue within the hamstring, it is more likely to get injured at this point – and this is what the studies show. In a review article by Petersen & Holmich (2005), they found that most hamstring injuries occur during either eccentric contraction (particularly during the later part of this contraction) or just before foot contact. Malliaropoulas et al. (2012) add that the largest musclo-tendonous stretch occurs in the hamstrings just before ground contact time and identify this as the most likely point of injury.
As I alluded to earlier, hamstring injuries are very common in sports that require running and kicking. In professional soccer, hamstring injuries account for roughly one in five of all injuries (Petersen & Holmich 2005). In high-level sprinters, this rate is higher; in a group of national level sprinters from Hong Kong, hamstring injuries accounted for 50% of all injuries (Yeung et al. 2009). The IAAF reports that 48% of all injuries within the 2011 World Athletics Championships were hamstring injuries (Alonso et al. 2012). Even more concerning is the re-injury rate for hamstrings, which in professional soccer is up to 30%, and in sprinters is 38%. What this essentially means is that if you injure your hamstring once, you are at an increased risk of injuring it again. We all know someone who has persistent hamstring injuries – hopefully that is not you! In terms of hamstring injury rate, Yeung et al. (2009) found in their sprinter subjects that hamstring injuries occurred roughly 0.87 times per 1000 training and competition hours. This means that a sprinter training two hours a day, five times per week, will likely have a hamstring injury once every two years. The rate in sprinters is much higher than that in other sports. Black et al. (2006) reported that in professional rugby union in the UK, the injury rate was 0.27 injuries per 1000 training hours, and in professional American Footballers, the rate is 0.77 per 1000 training hours (Elliot et al. 2011). For professional soccer players, the average training and game time missed per injury is 18 days (Woods et al. 2004) and 17 days for professional rugby players (Black et al. 2006). This is likely to be higher for sprinters because the loads placed on the hamstring are greater, and so longer rehabilitation time is likely to be required, although I can’t find any data to support this.
Most hamstring injuries occur in the biceps femoris, which is the most lateral of the hamstrings, situated towards the outside of the thigh. One of the mechanisms proposed for this increased injury rate is that the bicep femoris has a shorter moment arm in knee extension and so the musclo-tendonous stretch is significantly greater within that muscle (Malliaropoulas et al. 2005).
With hamstring injuries, there are different severities that can occur. A grade I tear is one where only a few fibres are torn or injured, accompanied with minor swelling and discomfort (Petersen & Holmich 2005). Range of motion will normally return within 24 hours although there may still be some pain on contraction (Pollock et al. 2014). These injuries are the most common, and see a quick return to sport, often in around 18 days (Lee et al. 2011).
Slightly more severe is a grade II hamstring injury, in which there is greater damage to the muscle and/or tendon (usually between 10-50% of muscle fibres). There tends to be a significant loss of strength associated with this type of injury, and range of motion will be impaired for longer than 24 hours. Return to sport with these injuries is often greater than 30 days. Grade III injuries are those in which greater than 50% of fibres are torn, and a grade 4 injury is one in which the muscle is completely torn – this tear can often be felt by hand and may need surgical repair.
Risk factors associated with a hamstring injury
There are many risk factors associated with a hamstring injury, including:
- Imbalance of Muscular Strength – Orchard et al. (1997) found that if the quadriceps were much stronger than the hamstrings, this increased the risk of a hamstring injury. They found that a ratio of below 0.6 for hamstring:quadracep strength increased the risk of injury. This ratio was mirrored in the Yeung et al. (2009) study on sprinters; the researchers found that if the ratio was below 0.6, then hamstring injury was seventeen times more likely to occur.
- Muscle Fatigue – Woods et al. (2004) found that significantly more hamstring injuries occur towards the end of a game, indicating that muscle fatigue plays a role in hamstring injury. Pinniger at al. (2000) demonstrated that repeated sprint bouts reduced hamstring function, meaning that the fatigued hamstring muscles could absorb less energy before reaching the level of stretch that caused injury.
- Hamstring tightness – Harting et al. (1996) found that hamstring flexibility reduced the risk of injury in a group of military recruits. This finding is a little controversial, as there are also a some studies that illustrate that lack of hamstring flexibility does not increase the risk of injury.
- Insufficient warm-up
- Previous Injury – Previous injury both within the hamstring muscles and surrounding muscles and structures increases the chance of injury. Koulouris et al. (2007) found that following anterior cruciate ligament (ACL) reconstruction surgery, the risk of a hamstring injury was significantly elevated. This is because the hamstrings play a role in stabilising the knee alongside the ACL – and if the ACL cannot perform this function, the hamstrings are placed under additional strain and load.
- Insufficient Recovery Period – Return from the previous injury before complete recovery
- Inadequate strength in hamstrings – Yeung et al. (2009) found that hamstring injuries were more likely to occur early in the season, when hamstring conditioning was not as high. In their study, 60% of hamstring injuries occurred within the first 100 hours of a training program.
Hamstring Injury Recovery
If we know that we are likely to injure our hamstrings once every two years (or even more if we are training with increased frequency or placing our hamstrings under increased load) then it is a good idea to know what to do when injury strikes. In their 2005 review article, Petersen & Holmich mention that they are very few randomised control studies (RCTs) in the area of hamstring injury rehabilitation. This is problematic, as RCTs are the gold standard of trials. Nevertheless, there are some studies examining hamstring rehabilitation best practice, and Petersen & Holmich proposed some ideas within their article.
In the acute phase of injury (depending on the severity, this can last up to seven days), they recommended utilising Rest, Ice, Compression and Elevation (RICE). This follows the typical recommendations that I came across in my career. The use of ice in soft tissue injury management has come under close scrutiny as of late. Reviews by Collins (2008) and Hubbard & Denegar (2004) indicate that there is insufficient evidence to suggest that the use of ice improves clinical outcome in soft tissue injury management – although it may reduce pain. Absence of evidence is not evidence of no effect, however, and many of the best soft tissue injury management programmes in the world do utilise ice during acute injury management. Similarly, the authors recommend the use of non-steroidal anti-inflammatory drugs (NSAIDs) during this phrase, but again they acknowledge the controversy regarding this: Recent research has indicated that delaying the use of NSAIDs until two to four days post injury may be better, as it doesn’t interfere with the early repair processes (Paoloni et al. 2009). Finally, the authors recommend early movement within pain-free range of motion, in order to decrease adhesions within the connective tissue.
During the sub-acute phase of injury (3-13 days, depending on the severity; this phase begins when inflammation has stopped), it is recommended to start pain free concentric strength exercises. Again, it is key to stay within an achievable range of motion and to ensure that the exercises are pain-free. These exercises will both prevent muscle atrophy, and also promote healing. During this phase, non-hamstring loaded training can begin, such as stationary bike sessions, swimming, and upper body circuits (provided they are pain-free!).
The next phase of rehabilitation is focused on muscle remodelling. In this phase, the injured hamstring should start to be stretched, which will reduce any loss of flexibility that may have occurred. It will also reduce muscle adhesion and scar tissue formation. In a 2004 study, Malliaropoulas et al. divided subjects with hamstring injuries into two groups. Group one conducted one stretching session per day, which consisted of four sets of 30-second hamstring stretches. Group two conducted four stretching sessions per day. The outcome was that the second group regained their range of motion in the injured leg much faster and also had a shorter overall rehabilitation period. During this phase, eccentric hamstring strengthening can also begin. Following this phase, the next goal is to return to full training. This should be comprised of a progressive increase in hamstring strength and flexibility exercises. Following a successful return to training, the athlete should maintain some rehabilitation exercises in the runup to return to competition. Competition is the final big test, as it represents an increase in intensity above that which occurs in training. Re-injury is a large risk in the return to the competition phase, so measures should be put in place to ensure that the hamstring is fully healed, and able to handle the increased demands placed upon it. During my time with British Athletes, I would be assessed by our doctor every 3 days, with an ultrasound scan to monitor how well the muscle was recovering. Following my early competitions, a followup with the doctor would take place, to ensure that no further injury had occurred.
Hamstring Injury Prevention
Now that we have looked at the best way to rehabilitate ourselves from a hamstring injury it is probably a good idea to look at how we can attempt to reduce the risk of hamstring injuries occurring in both our training and competition.
As I mentioned earlier, the role that hamstring flexibility plays within hamstring injuries is controversial. Harting et al. (1996) split military recruits into two groups. Group one conducted hamstring stretches three times per day for a period of 13 weeks. Group two didn’t do any hamstring stretching. Group one significantly increased their hamstring flexibility during the 13-week training period, and also had less hamstring injuries than the group that didn’t do any hamstring stretching.
The next thing to consider is strength training for the hamstrings. Askling et al. (2002) conducted a study in Swedish soccer players during their pre-season training. One group took part in a hamstring-strengthening programme, and one group didn’t. The hamstring-strengthening group had significant improvements compared to the group that didn’t do the exercises in both hamstring strength (unsurprisingly) and maximum running speed. The hamstring-strengthening group were also significantly less likely to suffer a hamstring injury. These hamstring strengthening exercises should also utilise eccentric movements. Mjolsnes et al. (2004) found that the addition of these types of exercise significantly increased the eccentric torque in the hamstring muscles. Whilst they didn’t directly measure injury prevalence post-training, they proposed that this would reduce the hamstring injury risk as athletes could tolerate the eccentric loading much better. Malliaropoulas et al. (2012) stated that eccentric hamstring exercises were useful as an injury prevention tool as they increased the load that the hamstring could tolerate before it failed as well as increasing the flexibility of the hamstring muscles. When designing a hamstring-strengthening programme, the authors recommended that exercises work both hip extension and knee extension, thus targeting both aspects of hamstring movement. They also recommended using both uni- and bilateral exercises in order to prevent muscle strength asymmetry. Finally, it was proposed that these strengthening exercises should occur at the end of a training session, in order to limit hamstring fatigue, which could increase injury risk if sprinting, were to follow. A final point regarding specific hamstring strengthening exercises is that they should provide a more favourable hamstring:quadricep strength ratio, further reducing the injury risk. Examples of hamstring specific exercise included single leg deadlifts, sliding leg curls, and Nordic hamstrings.
Finally, care should be taken to ensure that hamstring fatigue is well managed in the athlete. As mentioned earlier, a significant risk factor for hamstring injury is fatigue. Steps should be taken to reduce this fatigue; adequate training loads and recovery, soft tissue therapy, and placement of hamstring dominant exercises toward the end of the training session and training week to name a few.
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