Introduction
In today’s blog, we are going to be talking about hamstring muscle strains. Hamstring strains are most commonly seen in sprinting activities such as football, rugby, track and field events and account for up to 37% of all muscle strains. They can also be caused by over stretching the muscle, such as in dancing or kicking where the muscle is placed in its most elongated position of both hip flexion and knee extension. Sprinting strains however are more common and are thought to occur during the terminal swing phase of the gait cycle when the hamstring has to contract eccentrically in order to decelerate the leg prior to landing (as seen in Image 1 below)
Image 1 (Hiedersheit et al. 2010).
Anatomy
The hamstring muscle is bi-articular meaning it crosses two joints, which is what makes it more vulnerable to injury. It is comprised of three different muscles (Image 2): Semi-membranosus, Semi-tendinosus and the Biceps femoris, which has both a long and a short head. The Adductor Magnus also sits very close medially and can sometimes be referred to as the 4th hamstring. It is the Semi-membranosus which is mostly injured in stretching hamstring strains and this will usually require a longer recovery period in comparison to sprinting hamstring strains. In sprinting, it is the Biceps femoris which is activated more in the acceleration and terminal swing phase which means it is subject to more strain and therefore more commonly injured. It is important to therefore find out what the mechanism of injury was as this will have an impact on potential muscles involved as well as prognosis.
Classification
Hamstring strains are usually classified in accordance to the degree of pain, available range of motion and strength, resulting in grade I (mild), grade II (moderate) and grade III (severe). The British Athletics Muscle Injury Classification however, proposes a new system which they feel will further aid clinical decision making and prognostication. They look to grade the extent of muscle injury seen on MRI on a scale of O-4 , as well as identifying the site of injury as either a) myofascial, b) muscle-tendon junction (MTJ) or c) intratendinous. The most common site of injury is at the MTJ and both b) and a) have much more favourable prognosis than when the tendon (c) is involved.
If you are wondering, a grade 0 is an ‘MRI negative muscle injury’ whereby nothing could be detected on imaging however they are still presenting with focal soreness. The study by Macdonald et al, 2019, goes into great detail about the rehab for each class of injury and is well worth a read. The timeframe to return to play is a contentious topic as it can really vary between individuals, however people presenting with grade II a) muscle injury will typically return within 2-4 weeks, b) 4-8 weeks and 2-4 months if the tendon is involved: more proximal = more problematic. See image 2.
Image 2 (Macdonald et al. 2019)
Although this is accurate and gold standard, the problem with this form of classification is that it needs to be diagnosed using MRI within 24-48 hours from injury and for most clinicians, this may not be achievable. A thorough history and clinical examination is therefore very important.
Unfortunately, the data shows the re-injury rates of hamstring strains following return to play is high and currently sits around one in three, especially within the first two weeks. Now this could come down to a number of factors including returning too early (possibly secondary to pressures from players, coaches and financial reasons etc.) and therefore not allowing adequate healing time for the tissue, or, not adequately rehabilitating the player and exposing them to their required sporting loads prior to returning. This is something that we will come onto a little later.
Risk Factors
The main modifiable risk factors of hamstring strains are reduced Biceps femoris fascicle length and reduced eccentric hamstring strength. A combination of both reduced length and strength places the individual within the much talked about, ‘quadrant of doom’, as seen in Image 4. Players with short fascicle length and reduced eccentric strength were shown to be 4.1 and 4.4 times, respectively, more likely to suffer a future hamstring strain than those with longer and stronger hamstrings.
Image 3(Bourne et al. 2017)
It is also thought that increased contralateral hip flexor/knee extensor tightness can have an effect as it causes an increase in anterior pelvic tilt which subsequently puts the hamstring in a lengthened position. Other factors include fatigue as it is thought this can impair neuromuscular function, an altered quadriceps/hamstring strength ratio and reduced lumbo-pelvic stability. These are all things that therefore need to be assessed and addressed during the rehabilitation phase and may help in preventing the re-occurrence of injury.
Presentation
Players will usually complain of sudden pain in the posterior thigh following a hamstring strain. If there is an audible ‘pop’, an inability to weight bear and bruising that appears within an hour then it is likely that a grade 3/4 tear has occurred. If it is a complete rupture then often a palpable gap can be felt. Grade I and II strains can be slightly less obvious however will usually present with pain on palpation, reduction in muscle length (hip dominant: passive SLR and knee dominant: MHFAKE - maximum hip flexion and knee extension test) and reduction of strength on resisted testing. The Askling-H test can also be used and involves the player completing three rapid SLR’s and assesses for any pain and/or insecurities during movement. On palpation, you are trying to locate the point of maximum tenderness as this will indicate where the tear is. As previously mentioned, the greater distance from the Ischial Tuberosity, the better the prognosis. Take a look below at George North clutching onto his Bicep Femoris during 2019 Rugby World Cup.
It is also important to consider differential diagnoses which can include referred pain from the lumbar or gluteal region or an adductor strain. This will therefore require additional tests and a different treatment approach.
Rehabilitation
The usual concept of rehabilitation of muscle injuries will often start with isometric exercises followed by concentric and then eccentric exercises before return to play. The problem with this approach is that it may not expose the player to adequate eccentric loading prior to returning to play which could increase the risk of re-injury. Just doing concentric hamstring exercises has actually been shown to further reduce fascicle length where as eccentric training has been shown to improve both fascicle length and strength. As mentioned previously, it is important that we target the modifiable risk factors and ensure the player is opposite to the ‘quadrant of doom’ (i.e. long and strong).
There are a number of studies that look at what exercises/protocols are best for hamstring strains. The study by Askling et al (2013) compared eccentric lengthening exercises (the extender, the diver and the glider) to conventional hamstring exercises (contract-relax stretch, resisted hip extension and bridging). The study showed a significantly quicker recovery time and return to sport in the lengthening programme and there were no re-injuries reported following the lengthening programme. This may be because they are targeting those modifiable factors in eccentric strength and fascicle length in the exercises.
A study by Mendigucia et al (2017) produced a criteria based progressive algorithm for hamstring treatment and compared it to Askling’s treatment approach. The algorithm was comprised of two progressive stages (the regeneration and the functional phase) and incorporated many more exercises looking to target the multifactorial nature of hamstring strains. The results of this study actually showed a slightly increased return to play timeframe however demonstrated reduced injury re-occurrence and improved 10m running time and maximal sprinting speed than the control. So although return to play was slower, as it targeted more of the modifiable risk factors this is possibly why performance was enhanced and re-injury was lower.
The above study however did not introduce eccentric exercises until the second phase and players were only allowed to progress exercises when they were pain free and other criteria was met. This could potentially put a delay on the return to play. A study by Hickey (2019) looked to incorporate end stage rehab exercises at the start of rehabilitation at a sub maximal intensity. The initial exercises were a double leg bridge, hip extensor and bilateral sliding leg curl. These exercises were then progressed to single leg (+/- external load) and nordics were also gradually introduced. They also used a progressive running programme that was based on Silder et al’s (2013) work. Both groups completed the same exercise programme however one group was instructed to complete the exercises pain free and the other group were allowed a tolerable level of pain during the exercise (4/10 on VAS). The results of this study actually showed no difference in return to play between the two groups. The players that were able to exercise with tolerable pain however, demonstrated an increased knee extensor strength, a reduced fear of movement and greater Biceps femoris fascicle length than the pain free group. This therefore demonstrates that we do not necessarily need to completely avoid pain during rehabilitation exercises.
The main goal following a hamstring strain is to return to play as quickly as possible and minimise the risk of re-injury. The above studies demonstrate that it is important to incorporate an eccentric element to rehabilitation and it is thought that both a knee dominant and hip dominant approach is best. It is also critical to ensure a progressive running/sprinting phase is implemented. The return to play criteria that was used in the study completed by Hickey included: no pain on palpation, full pain free symmetrical range of motion, pain free knee flexor isometric strength (comparable to contralateral side) and no pain/apprehension during sprint testing.
As one of the biggest risk factors to developing a hamstring strain is a previous hamstring strain, prevention is definitely better than cure. It is probably not possible to prevent all hamstring injuries, however, if we can ensure our players warm up well, have good range of movement in both hamstrings + hip flexors/quads and have hamstrings like Gareth Bale, then we can hopefully help reduce the occurrence of hamstring strains.
Thank you for reading. By Josh Petty.
References
Askling et al 2013, Acute hamstring injuries in Swedish elite football: a prospective randomised controlled trial comparing two rehabilitation protocols
Bourne et al 2017, An evidenced based framework for strengthening exercises to prevent hamstring injury
Heiderscheit et al 2010, Hamstring strain injuries: recommendations for diagnosis, rehabilitation and injury prevention
Hickey et al 2019, Pain free vs pain-threshold rehabilitation following acute hamstring strain: a randomised control trial
Macdonald et al 2019, Hamstring rehabilitation in elite track and field athletes: applying the British Athletics Muscle injury classification in clinical practice
Mendiguchia et al 2017, A multifactorial, criteria based progressive algorithm for hamstring injury treatment
Pollock et al 2014, British athletics injury classification: a new grading system
Timmins et al 2015, Short bicep femoris fascicles and eccentric knee flexor weakness increase the risk of hamstring injury in elite football (soccer): a prospective cohort study
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