Hamstring injuries can be frustrating injuries. The symptoms are typically persistent and chronic. The healing can be slow and there is a high rate or exacerbation of the original injury (Petersen J et al. 2005).
The classical hamstring injury is most commonly found in athletes who indulge in sports that involve jumping or explosive sprinting (Garrett W E Jr. 1996) but also have a disproportionately high prevalence in activities such as water skiing and dancing (Askling C et al. 2002).
A brief overview of the literature on the subject shows that the majority of the epidemiological studies in this area have been done in the high-risk areas of Australian and English professional football teams. Various studies have put the incidence of hamstring strain injuries at 12 – 16% of all injuries in these groups (Hawkins R D et al. 2001). Part of the reason for this intense scrutiny of the football teams is not only the high incidence of the injury, which therefore make for ease of study, but also the economic implications of the injury.
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Some studies (viz. Woods C et al. 2004) recording the fact that hamstring injuries have been noted at a rate of 5-6 injuries per club per season resulting in an average loss of 15 -21 matches per season. In terms of assessing the impact of one hamstring injury, this equates to an average figure of 18 days off playing and about 3.5 matches missed. It should be noted that this is an average figure and individuals may need several months for a complete recovery. (Orchard J et al. 2002). The re-injury rate for this group is believed to be in the region of 12 – 31% (Sherry M A et al. 2004).
The literature is notable for its lack of randomised prospective studies of treatment modalities and therefore the evidence base for treatment is not particularly secure.
If one considers the contribution of the literature to the evidence base on this subject, one is forced to admit that there is a considerable difficulty in terms of comparison of various differences in terminology and classification. Despite these difficulties this essay will take an overview of the subject.
Classification of injuries
To a large extent, the treatment offered will depend on a number of factors, not least of which is the classification of the injury. In broad terms, hamstring injuries can have direct or indirect causation. The direct forms are typically caused by contact sports and comprise contusions and lacerations whereas the indirect variety of injury is a strain which can be either complete or incomplete. This latter group comprises the vast majority of the clinical injuries seen (Clanton T O et al. 1998).
The most extreme form of strain is the muscle rupture which is most commonly seen as an avulsion injury from the ischial tuberosity. Drezner reports that this type of injury is particularly common in water skiers and can either be at the level of the insertion (where it is considered a totally soft tissue injury) or it may detach a sliver of bone from the ischial tuberosity (Drezner J A 2003). Strains are best considered to fall along a spectrum of severity which ranges from a mild muscle cramp to complete rupture, and it includes discrete entities such as partial strain injury and delayed onset muscle soreness (Verrall G M et al. 2001). One has to note that it is, in part, this overlap of terminology which hampers attempts at stratification and comparison of clinical work (Connell D A 2004).
Woods reports that the commonest site of muscle strain is the musculotendinous junction of the biceps femoris (Woods C et al. 2004).
In their exemplary (but now rather old) survey of the treatment options of hamstring injuries, Kujala et al. suggest that hamstring strains can usefully be categorised in terms of severity thus:
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Mild strain/contusion (first degree): A tear of a few muscle fibres with minor swelling and discomfort and with no, or only minimal, loss of strength and restriction of movements.
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Moderate strain/contusion (second degree): A greater degree of damage to muscle with a clear loss of strength.
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Severe strain/contusion (third degree): A tear extending across the whole cross section of the muscle resulting in a total lack of muscle function.
(Kujala U M et al. 1997).
There is considerable debate in the literature relating to the place of the MRI scan in the diagnostic process. Many clinicians appear to be confident in their ability to both diagnose and categorise hamstring injuries on the basis of a careful history and clinical examination. The Woods study, for example, showing that only 5% of cases were referred for any sort of diagnostic imaging (Woods C et al. 2004). The comparative Connell study came to the conclusion that ultrasonography was at least as useful as the MRI in terms of diagnosis (this was not the case if it came to pre-operative assessment) and was clearly both easier to obtain and considerably less expensive than the MRI scan (Connell D A 2004).
Before one considers the treatment options, it is worth considering both the mechanism of injury and the various aetiological factors that are relevant to the injury, as these considerations have considerable bearing on the treatment and to a greater extent, the preventative measures that can be invoked.
It appears to be a common factor in papers considering the mechanisms of causation of hamstring injuries that the anatomical deployment of the muscle is a significant factor. It is one of a small group of muscles which functions over two major joints (biarticular muscle) and is therefore influenced by the functional movement at both of these joints. It is a functional flexor at the knee and an extensor of the hip. The problems appear to arise because in the excessive stresses experienced in sport, the movement of flexion of the hip is usually accompanied by flexion of the knee which clearly have opposite effects on the length of the hamstring muscle.
Cinematic studies that have been done specifically within football suggest that the majority of hamstring injuries occur during the latter part of the swing phase of the sprinting stride (viz. Arnason A et al. 1996). It is at this phase of the running cycle that the hamstring muscles are required to act by decelerating knee extension with an eccentric contraction and then promptly act concentrically as a hip joint extensor (Askling C et al. 2002).
Verrall suggests that it is this dramatic change in function that occurs very quickly indeed during sprinting that renders the hamstring muscle particularly vulnerable to injury (Verrall G M et al. 2001).
Consideration of the aetiological factors that are relevant to hamstring injuries is particularly important in formulating a plan to avoid recurrence of the injury.
Bahr, in his recent and well-constructed review of risk factors for sports injuries in general, makes several observations with specific regard to hamstring injuries. He makes the practical observation that the older classification of internal (intrinsic) and external (extrinsic) factors is not nearly so useful in clinical practice as the consideration of the distinction between those factors that are modifiable and those that are non-modifiable (Bahr R et al. 2003).
Bahr reviewed the evidence base for the potential risk factors and found it to be very scanty and “largely based on theoretical assumptions” (Bahr R et al. 2003 pg 385). He lists the non-modifiable factors as older age and being black or Aboriginal in origin (the latter point reflecting the fact that many of the studies have been based on Australian football).
The modifiable factors, which clearly have the greatest import for clinical practice, include an imbalance of strength in the leg muscles with a low H : Q ratio (hamstring to quadriceps ratio) (Clanton T O et al. 1998), hamstring tightness (Witvrouw E et al. 2003), the presence of significant muscle fatigue, (Croisier J L 2004), insufficient time spent during warm-up, (Croisier J L et al. 2002), premature return to sport (Devlin L 2000), and probably the most significant of all, previous injury (Arnason A et al. 2004).
This is not a straightforward additive compilation however, as the study by Devlin suggests that there appears to be a threshold for each individual risk factor to become relevant with some (such as a premature return to sport) being far more predicative than others (Devlin L 2000).
There is also some debate in the literature relating to the relevance of the degree of flexibility of the hamstring muscle. One can cite the Witvrouw study of Belgian football players where it was found that those players who had significantly less flexibility in their hamstrings were more likely to get a hamstring injury (Witvrouw E et al. 2003).
If one now considers the treatment options, an overview of the literature suggests that while there is general agreement on the immediate post-injury treatment (rest, ice, compression, and elevation), there is no real consensus on the rehabilitation aspects. To a large extent this reflects the scarcity of good quality data on this issue. The Sherry & Best comparative trial being the only well-constructed comparative treatment trial, (Sherry M A et al. 2004) but even this had only 24 athletes randomised to one of two arms of the trial.
In essence it compared the effects of static stretching, isolated progressive hamstring resistance, and icing (STST group) with a regime of progressive agility and trunk stabilisation exercises and icing (PATS group). The study analysis is both long and complex but, in essence, it demonstrated that there was no significant difference between the two groups in terms of the time required to return to sport (healing time). The real significant differences were seen in the re-injury rates with the ratio of re-injury (STST : PATS) at two weeks being 6 : 0, and at 1 year it was 7 : 1.
In the absence of good quality trials one has to turn to studies like those of Clanton et al. where a treatment regime is derived from theoretical healing times and other papers on the subject. (Clanton T O et al. 1998). This makes for very difficult comparisons, as it cites over 40 papers as authority and these range in evidential level from 1B to level IV. (See appendix). In the absence of more authoritative work one can use this as an illustrative example.
Most papers which suggest treatment regimes classify different phases in terms of time elapsed since the injury. This is useful for comparative purposes but it must be understood that these timings will vary with clinical need and the severity of the initial injury. For consistency this discussion will use the regime outlined by Clanton.
Phase I (acute): 1–7 days
As has already been observed, there appears to be a general consensus that the initial treatment should include rest, ice, compression, and elevation with the intention to control initial intramuscularly haemorrhage, to minimise the subsequent inflammatory reaction and thereby reduce pain levels. (Worrell T W 2004)
NSAIAs appear to be almost universally recommended with short term regimes (3 – 7 days) starting as soon as possible after the initial injury appearing to be the most commonly advised. (Drezner J A 2003). This is interesting as a theoretically optimal regime might suggest that there is merit in delaying the use of NSAIAs for about 48 hrs because of their inhibitory action on the chemotactic mechanisms of the inflammatory cells which are ultimately responsible for tissue repair and re-modelling. (Clanton T O et al. 1998).
There does appear to be a general consensus that early mobilisation is beneficial to reduce the formation of adhesions between muscle fibres or other tissues, with Worrell suggesting that active knee flexion and extension exercises can be of assistance in this respect and should be used in conjunction with ice to minimise further tissue reaction (Worrell T W 2004).
Phase II (sub-acute): day 3 to >3 weeks 0
Clanton times the beginning of this phase with the reduction in the clinical signs of inflammation. Goals of this stage are to prevent muscle atrophy and optimise the healing processes. This can be achieved by a graduated programme of concentric strength exercises but should not be started until the patient can manage a full range of pain free movement (Drezner J A 2003).
Clanton, Drezner and Worrell all suggest that “multiple joint angle, sub-maximal isometric contractions” are appropriate as long as they are pain free. If significant pain is encountered then the intensity should be decreased. Clanton and Drezner add that exercises designed to maintain cardiovascular fitness should be encouraged at this time. They suggest “stationary bike riding, swimming, or other controlled resistance activities.”
Phase III (remodelling); 1–6 weeks
After the inflammatory phase, the healing muscle undergoes a phase of scar retraction and re-modelling. This leads to the clinically apparent situation of hamstring shortening or loss of flexibility. (Garrett W E Jr. et al. 1989). To minimise this eventuality, Clanton cites the Malliaropoulos study which was a follow up study with an entry cohort of 80 athletes who had sustained hamstring injuries.
It was neither randomised nor controlled and the treatment regime was left to the discretion of the clinician in charge. It compared regimes which involved a lot of hamstring stretching (four sessions daily) or less sessions (once daily). In essence the results of the study showed that the athletes who performed the most intensive stretching programme were those who regained range of motion faster and also had a shorter period of rehabilitation. Both these differences were found to be significant. (Malliaropoulos N et al. 2004)
Verrall suggests that concentric strengthening followed by eccentric strengthening should begin in this phase. The rationale for this timing being that eccentric contractions tend to exert greater forces on the healing muscle and should therefore be delayed to avoid the danger of a rehabilitation-induced re-injury. (Verrall G M et al. 2001). We note that Verrall cites evidence for this from his prospective (un-randomised) trial
Phase IV (functional): 2 weeks to 6 months
This phase is aimed at a safe return to non-competitive sport. It is ideally tailored to the individual athlete and the individual sport. No firm rules can therefore be applied. Worrell advocates graduated pain-free running based activities in this phase and suggests that “Pain-free participation in sports specific activities is the best indicator of readiness to return to play.” (Worrell T W 2004)
Drezner adds the comment that return to competitive play before this has been achieved is associated with a high risk of injury recurrence. (Drezner J A 2003)
Phase V (return to competition): 3 weeks to 6 months
This is the area where there is perhaps the least agreement in the literature. All authorities are agreed that the prime goal is to try to avoid re-injury. Worrell advocates that the emphasis should be on the maintenance of stretching and strengthening exercises (Worrell T W 2004).
For the sake of completeness one must consider the place of surgery in hamstring injuries. It must be immediately noted that surgery is only rarely considered as an option, and then only for very specific indications. Indications which the clinician should be alert to are large intramuscular bleeds which lead to intramuscular haematoma formation as these can give rise to excessive intramuscular fibrosis and occasionally myositis ossificans (Croisier J L 2004).
The only other situations where surgery is contemplated is a complete tendon rupture or a detachment of a bony fragment from either insertion or origin. As Clanton points out, this type of injury appears to be very rare in football injuries and is almost exclusively seem in association with water skiing injuries (Clanton T O et al. 1998).
It is part of the role of the clinician to give advice on the preventative strategies that are available, particularly in the light of studies which suggest that the re-injury rate is substantial (Askling C et al. 2003).
Unfortunately this area has an even less substantial evidence base than the treatment area. For this reason we will present evidence from the two prospective studies done in this area, Hartig and Askling
Hartig et al. considered the role of flexibility in the prophylaxis of further injury with a non-randomised comparative trial and demonstrated that increasing hamstring flexibility in a cohort of military recruits halved the number of hamstring injuries that were reported over the following 6 months (Hartig D E et al. 1999).
The Askling study was a randomised controlled trial of 30 football players. The intervention group received hamstring strengthening exercises in the ten week pre-season training period. This intervention reduced the number of hamstring injuries by 60% during the following season (Askling C et al. 2003). Although this result achieved statistical significance, it should be noted that it involved a very small entry cohort.
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Conclusions.
Examination of the literature has proved to be a disappointing exercise. It is easy to find papers which give advice at evidence level IV but there are disappointingly few good quality studies in this area which provide a substantive evidence base. Those that have been found have been presented here but it is accepted that a substantial proportion of what has been included in this essay is little more than advice based on theory and clinical experience.
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