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Curtin University of Technology
School of Physiotherapy

Predisposition to hamstring injury cannot be determined!

Proposition for Debate - by Jeremy Hunter

Contents

Statement of the Topic

Can a predisposition to hamstring injuries be determined with particular reference to muscle strength and muscle balance?

Introduction

Orchard et al. (1997) suggests that hamstring strains are the most prevalent injury in Australian Rules Football accounting for 16% of playing time missed as a result of injury. Kujala et al. (1997) supports this by suggesting that hamstring injuries are the most common muscle injuries in athletes.

Kujala et al. (1997) states that a hamstring tear is typically partial and commonly takes place during the eccentric phase of muscle usage when the muscle develops tension while lengthening. Garrett (1996) suggests that most strain injuries occur near the musculotendinous junction and the most commonly injured hamstring muscle is the biceps femoris. The diagnosis is usually based on typical injury mechanism and clinical findings of local pain and loss of function. X-rays and more so ultrasonography and magnetic resonance imaging may be useful for differential diagnosis.

Toomey (1995) states that with the forceful flexion of the hip and extension of the knee during the swing phase of sprinting, the hamstring muscle groups are put under extremely high loads in a lengthened position. Brukner and Kahn (1993) also state that many hamstring injuries occur during the last part of swing phase or at foot strike. At both of these phases of running the hamstrings generate peak torque values and work maximally eccentrically to decelerate the leg. This factor of the hamstrings working to decelerate knee extension is important to consider when evaluating studies involving isokinetic profiles of the hamstrings.

According to Brukner and Kahn (1993) predisposing factors to hamstring injuries include:

It is obvious that there are many possible factors that can attribute to a hamstring strain but the question is whether any of these factors can be predetermined. As physiotherapists, if we can accurately and reliably measure any of these factors we may be able to implement programs to prevent them and therefore reduce the incidence and risk of hamstring strain and lost playing time. Knapik (1992) defines injury risk as the number of athletes injured divided by the population at risk for the injury.

The aim of this paper is to review the scientific evidence to determine if muscle power and muscle balance can be measured accurately and reliably in order to predetermine hamstring injuries.

Definitions of Muscle Strength, Muscle Balance and Muscle Power

Before reviewing the literature it is important to define muscle balance and muscle strength. Muscle balance is a term used to describe the relationship between either:

  1. Agonist to antagonist muscle groups
  2. One limb to another agonist muscle groups
  3. Eccentric to concentric muscle actions.

The muscle balance can be in reference to many variables. For the purpose of this paper it applies to muscle strength. Knapik et al. (1992) support this by stating that imbalances involve either difference between the strength in muscles on the left and right side of the body or a reduced ratio between an agonist muscle group and its antagonist counterpart.

McArdle (1986) defines muscular strength as the maximum tension generated by a muscle or group in a single voluntary contraction. Sapega (1990) explains that it is incorrect to measure the development of muscle tension as power. Power has it's own specific definition of the rate of work output (measured in Kcal/kg/sec). Clinical testing of an individual�s strength is to evaluate the muscles ability to develop maximum voluntary tension and should be measured in units of either linear or rotational force (torque). Considering that power is directly proportional to strength it provides little additional information about a muscles ability to perform a single maximal voluntary contraction.

Types of Muscular Contraction Tested

There are three classifications of muscle contractions as described by McArdle (1986). Concentric muscle contraction is where the muscle overcomes resistance to shorten as it develops tension; an eccentric contraction where the muscle lengthens while developing tension due to the external force overcoming the active muscle; and an isometric contraction is where the muscle does not change in length as it develops tension against an external force.

Sapega (1990) describes another muscle contraction that only occurs in the physiology laboratory. These are isotonic contractions and occur when a muscle moves a fixed load through a range of movement and maintains a constant muscular tension regardless of the length of the muscle. Furthermore testing can be performed isokinetically, where by at a constant angular velocity an accommodating resistance is applied. This resistance will vary to meet the subjects changing capability for maximal strength as the joint changes position. This will give an improved clinical picture of the muscles function through range, as it demonstrates a muscles own inherent length-tension and force ?velocity relationships.

Literature Review

There is a moderate amount of literature studying possible predetermining factors of hamstring injuries. Some studies specifically review the relationship between strength, measured in various methods; the goal of these studies is to verify if a particular strength profile is in existence in subjects who suffer initial or recurrent hamstring strains. Others observe the correlation between flexibility and hamstring muscle strains and few studies look at both of these variables. All of these studies have different methodology and varying outcomes making some more valid than others and comparing data between studies difficult.

Studies on Strength Imbalances and Injuries

Arguments Against

The overall relationship between strength and the risk of hamstring injuries is not made clear by reviewing the studies available. Several studies indicate no relationship, although at times this due to poor study design. As early as 1970 Burkett (as cited in Knapik 1992) measured how much weight male footballers could lift from the floor and found that the risk of injuries was the same in strong and weak athletes. This was obviously a gross strength test and not as specific as more current studies.

Heiser et al. (1984) performed a retrospective study on intercollegiate American footballers. The authors compared player groups prior to and after the acquisition of an isokinetic dynamometer (Cybex 2) and the incidence of initial and recurrent hamstring injuries. Preseason baseline testing of players was performed with the isokinetic dynamometer and those players with a 10% or greater discrepancy between left and right hamstrings or quadriceps were felt to have a relative deficit. Similarly any player with a ratio of hamstrings to quadriceps of 0.50 or less was commenced on an isokinetic rehabilitation program until this deficit was corrected to 0.60 or greater. The authors claim that 7.7% of players with a 31.7% incidence of recurrence occurred prior to obtaining the Cybex 2. During the period the Cybex was used for baseline testing the incidence of hamstring injury was 1.1% with no recurrence. It is very difficult to draw the conclusion that the use of the Cybex 2 was the reason for the reduced incidence of hamstring strains due to many other factors changing during the periods before and after obtaining the Cybex 2. While it possible that the baseline testing and subsequent testing may have contributed to the reduced injury rate there is no statistically analysis was performed and therefore the conclusion is only anecdotal.

Worrell et al. (1991) study of university athletes matched sixteen athletes with a history of hamstring injury to sixteen without a history of injury. These athletes were further matched by motor dominance, sport, and position. Subjects were tested for concentric and eccentric quadriceps and hamstring peak torque and reciprocal muscle group ratios on a KIN-COM dynamometer. The authors found that no statistically significant difference existed within or between the two groups regardless of velocity and contraction type. It also found that the previously injured groups hamstring to quadriceps ratio not significantly different from the nonhamstring-injured group. This was also the same for differing velocities and contraction types. Within this paper the authors cite that Lieholm (1978) and Paton et al. (1989) performed similar research observing hamstrings/quadriceps ratios and they also found that they were not significantly different.

Many authors have commented on the limitations of isokinetic testing. Kibler et al. (1988) stated that sports specific or activity specific testing is more appropriate for evaluating an athlete. Zachazewski (1996) commented that for a test to have a predictive value it must incorporate some of the dynamic characteristics specific to that sport. Lephart (1992) states that low peak torque values are not necessarily related to functional capacity.

Crosier and Crielaard (2000), in a study that seems to confirm the relationship between strength a hamstring muscle strains, themselves state limitations in isokinetic testing as:

Arguments For

More recently detailed studies have further evaluated the relationship between strength and the occurrence or reoccurrence of hamstring strains. It has been suggested that poor eccentric strength in the hamstring muscle group might be a causative factor in hamstring strains (Stanton and Purdam 1989). Jonhagen et al. (1994) evaluated this by comparing eleven sprinters with recent hamstring injuries with nine uninjured runners. It was found that the sprinters with a history of injury are weaker in eccentric contractions at all velocities and weaker quadriceps concentric contractions at low velocities. The authors postulate that a possible reason for hamstring injuries occurring in sprinters could be due to poor eccentric strength, particularly at high angular velocities (180 and 230 deg/sec). This is because such high angular velocities occur in sprinters and the most marked differences were found at the high angular velocities. Comparing this study to Worrell et al. (1991), the athletes chosen by Jonhagen et al. (1994) were only sprinters and not from differing sports and the severity of the hamstring injuries was greater with the average absence being 2 months compared to 2 weeks.

In 1997 Orchard et al. performed preseason measurements on 37 professional Australian rules footballers of hamstring and quadriceps muscle concentric peak torque at 60, 180 300 deg/sec. Players were then studied if they injured their hamstring during that season. Six players were clinically diagnosed as having sustained hamstring muscle strains. According to their statistical analysis the injury was significantly associated with a low hamstring to quadriceps muscle peak torque ratio on the injured leg and a reduced peak torque of injured to uninjured side. These were both at 60 deg/sec and the authors suggest that slower speeds such as 60 deg/sec may more show more "accurate ratio deficits, despite not being close to the physiolologic speeds of sprinting (Orchard et al., 1997)". Eccentric testing was not performed because the authors state that it involves fewer natural movements and is more difficult to perform which may form problems in regards to reliability and validity.

Crosier and Crielaard (2000) performed a thorough study of twenty-three top-level athletes to determine if athletes with previous hamstring tears have a specific isokinetic profile. Despite the fact that some cases presented with no changes, statistical analysis of the concentric and eccentric data demonstrated several particular features:

Crosier and Crielaard (2000) note that some of the subjects with a history of hamstring injuries did not demonstrate asymmetries or agonist/antagonist imbalances and that this fact confirms the multifactorial origin of hamstring strains.

Clinical Implications

The literature provides the clinician with many pieces of information that should be applied as part of an athletes preseason fitness program and if an athlete suffers a hamstring injury during the season. While the research presented is not 100% conclusive components are noteworthy. Physiotherapists should pay more attention to eccentric muscle strengthening in the maximal muscular elongated position particularly in the final therapeutic phase of treatment after a hamstring muscle strain has occurred. Heiser et al. (1984) suggests that isokinetic exercise appears to be an important factor in the reduction of recurrent hamstring strains by allowing the safe application of low loads at controlled speeds during the critical healing phase. It also enables the physiotherapist to objectively assess the players� muscle strength to be used in the decision of when it is safe for the player to return to sport. This knowledge is of greater benefit to the clinician who has performed preseason testing to provide normative values of each player�s strength. The major problem encountered when utilizing isokinetic testing to decide when the player is to return to sport is determining what percentage of normal strength is the value that the athlete must achieve. Heiser et al. (1984) states that intercollegiate footballers were allowed to return once they had reached a level of 95% of baseline. Comparatively, Sapega (1990) claims that 80 or 90 per cent of the measured capability in the uninjured limb is sufficient as a standard for the patient to achieve before returning to activity. Insufficient scientific data exists on this topic.

Conclusion

From the literature reviewed it is difficult to categorically claim that strength testing provides an accurate indicator of the predisposition for initial or recurrent hamstring strains. This is impart due to the fact that there are few well conducted studies examining the relationship between strength testing and hamstring strains. More recent studies by Orchard et al. (1997) and Croisier and Crielaard (2000) do present statistically significant results confirming the relationship between strength and hamstring injuries, however their sample size was small and this needs to be considered when interpreting the results. It is obvious that further studies of a similar prospective nature as Orchard et al. (1997) but with a greater sample size are required to further validate strength testing as a possible predictor of hamstring strains. Lack of specificity to sport and error factors makes it impossible to utilize strength testing as a definitive predictive tool of hamstring injuries however, it may be a useful adjunct to the whole clinical picture.

References

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Burkett, L.N. (1970)
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Hamstring muscle tear with recurrent complaints: An isokinetic profile. Isokinetic and Exercise Science, 8, 175-180.
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Prophylaxis and management of hamstring muscle injuries in intercollegiate football players. American Journal of Sports Medicine, 12 (5), 368-370.
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Hamstring injuries in sprinters: The role of concentric and eccentric hamstring muscle strength and flexibility. American Journal of Sports Medicine, 22 (2), 262-266.
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Exercise Physiology Educational Resources 2001