Whilst generally viewed as a somewhat avoidable soft tissue injury, hamstring strain injuries continue to plague the epidemiology in team sports.  There are an endless amount of statistics in the literature regarding their prevalence but perhaps most worryingly of all are the recent findings from Professor Jan Ekstrand’s European football surveillance research group that suggests hamstring injuries have increased by 4% annually since 2001 (Ekstrand et al, 2016).  Dr John Orchard showed many years ago that for muscle strains the strongest risk factor is previous injury (2001) and the highest risk for injury recurrence is within the first two weeks of returning to the sport (Orchard and Best, 2002).  Given these statistics though, do we need to reflect on the evidence base and current applied practice of the Return To Play (RTP) decision?  Hamstring injuries cause considerable time lost from training and competition, resulting in financial loss and reduced athletic performance (Opar et al, 2012) therefore ensuring time efficient and successful RTP is a high priority of the Sports Medicine team.

 

I believe my role as a Sport Scientist is to add objective data to the multifactorial, multi-disciplinary decision making Return to Play process.  The complexity of RTP has already been discussed at length in this previous blog (http://sportsdiscovery.net/journal/2015/07/12/can-end-stage-rehab-be-assessed-truly-scientifically/) but in this article I will discuss some of the evidence in the literature for various RTP criteria, specifically for hamstring strain injuries.  It is a chance to reflect on what RTP milestones we are using and why; have they just been inherited over time, are they based on anecdotal experience and/or do they have support from the evidence base?

 

Remember it is important to consider the type of hamstring injury (see Carl Askling’s work (2006) and Pollock et al (2015) for some insight) – this blog is already long enough without differentiating but definitely needs considering!

 

Pain

Complete pain relief, regardless of injury, is widely recommended by RTP experts (Kvist, 2004) and was selected as the most important RTP criterion for hamstring strain injuries in a survey of physicians in French and Belgian professional soccer clubs (Delvaux et al, 2014).  Maximum pain score using a visual analogue scale and length of hamstring tenderness were independently associated with the time to return to sport after acute hamstring injury in a recent multivariate analysis (Wangensteen et al, 2015).  In a separate multivariate analysis, localised discomfort on hamstring palpation just after RTP was associated with almost four times higher risk of reinjury (De Vos, 2014).

 

However, focusing solely on ‘pain free movements’ in RTP may lead to deficits in neuromuscular control, strength, flexibility, ground reaction force attenuation and production and asymmetries between legs (Mendiguchia and Brughelli, 2011).  This reinforces the importance of a multi-faceted approach to Return To Play so what other factors should we include alongside pain relief?

 

MRI

The use of MRI (Magnetic Resonance Imaging) is a widely used method for the diagnosis and prognosis of acute hamstring injuries in the elite population however; its use at the point of RTP has not been validated.  In fact, there appears to be some consensus in the literature that clinical and functional tests seem to be better associated with re-injury rates than MRI findings at RTP.  One study showed 89% of clinically recovered hamstring injuries presented increased intramuscular signal intensity on MRI at RTP; therefore normalisation of this oedema is not necessarily a requirement for return to sport (Reurink et al, 2014).  A number of recent multivariate analyses found MRI parameters were not associated with time to RTP (Moen et al, 2014; Wangensteen et al, 2015) or reinjury risk (De Vos, 2014).

 

Isokinetic Dynamometry

Strength assessments may be used as RTP markers, whether that is preinjury baseline strength comparisons and/or contralateral imbalances with the uninjured side.  Isokinetic dynamometry is a popular assessment for muscle strength in the applied setting; muscle strength performance was ranked the second most important RTP criteria behind compete pain relief in the survey of physicians (Delvaux et al, 2014).  There is however, a lack of consensus amongst practitioners regarding the choice of assessment device (isokinetic, manual or power machine), the muscles and type of contraction assessed, the type of comparison and the level of tolerable strength differences.  Perhaps this variety of practice in the applied world is limiting our evidence base – achieving such a consensus would be a difficult task so this emphasises the need for in house validation.

 

There have been a number of suggestions in the literature for objective milestones for Return To Play with isokinetic dynamometry, including:

• Side to side comparison of less than 5% (Croisier et al, 2002).
• Within 5% bilateral deficit in eccentric hamstring to concentric quadriceps strength ratio (30°/s: 240°/s) and a similar knee flexion angle for peak concentric knee flexion torque between sides (Heiderscheit et al, 2010).
• The importance of including the eccentric component in the isokinetic assessment was highlighted by Croisier and Crielaard (2000) who found an ordinary concentric protocol would not have revealed exclusive eccentric deficits in 23% of patients (Figure 1). More on eccentric strength shortly…

 

However, a recent isokinetic study found 67% of clinically recovered hamstring injuries had at least one ipsilateral deficit of more than 10% during isokinetic testing (Tol et al, 2014).  This cohort of professional footballers had worked through a progressive, criteria-based rehabilitation programme and completed a sport specific functional field test, a similar programme to what may be used in the High Performance environment.  Whilst full return of hamstring isokinetic function does not seem to be an automatic outcome of completing such a specific rehabilitation programme, the consequential risk of reinjury is unknown.  So we can conclude currently there is no consensus in the literature on the necessary outcomes of isokinetic dynamometry symmetries for Return To Play.

 

An alternative or additional method of strength assessment to assess lower limb isometric strength using a force plate was put forward last year by McCall et al (2015) . This test demonstrated good to high reliability and sensitivity to detect changes after a competitive match, however further research is needed to determine any relationship between the outcome measures and hamstring strain injury risk and therefore use as a RTP milestone.

 

Eccentric Strength

The importance of eccentric strength on hamstring injuries is not necessarily a new concept, as demonstrated by the already discussed isokinetic profile findings 15 years ago, but it is currently getting a lot of press.  One research group has put forward a new conceptual framework that suggests neuromuscular inhibition following hamstring injuries may hinder rehabilitation and add to reinjury risk (Opar et al, 2012).  This inhibition causes maladaptations that may make athletes more susceptible to hamstring reinjury; one maladaptation proposed is eccentric hamstring weakness.  This research group have also developed a novel device used during the Nordic hamstring exercise (the Nordbord) to assess eccentric knee flexor strength and it has been shown as a reliable method to assess strength asymmetry and residual weakness in previously injured elite athletes (Opar et al, 2013).

 

Evidence suggests athletes who have suffered hamstring strain injury lack strength when the muscle is utilized in a lengthened state, as seen during high speed running, so this provides further evidence for the importance of eccentric strength during RTP (Schmitt et al, 2012).  In addition, new evidence suggests assessing the muscle architecture with ultrasound may also provide monitoring and possibly RTP criteria.  Timmins et al (2015a; 2015b) have shown short Bicep Femoris long head fascicle length (as well as low levels of eccentric strength) in soccer players increases the risk of hamstring strain injury.  Recent evidence also suggests that high levels of eccentric hamstring strength may mitigate reinjury risk with so-called unmodifiable factors of age and previous injury (Opar et al, 2015).  Perhaps ‘long’ and ‘strong’ holds the key to hamstring injury prevention and we may now have tools to objectively assess this.

 

 

While the debate rages on whether the nordic exercise is functional enough to truly assess eccentric strength, this component of strength is clearly important to hamstring reinjury risk and therefore these tools may add valuable objective data for use as RTP criteria and an ongoing monitoring tool.  Meanwhile also remember if the baseline level of strength was not enough to protect an athlete from the original injury should we really be using that as our RTP target??

 

Other Clinical Markers

There are a variety of other clinical markers that are used in practice during hamstring rehabilitation, some of which have been validated in the literature.  Active knee extension deficit and isometric knee force deficit at 15° just after RTP (as well as palpation and the number of previous hamstring injuries) were associated with a higher hamstring re-injury rate (De Vos, 2014).  In another multivariate analysis self-predicted time to RTP and passive straight leg deficit were independently associated with time to RTP (Moen et al, 2014).  These studies provides potential Return to Play markers for clinicians to use within their setting and importantly the authors emphasise monitoring the athlete in the first week post RTP as well as at the point of RTP.

 

Flexibility is another important clinical marker for hamstring injuries as it has been known for some time that a reduction in range of motion is related to injury risk (Witrouw et al, 2003).  More recently a new active ballistic hamstring flexibility test was shown to be highly reliable and more sensitive at detecting differences between the injured and uninjured side than other passive clinical markers (Askling et al, 2010).  As with strength markers, it seems important to include more active and ballistic flexibility markers in your RTP criteria that more closely replicate the actual sporting demands compared to only passive movements that may mask contralateral differences.

 

Functional movements

Heiderscheit et al (2010) included the ability to perform functional movements, such as jumping, running and cutting, without stiffness or pain as a RTP criteria for hamstring injuries but also admit this may be too vague as the risk of reinjury persists for longer than stiffness and pain.  Whilst sport specific functional fitness tests may be included in RTP models (Creighton et al, 2010) and are popular in actual practice (Delvaux et al, 2014), the evidence base behind a valid and reliable functional test to determine or confirm RTP is lacking and is quite possibly our biggest challenge.

 

I believe it is now common practice to use GPS as a tool to objectively set individualised functional targets based on training and match data and to continually assess throughout the rehabilitation in elite level sport.  Given the importance of eccentric strength and the function of the hamstring in a lengthened state, high speed running and peak speeds will be a particularly important component for this particular injury.  This may be represented as something like this:

 

 

But the complexity arises when setting these targets… What variables do you use? How do you set the “target” - do you use the highest output, the third highest to mitigate “outliers”, the top 5% of each variable, the average etc? How much of a match do they have to achieve?  Do you include pre season training? Do you include training matches or sessions with other coaches or age groups? What happens if they reach milestones in some variables but not others? Do they have to achieve all GPS targets in one session or is this creating an unrealistic output?

 

What about the variables they cannot reach?  Can you ever truly replicate the demands of an unpredictable reactive training session with 20 or so other players, compared to a 1 on 1 rehabilitation setting? Even if you hit these targets physically, does it represent the mental and technical demands of a training session?

 

Should you aim to replicate a whole training week in a rehab setting? (This is a philosophy I know is used in practice)  Or do we then leave ourselves susceptible to keeping them out for too long and being too cautious, after all we are trying to return them as soon as possible!  This debates links to a very important paper published recently by Peter Blanch and Tim Gabbett (2015): “Has the athlete trained enough to return to play safely?”.  Whilst that paper will probably be the topic of a entire blog post itself, it reinforces the importance of considering the training load in rehabilitation compared to the load demands after RTP and not only focussing on hitting functional one-off targets and progressions.

 

Conclusion

I think the length and depth of this post gives some insight into the complexity of RTP criteria for hamstring strain injuries both in applied practice and in the evidence base; I am sure there will be plenty of other criteria and key research papers that I have not even mentioned.  Further to that, the high injury and recurrence rates we continue to see with hamstring injuries, despite the increased body of research, reinforces the complexity of the decision.  Our major challenges are coming to consensus in the literature as well as employing best practice in the applied setting - despite the evidence in support of an eccentric based injury prevention programme the majority of football teams surveyed in the Champions League and Norwegian Premier League did not implement it during the 2012-2014 seasons (Bahr et al, 2015).

 

Given the research I have presented here I think these is consensus that the following criteria should be including in the RTP decision for hamstrings: pain free, strength levels (including the eccentric component), active flexibility, functional movements (including high speed running) and load management. But of course the complexity is within the detail and how these criteria come together.  We must also remember the individual nature of rehabilitation and how players will have undulating responses to the process.  Ultimately the RTP decision is made using a mixture of objective milestones and subjective knowledge and interpretation across the interdisciplinary team.

 

If nothing else I hope this post has inspired you to reflect on the evidence and reason behind the RTP criteria you use and provided some sources of reading in the literature on hamstring injuries!

 

Jo Clubb

 

References

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