It is well recognised that hamstring strains are one of the most common –and impactful- injuries in sports, resulting in missed training, games, and events.  Given the potential direct and indirect costs of this injury can be high for athlete, club, or competition, there has been a considerable amount of research into risk factors for hamstring injuries.

For the clinician, sifting through the abundance of studies and combining the results to form a clear picture about risk factors may be difficult task. The large number of studies with often conflicting outcomes and methodologies can overwhelm.   Our study in BJSM (Freckleton & Pizzari, 2013) endeavours to synthesis the literature and combine results using meta-analyses where possible to simplify the research.

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Not surprisingly, we identified commonly accepted risk factors of older age and past history of hamstring injury as risk factors for hamstring strain.  Interestingly, a number of the isokinetic strength measures frequently utilised for screening athletes (and assumed to be implicated in hamstring injury risk) were not predictive of hamstring injury.

Conventional isokinetic concentric hamstring : quadriceps (H:Q) ratio and hamstring peak torque were not supported as a risk factors.  The quadriceps peak torque was the only strength measure to be identified as a significant risk factor using a meta-analysis.

The analysis included four studies (195 participants) and found that athletes with an increased quadriceps peak torque were at risk of subsequent hamstring injury.   Hamstring to opposite hamstring (H:^oppH) results could not be included in a meta-analysis, however the three studies that looked at this strength measure all showed some evidence for a lower ratio being predictive of injury.

The limitation of using isokinetic testing to evaluate risk could be that the position of the lower limb during the testing does not reflect the position where the hamstring injury is most vulnerable to strain in running (Chumanov, Schache, Heiderscheit, & Thelen, 2012).

Evidence for the advantage or disadvantage of hamstring flexibility has been debated for a long time and the analysis showed few results to support flexibility as a risk factor for injury.  The one test that was close to being a significant risk factor was the active knee extension test.  It is noteworthy that this test has also previously been identified as a potentially useful predictor of recurrence of hamstring injury (Warren, Gabbe, Schneider-Kolsky, & Bennell, 2010) and deserves further investigation as a predictor of initial injury.

Flexibility of the quadriceps, hip flexors and ankle joint dorsi-flexion had some evidence for risk, but could not be included in a meta-analysis.

With regard to the meta-analyses only age, past history, and quadriceps peak torque were identified as significant risk factors.  The following factors showed some promise but failed to have the study power to show a meta-analysis result or could not be included in a meta-analysis:  the AKE test, athlete weight, hip flexor flexibility, quadriceps flexibility, ankle dorsi-flexion lunge range of motion, playing position, lower limb joint position sense, and H:H ratio.

Factors that showed little prediction strength for injury included the following: BMI, height, passive length of hamstring, leg dominance, abdominal strength, VO² max, peak O² uptake, anaerobic fitness, slump test, single and double leg counter-movement jump, player exposure, jumping ability and height, knee laxity and running speeds.

It should be noted that this review excluded studies aimed at prevention, although such studies might allow some inferences regarding risk factors.  The limitation of including such studies is that they assume that the prevention strategy is in fact targeting a known risk factor and that the strategy actually alters that risk.

There are a number of perceived risk factors that have not yet been studied in the literature, due mostly to the difficult methodology involved in such evaluation.  The posterior thigh pain chapter in Clinical Sports Medicine identifies fatigue as being a relevant consideration in hamstring strain.

The biomechanical changes that occur with fatigue and the timing of hamstring injuries (late in games) is reasonably used to infer fatigue is factor in hamstring injury (Opar, Williams, & Shield, 2012).  Other measures, such as ground conditions, pre-season participation, player workload, and concomitant injury also require further analysis.

The risk for hamstring injuries is recognised as being multi-factorial in nature, however ongoing research will help to identify areas that should be addressed by clinicians and areas that can be discounted when screening, evaluating, and managing athletes.

Dr. Tania Pizzari is a lecturer and researcher in the Department of Physiotherapy at La Trobe University.  She is also a physiotherapist and director of Mill Park Physiotherapy Centre in Melbourne, Australia.

References

1. Chumanov, E. S., Schache, A. G., Heiderscheit, B. C., & Thelen, D. G. (2012). Hamstrings are most susceptible to injury during the late swing phase of sprinting. Br J Sports Med, 46(2), 90. doi: 10.1136/bjsports-2011-090176
2. Freckleton, G., & Pizzari, T. (2013). Risk factors for hamstring muscle strain injury in sport: a systematic review and meta-analysis. Br J Sports Med, 47(6), 351-358. doi: 10.1136/bjsports-2011-090664
3. Opar, D. A., Williams, M. D., & Shield, A. J. (2012). Hamstring strain injuries: factors that lead to injury and re-injury. Sports Med, 42(3), 209-226. doi: 10.2165/11594800-000000000-00000
4. Warren, P., Gabbe, B. J., Schneider-Kolsky, M., & Bennell, K. L. (2010). Clinical predictors of time to return to competition and of recurrence following hamstring strain in elite Australian footballers. Br J Sports Med, 44(6), 415-419. doi: 10.1136/bjsm.2008.048181