Hamstring Injuries: What is the best method to monitor hamstring strength?
Hamstring strain injuries (HSIs) are a common and complex injury in many field sports. This review discusses the best practices for professional sporting environments when monitoring hamstring strength in order to quantify the likelihood of a player experiencing a HSI.
Pressure test devices are deemed to be the most cost effective and efficient way to measure a player’s hamstring strength. However, it is important to acknowledge that hamstring strength is just one of many variables related to HSIs and without aggregating different variables the true risk of injury may remain unknown.
Hamstring strain injuries are prevalent in sports that involve sprinting such as soccer, rugby, and Australian Football (Sconce et al., 2015). HSIs are reported to account for 12-16% of all injuries sustained by athletes in these sprint-related team sports (Heiderscheit et al., 2010). HSIs have also been reported by Liu et al. (2012), to account for 12% of all injuries in a competitive season of English professional soccer resulting in an average of 17 days missed playing/training time. HSI rates for the above mentioned sports have been reported by Bahr et al. (2015) to have increased over the last two decades from 7% to 17%. It was stated by Bahr et al. (2015) that HSIs are putting increased financial pressure on professional sports clubs due to their prolonged recovery time and high recurrence rates.
HOW DO THEY HAPPEN? Hamstring injury mechanisms
High-speed running, particularly sprinting, has been widely identified in the literature as the primary mechanism for HSIs. The late swing phase is believed to be the most ‘at risk’ phase of gait due to the level of high-velocity eccentric force absorbed by the hamstring muscles to control the deceleration of a rapidly extending limb in preparation for heel strike. Eccentric knee flexor strength is important for performance as it allows for greater control of the descending limb during sprinting and jumping which leads to a faster change over from eccentric deceleration to concentric acceleration. It was proposed by Clark et al. (2005) that eccentric knee flexor strengthening improves the angle of peak torque which contributes to increased knee joint stability during deceleration prior to a jump performance.
WHY DO THEY OCCUR? Risk factors of hamstring injuries
Several modifiable HSI risk factors have been identified (Liu et al. 2012); these include shortened optimum muscle length, insufficient muscle flexibility and strength imbalances between right and left limbs. It is important that these modifiable risk factors can be objectively measured for regular monitoring to determine if an athlete is at an increased risk of incurring a HSI and to monitor any adaptations resulting from strength training and/or injury rehabilitation. Croisier et al. (2008) have proposed screening that hamstring strength weakness and imbalances can play an important role in identifying risks of incurring a HSI. Eccentric knee flexor strength imbalances between right and left limbs of greater than or equal to 15% and greater than or equal to 20% were observed by Bourne et al. (2015), with the findings correlating to an athlete being 2.4 and 3.4 times, respectively, more likely to suffer a HSI.
METHODS TO ASSESS HAMSTRING STRENGTH
Various devices and methods have been used to record objective strength measures for research purposes, athletic performance, assessment and medical rehabilitation monitoring. The most common methods used for objective hamstring strength measurement are isokinetic dynamometry and hand-held dynamometry testing.
The gold standard measure for the assessment of knee flexor strength, concentrically, isometrically and eccentrically, is isokinetic dynamometry. An isokinetic dynamometer has the ability to record several measurements simultaneously making it a useful and effective method of muscle strength assessment. However, this means of assessment is not readily accessible in all conditioning or clinical settings due to the large size and high cost of the device. The isokinetic device is also not practical when assessing a large group of athletes regularly as it requires considerable time to set up and execute the testing protocol (Opar et al., 2013) while also requiring expertise in interpreting results to determine what aspects of the data collected will need to be addressed.
Another objective method of hamstring strength assessment is hand-held dynamometry. A hand-held dynamometer (HHD) was created as a more precise and objective alternative to manual muscle testing by giving a quantifiable measurement of force in Newtons (N) or pressure (mmHg) between the therapist’s hand and the patient’s limb being tested. The HHD’s limitations include the inability of the device to control the speed of the contraction and the device also depends on the tester’s ability to generate enough force to match or overcome the subject’s muscle contraction force. As mentioned by Stark et al. (2011) this may put the tester at risk of injury while carrying out the required manual resistance. Intra and inter-rater reliability is a further limitation of using hand-held dynamometry for the objective assessment of hamstring strength.
Using pressure feedback systems is a relatively new concept for conveniently assessing hamstring strength and between-limb imbalance within an individual or team setting. This newly developed method of objective hamstring strength assessment has an increasing body of evidence supporting its effectiveness in identifying and addressing hamstring injury risk factors. It was proposed by Opar et al. (2013), that a novel device that can monitor the eccentric strength of each limb independently, during performance of the Nordic hamstring exercise (NHE), could make screening of eccentric strength and unilateral imbalances more efficient and feasible in a group setting. Pressure feedback systems have their limitations; including the inability of the device to control the speed of the contraction and the feedback provided also lacks important details like angle of peak torque that an isokinetic dynamometer can provide. The literature supports the use of such a pressure feedback system as a screening tool to regularly monitor eccentric hamstring strength to identify any unexplained drops that may put the athlete at risk of incurring a HSI.
BEST PRACTICE: Our recommendations for cost effective & efficient screenings
With such a high prevalence of HSIs in sprint-related team sports, regular hamstring screening has become an essential component of an athlete’s weekly routine. Screening athletes for hamstring weakness and imbalance is necessary in order to monitor and quantify an athlete’s risk of incurring a HSI. Regular objective screening with a pressure feedback device allows some of these modifiable risk factors to be monitored and addressed onsite in a convenient and cost effective manner.
CREATING A BIGGER PICTURE OF RISK: The Kitman Labs Application
HSI mechanisms are multifactorial in nature, there are a variety of risk factors that may result in an athlete experiencing a HSI. With this in mind, it is important that coaches and medical staff aim to target the modifiable risk factors with the goal of reducing the likelihood of their athletes suffering from HSI.
It is important to note that pressure feedback platforms are one of many means to measure the risk of HSI but without more information it can be challenging for experts to understand the additional factors that lead to injury. Athlete Optimisation Systems such as Kitman Labs’ can enhance an expert’s view of the collected data by aggregating it into one platform and mapping data trends from historical injuries to highlight any relationships in the data that were once disconnected.
The common occurrence and multifaceted nature of hamstring injuries in field sports highlights the importance of aggregating data to quantify new relationships and understand risk factors that have previously lead to injury. Through combining professional experience and knowledge with objective evidence, experts can continue to unlock the mysteries of injury risk.
SCONCE, E., JONES, P., TURNER, E., COMFORT, P. & GRAHAM-SMITH, P. 2015. The Validity of the Nordic Hamstring Lower for a Field-Based Assessment of Eccentric Hamstring Strength. Journal of Sport Rehabilitation, 24, 13-20.
HEIDERSCHEIT, B. C., SHERRY, M. A., SILDER, A., CHUMANOV, E. S. & THELEN, D. G. 2010. Hamstring Strain Injuries: Recommendations for Diagnosis, Rehabilitation, and Injury Prevention. Journal of Orthopaedic & Sports Physical Therapy, 40, 67-81.
LIU, H., GARRETT, W. E., MOORMAN, C. T. & YU, B. 2012. Injury rate, mechanism, and risk factors of hamstring strain injuries in sports: A review of the literature. Journal of Sport and Health Science, 1, 92-101.
BAHR, R., THORBORG, K. & EKSTRAND, J. 2015. Evidence-based hamstring injury prevention is not adopted by the majority of Champions League or Norwegian Premier League football teams: the Nordic Hamstring survey. British Journal of Sports Medicine.
CLARK, R., BRYANT, A., CULGAN, J.-P. & HARTLEY, B. 2005. The effects of eccentric hamstring strength training on dynamic jumping performance and isokinetic strength parameters: a pilot study on the implications for the prevention of hamstring injuries. Physical Therapy in Sport, 6, 67-73.
CROISIER, J. L., GANTEAUME, S., BINET, J., GENTY, M. & FERRET, J. M. 2008. Strength imbalances and prevention of hamstring injury in professional soccer players - A prospective study. American Journal of Sports Medicine, 36, 1469-1475.
BOURNE, M. N., OPAR, D. A., WILLIAMS, M. D. & SHIELD, A. J. 2015. Eccentric Knee Flexor Strength and Risk of Hamstring Injuries in Rugby Union: A Prospective Study. The American Journal of Sports Medicine.
OPAR, D. A., PIATKOWSKI, T., WILLIAMS, M. D. & SHIELD, A. J. 2013. A Novel Device Using the Nordic Hamstring Exercise to Assess Eccentric Knee Flexor Strength: A Reliability and Retrospective Injury Study. Journal of Orthopaedic & Sports Physical Therapy, 43, 636-640.
STARK, T., WALKER, B., PHILLIPS, J. K., FEJER, R. & BECK, R. 2011. Hand-held Dynamometry Correlation With the Gold Standard Isokinetic Dynamometry: A Systematic Review. PM&R, 3, 472-479.