An investigation into the acute effects of utilising a live strength feedback system to ensure equal bilateral force distribution when performing the Nordic hamstring exercise, a randomised control trial

Abstract

Hamstring strain injuries (HSIs) are prevalent in sports that involve sprinting such as soccer, rugby, and athletics (Sconce et al., 2015). HSIs are reported to account for between 12% & 16% of all injuries sustained by athletes in sprint related sports such as track & field (Yeung et al., 2009), soccer (Croisier, 2004), Australian football (Ruddy et al., 2016) and rugby (Moore et al., 2015). 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 of missed playing/training time. HSI rates in professional soccer have been reported by Bahr et al. (2015) to have increased over the last two decades from 7% to 12-17%. It was also 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. The implications of HSIs in amateur sport do not only include physical and emotional stress but also include financial strain on amateur clubs that very often have to operate on minimal budgets when compared the their professional counterparts. High-speed running, particularly sprinting, has been widely identified in the literature as the primary offending mechanism for HSIs (Ruddy et al., 2016). The late swing phase is believed to be the most ‘at risk’ phase of gait for HSIs due to the level of high-velocity eccentric force absorbed by the hamstring muscle to control the rapid deceleration of the extending limb in preparation for heel strike (Liu et al., 2017). The Hamstring Solo Elite (HSE) is a pressure feedback device that utilises load cell technology to monitor eccentric knee flexor strength. The HSE is a device with a cushioned inclined surface on which the subject kneels with their heels fixed beneath cushioned load cells; these provide instant feedback on the force being exerted by each limb during repetitions of the Nordic Hamstring Exercise (NHE). The HSE is one of the first devices designed to provide live objective feedback for hamstring strength and individual limb force distribution when performing the NHE. The accessibility and convenience of the device as an in-house method of quantifying hamstring strength makes the HSE a more desirable method when compared to much less accessible methods such as isokinetic dynamometry. Validity and/or reliability testing is carried out to determine the validity of a measurement device or a tester’s ability to consistently measure a variable (Bruton et al., 2000). Reliability is essential in order for data to be recorded consistently and used for analysis. A strength measuring device, such as the HSE, must be reliably consistent in order to determine if any adaptations to training can be solely attributed to the training 5intervention and not to an inconsistent error associated with the device (Pincivero et al., 1997). Accuracy is essential when assessing performance, so it is vital that the HSE is both a reliable and a valid device. A device that lacks validity and reliability is flawed and practitioners cannot confidently use the device to monitor periodised gains. When a new device such as the HSE is introduced it is essential that the device’s validity is first established against a gold standard device that measures the same information such as an isokinetic dynamometer. It is also essential that the device be reliable in a test-retest scenario as any inconsistencies compromise the quality of the data obtained from the device. This is particularly important when the device is used for clinical rehabilitation, as any undetected weakness or misreading could potentially put the patient/athlete at risk of further injury or result in the misinterpretation of clinical findings. Once found to be valid and reliable, the HSE device has the potential to be utilised in conjunction with a NHE intervention to address between-limb eccentric hamstring strength imbalances. Between-limb imbalances are regarded as a significant risk factor for HSI (Yeung et al., 2009). In support of this finding, eccentric knee flexor strength imbalances between right and left limbs of ≥15% and ≥20% were observed by Bourne et al. (2015), to increase the risk of an athlete sustaining a HSI by 2.4 fold and 3.4 fold, respectively. With the serious implications that a between-limb eccentric strength imbalance can have on HSI risk, it has been proposed by Croisier et al. (2008) that pre-season screening for hamstring strength weakness and imbalances is fundamental for identifying the potential risks of incurring a HSI. The NHE has been strongly supported in the literature as being extremely effective in reducing HSI occurrences (Al Attar et al., 2016, van der Horst et al., 2015, Thorborg, 2012, Petersen et al., 2011). While the NHE is an effective method for developing eccentric hamstring strength, it is a double leg exercise that can be limited by limb dominance leading to unequal strength gains between the left and right limbs leading to an increase in between-limb strength imbalances. This study investigates the implementation of a pressure feedback system for eccentric hamstring conditioning such as the HSE could be used to identify between-limb eccentric strength imbalances and also be used as monitoring tool to ensure equal force distribution between both limbs during eccentric conditioning exercises such as the NHE.