Quantifying Eccentric Hamstring Strength in Elite Academy Footballers: Analysis on the NordBord and IKD

Context: Hamstring injury is a prevalent muscle injury in sports, reduced hamstring strength is associated with such injury risk but has also been shown to impact on knee function. Hamstring injury-risk assessment and methods of quantifying eccentric hamstring strength for return to play criteria following injury have primarily been investigated using isokinetic dynamometry (IKD). However, practical issues such as cost and availability limit the widespread application of isokinetics for injury-risk assessment; thus, a field-based alternative for assessing eccentric hamstring strength (NordBord) has been developed. The use of the NordBord is growing within elite performance environments, but questions remain with regards the parameters utilised to quantify eccentric hamstring strength and whether it has the potential to replace the IKD as a gold standard alternative. Objective: The aim of this study was to investigate the reliability of each device in elite youth soccer, whilst aiming to further investigate the correlation between the two pieces of equipment to see if similarities or differences occurred when comparing the parameters of average torque (AvT), peak torque (PkT), breaking angle/angle of PkT (Ɵ). Intentions of the study were to guide practitioners in the use of each in an elite applied environment. Participants: Thirty-four male elite footballers from Premier League Category 1 Academies (mean ± SD age 17.60 ± 0.76 y, height 179.23 ± 7.8 cm and body mass 72.7 ± 9.9 kg) acted as subjects for the study. Methods: Participants underwent eccentric knee flexor testing on the IKD at varying testing speeds (60°·s-1 and 150°·s-1) to determine PkT, AvT, Ɵ. Likewise participants completed the NordBord to determine results of PkT, AvT and break point angle. The Nordic break-point angle (the point at which the subject can no longer resist the increasing gravitational moment during a Nordic hamstring lower) was measured using video analysis. Study 1 was designed to determine the test-retest reliability of each modality of assessment. Study 2 continued to analyse the relationship between the results of the IKD and NordBord to determine where, if any correlations occurred. Results: The results from study 1 revealed excellent test-retest ICC scores for the IKD across both testing speeds for all parameters (ICC = 0.87 - 0.91). In contrast to the NordBord which found fair to moderate ICC values for PkT and AvT (ICC = 0.56 – 0.76) yet excellent for break point angle (ICC = 0.92). In study 2, when identifying correlations between the two testing devices, the present study acknowledged very weak relationships between the NordBord and the IKD when analysing the parameters of AvT and PkT at the slower speed of 60°·s-1. This trend continues with the IKD at 150°·s-1 for the right leg, but interestingly a strong correlation was seen for the left leg. Conclusion: Quantification of eccentric hamstring strength in elite academy footballers is a contemporary concern. Prior to identifying any potential injury risk, the outputs observed across the IKD and NordBord must be reliable and clearly relate to injury mechanism. In study 1 the IKD was demonstrated to be a reliable measure of all parameters observed (PkT, AvT and Ɵ). In contrast parameters of PkT and AvT determined via the NordBord must be interpreted with caution. Contrastingly, break angle when performing the NHE on the NordBord displayed excellent reliability and should be considered in practice to determine the muscle architecture of elite academy footballers. The speed and ease of testing when utilising the NordBord is a huge advantage to practitioners in the field and study 2 identified clear relationships between Ɵ and break angle. Highlighting, the use of the NordBord in practice for establishing the muscle architecture of individual athletes