Unilateral leg strength: Relevent to Rugby League Speed?

Rugby League (RL) is an intermittent, high intensity, collision sport characterised by frequent accelerations and changes of direction (Gabbett et al., 2008). Leg strength has been associated with improved performance in both linear and change of direction speed (CODS) (Bret et al., 2002, Wisløff et al., 2004, Cronin and Hansen, 2005, McBride et al., 2009). Previous research has used bilateral methods of investigating this association, yet few studies have taken a unilateral approach to strength assessment. The aim of this study was to investigate the relationships between unilateral leg strength speed, momentum, change of direction speed and change of direction deficit, in RL players. Further understanding the interaction to different forms of strength and performance measures may better enable practitioners to prepare their athletes. It was hypothesized that increased leg strength would positive influence performance in sprint and CODS measures. With institutional ethical approval testing was conducted at the respective facilities of three participating Rugby League clubs (n = 50). Physiological testing was separated into two sessions the first being speed and change of direction assessment and the second collection of strength data. CODS was measured using the modified 5-0-5 test, speed was tested using a 20m linear sprint from a standing start. For both tests three trials were performed and the best result used for analysis. Unilateral leg strength was measured using the rear foot elevated split squat (RFESS) 5RM (Helme et al, under review). Strength data was calculated in both absolute and relative terms, linear speed was analysed at both 10m and 20m for time, mean velocity and mean momentum for each 10m section. CODS was analysed for time and also for change of direction deficit, (Nimphius et al., 2016). The mean absolute load achieved in the RFESS 5RM was 88.92 ±12.59kg, when expressed relative to body mass was 1.01 ±0.17kg/kg. Possibly to likely small negative associations were found to both change of direction times and change of direction deficits. Possibly small positive associations were observed between momentum and velocity, for the 10-20m section, but not for 0-10m. No correlations reached the alpha level of significance (p = <0.05) for absolute strength. A most likely or almost certain moderate negative relationship was found between CODS and relative leg strength (p= <0.05) and a likely moderate positive relationship to linear mean velocity between 10 and 20m. Almost certainly moderate (10-20m) or large (0-10m) negative relationships were observed with momentum in a linear sprint (p = >0.05). The data from this study suggests that absolute unilateral leg strength is not significantly related to sprinting or CODS performance. However, increasing relative unilateral leg strength is positively associated with increased change of direction speed and velocity, specifically between 10m and 20m. Furthermore, almost certainly large negative correlations were found between relative leg strength and momentum, which is at odds with the other findings of this study. Baker and Newton (2008) identified momentum as a significant contributor to performance in RL, particularly in winning collisions, which are in turn, essential for successful match outcomes. Practitioners may consider the use of relative unilateral leg strength to better understand the development of velocity and CODS in their athletes. This should be taken with caution though, as further investigation is required to better understand the interaction between relative strength and momentum.