Rugby injuries are frequent and often severe. Injury to the anterior cruciate ligament (ACL) is devastating and can cause serious hardship. Differences between an athlete’s two legs, also known as an asymmetry, can increase injury risk. The question of interest in this thesis is does lower-extremity symmetry matter for ACL injury risk in male rugby union athletes. A review of the literature describing the aetiology and mechanisms of ACL injuries was performed in conjunction with a pilot investigation analysing rugby match footage to support the rationale of the thesis of investigating the sidestep manoeuvre in rugby. An in-depth systematic review and meta-analysis of knee mechanics during sidestepping concluded that weight acceptance is the most important phase to examine abduction moments of the knee when assessing ACL injury risk. A theoretical ACL injury model was conceived from our examination of the sidestep manoeuvre in rugby and contained elements of strength, balance and sprint kinetics. Laboratory-based practical assessment tools within our theoretical model were used to evaluate thirty male academy-level rugby athletes. The preferred legs were stronger (ES = 0.21 – 0.37), had better balance (ES = 0.63 – 1.0), produced greater sprint kinetics (ES = 0.32 – 0.67) and experienced a smaller knee abduction moment, a more flexed knee, less trunk lateral flexion and less distance between the centre-of-mass and the ankle-joint-centre while sidestepping (ES = -0.26 – -0.97) compared to the non-preferred legs. Forwards were stronger (ES = 0.50 – 0.66), had worse balance with larger asymmetries (ES = -0.66 – -1.8) and produced greater sprint kinetics with larger asymmetries (ES = 0.74 – 0.81) compared to backs. A hierarchical multiple regression was used to examine the contribution of each leg in determining increased knee abduction moments during sidestepping. While single-leg balance did not contribute to increased ACL injury risk (Ŕ² = < 1 – 4%), lower-extremity strength and sprint kinetics did (Ŕ² = < 1 – 31%). The preferred (Ŕ² = 41%) and non-preferred (Ŕ² = 8%) legs independently contribute to increased ACL injury risk with unique distributions of strength and sprint kinetics, however these contributions all appear linked with posterior-chain strength.

In summary, criterion and practical laboratory-based assessment tools to measure and assess ACL injury risk factors in rugby athletes were identified in this thesis. Assessment tools were used to quantify the differences in strength, balance, sprint kinetics and three-dimensional sidestepping mechanics between the preferred and non-preferred legs and between forwards and backs in amateur academy-level male rugby athletes. Normative values, symmetry angle scores and a discussion of assessment components and training recommendations were provided. A new model and framework for assessing ACL injury risk were outlined to guide the progression of ACL injury prevention strategies in rugby athletes.