Background: Australian Football League (AFL) and National Rugby League (NRL) are elite Australian competitions for rugby league and Australian rules football. Non-contact injuries, like muscle strains and ligament sprains, are very common but the cause of these injuries difficult to identify. There is preliminary evidence in AFL athletes, that reduced cross-sectional area of the lumbar multifidus at the fifth vertebrae and increased cross-sectional area of the quadratus lumborum measured via ultrasound imaging, is a risk factor for sustaining any non-contact lower limb injuries. The lumbar multifidus and quadratus lumborum are both involved in trunk extension, rotation, and proprioception and believed to influence hip and trunk kinematics during exercise. However, there is limited evidence showing a mechanistic link between lumbar muscle cross-sectional area and pooled noncontact lower limb injuries. Assessing the force generating capacity of lumbar spine muscles may provide a better understanding to why the lumbar multifidus at the fifth lumbar vertebral level and quadratus lumborum are potential risk factors for lower limb injuries. Previous studies have only assessed lumbar muscle morphology in single sport cohorts (Australian rules football) that mainly consisted of small sample sizes. With most the literature reporting the same sport, there are uncertainties about the applicability in other sports.

This thesis aims to (1) develop a novel trunk and hip extensor strength test and assess its reliability, (2) replicate a previously established model to assess non-contact lower limb injury risk in elite team sports based on lumbar muscle morphology, (3) develop a predictive model for hamstring strain and knee ligament injury risk based on lumbar muscle morphology and strength, and (4) systematically review and meta analyse the literature on lumbar muscle characteristics as a risk factor for non-contact lower limb injuries. [...]