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Relationships between trunk morphology and strength with non-contact lower limb injuries in elite rugby league and Australian football players

      Background: Non-contact lower limb injuries (NCLLI) are common within the National Rugby League (NRL) and Australian Football League (AFL). Smaller lumbar multifidus at L5 (LM L5) cross-sectional area (CSA) and lower levels of trunk and hip extension and knee flexion strength have been associated with increased NCLLI risk in AFL players. However, such associations have not been reported in the NRL. This study aims to explore relationships between NCLLI, trunk morphology and isometric trunk and hip extension strength in NRL and AFL players.
      Methods: NRL and AFL players had their LM L2-L5 and quadratus lumborum (QL) CSA (n=238), and isometric trunk and hip extension strength (n=153) measured using ultrasonography and a novel field-based test in preseason. Isometric trunk and hip extensor strength were measured using a 65s maximal voluntary isometric contraction (MVIC) protocol (5s MVIC, 5s off, 45s MVIC, 5s off, 5s MVIC). Team medical staff reported all NCLLIs sustained in the subsequent competitive season. The associations between LM and QL CSA, 5s and 45s MVIC, age, and previous injury, and prospectively occurring NCLLI were analysed using univariable and then multivariable logistic regression. Logistic regression analysis was also performed for the most sustained NCLLI (hamstring and knee).
      Results: Seventy-two players sustained a NCLLI in the 2020 competitive seasons (51 NRL, 21 AFL). From univariable logistic regression previous injury increased the risk of any NCLLI in NRL (OR=2.43, 95%CI=1.16-5.09, p=0.019). Reduced 5s MVIC (OR=1.00, 95%CI=1.00-1.01, p=0.012), increased QL CSA (OR=1.55, 95%CI=1.05-2.38, p=0.033) and lower ratio between LM and QL CSA (OR=0.03, 95%CI=0.01-0.60, p=0.02) increased NCLLI risk in AFL. Pooled hamstring injuries (n=28) were most common and were associated with lower 5s MVIC (OR=0.99, 95%CI=0.99-1.00, p=0.02) and 45s MVIC (OR=0.99, 95%CI=0.99-1.00, p=0.001). Pooled knee injuries (n=21) were associated with both reduced LM CSA (OR=1.35, 95%CI=1.08-1.70, p=0.018) and LM to QL ratio (OR=3.62, 95%CI=1.41- 9.66, p=0.043).
      Conclusions: Larger QL CSA and reduced levels of trunk and hip extension strength are observed in AFL players in preseason who sustain a NCLLI in the subsequent playing season. Only previous NCLLI was associated with future risk of NCLLI for NRL players. These results suggest that risk factors for NCLLI are football code specific, and prevention strategies should be tailored accordingly.
      Conflict of interest statement: My co-authors and I acknowledge that we have no conflict of interest relevant to the submission of this abstract.