Introduction: In the Australian Football League (AFL), players with a cross-sectional area (CSA) of the lumbar multifidus muscle below 8.2cm2, significantly increase the prospective likelihood of a time loss lower limb injury. A subsequent intervention study, implementing a functional resistance training program involving players with a multifidus CSA below 8.2cm2, increased this CSA value. The aim of this current study, as a final review of the Translating Research into Injury Prevention Practice (TRIPP) protocol, is to ascertain whether this increase in multifidus muscle size then leads to a reduction of time loss lower limb injury risk. Methods: Forty-seven players were recruited from one AFL club and underwent a validated assessment of their multifidus muscle size using ultrasound imaging. Based on previous results, players participated in a functional resistance program if their multifidus muscle CSA was below a clinical cut-off value of 8.2cm2, determined using receiver operating characteristic (ROC) curves. Players with a CSA above 8.2cm2 continued with normal pre-season training. Ultrasound imaging of multifidus muscle size was conducted at the end of the intervention/pre-season training. Lower limb injuries were recorded across the playing season. For analysis, players were classified into `intervention group - but CSA remained below 8.2cm2 (BInt)`, `intervention group - CSA increased above 8.2cm2 (AInt) and `no intervention required group`. Cross-tabulation of all groups was conducted with relative risk (RR) ratios used to compare groups. Results: A RR ratio of 1.9 (95% CI 1.0 - 3.4) was found when comparing the `Bint` group with the `no intervention required group`. Combining the `AInt group` with the `no intervention required group` (i.e., all players starting the season above 8.2cm2 cut-off) and comparing with players below the 8.2cm2 cut-off resulted in a RR ratio of 1.8 (95% CI 1.0 - 3.3). Within the intervention group, comparing the `AInt group` with the `BInt group` resulted in a RR ratio of 1.9 (95% CI 0.8 - 4.3). Discussion: A smaller multifidus muscle size is a lower limb injury risk factor for AFL players. Using a functional resistance training program, the size of the lumbar multifidus muscles can be increased. Players that remained below the 8.2cm2 clinical cut-off value were 1.8 times more likely to sustain an in-season lower limb injury than those players above 8.2cm2 at the start of the season. There was a higher risk of injury for players below the cut-off compared with players who did not require intervention. There was no difference in the injury risk between the players who were able to increase their multifidus muscle size above the cut-off and those that did not require intervention. Implementing a functional resistance training program aimed at increasing the size of the multifidus muscle can reduce the lower limb injury risk for AFL players. Impact/Application to the field: A functional resistance training program aimed at increasing the size of the lumbar multifidus muscle is effective at reducing lower limb injury risk in AFL players with a small multifidus muscle size.
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