## 4 Discussion
### Modifiable Risk Factors
#### 4.1.2 Biomechanical--Kinematics and Kinetics  
Three studies included report greater knee valgus angles and knee abduction moments during 90\({}^{\circ}\) COD, unplanned side-stepping, and single-leg DVJ as inferred risk factors for ACII in elite male field-based team sports based on the existing literature on ACL loading mechanisms [36; 37; 38]. Findings should be carefully interpreted as these retrospective studies do not establish causation for greater knee valgus angles and knee abduction moments to ACII risk by prospectively investigating ACII incidence. Particularly, findings from [PERSON] and co-workers warrant caution for three reasons: (1) none of the athletes recruited sustained an ACLI, rather soft tissue injuries or ankle sprains [37]; (2) the cohort (n = 21) was stratified into two groups, resulting in an achieved power (1-\(\beta\)) of 0.49 and 0.68 for knee valgus angle and knee flexion angle, respectively, which is substantially less than a recommended 0.8 [37]; (3) 2D motion analysis has reduced precision and ability to identify poor knee positions compared to 3D motion analysis [47; 48].  
More recent studies investigating correlation between double-leg DJ and ACLI risk in female sporting populations report knee valgus angle and knee abduction moment did not show any significant association to increased ACLI risk when using 2D or 3D motion analysis [7,49]. A recent investigation in professional female soccer and handball players concluded that 2D motion analysis of DJ should not be used to screen for ACLI risk because it could not identify individuals with an increased risk of non-contact ACLI [7]. In contrast, [PERSON] et al. [50] reported knee valgus angles and a knee abduction moment during DJ to predict ACLI, and [PERSON] et al. [51] in their prospective study with mixed sex collegiate athletes reported knee valgus and knee flexion angles at peak contact during double-leg DJ to be excellent predictors of ACLI [51].  
The inconsistency in findings and techniques makes determining the validity of DJ as an ACLI screening test difficult. No research investigating the correlation between DJ and ACLI risk in elite male field-based team sport was identified. Recent findings of the prospective study in professional female athletes indicate future research should place less emphasis on investigating DJ and ACLI risk [7]. This would appear reasonable, given ACL rupture occurs prior to adopting excessive knee valgus and tibial external rotation positions [52,53]. This gap in the literature is surprising given that the DJ is considered to challenge fundamental jumping and landing patterns, with potential to clearly identify deficient movement strategies as a precursor to injury [54]. Further investigation in elite male athletes is necessary before clubs could confidently utilize the DJ to screen for ACLI risk. Whilst the feasibility of 3D motion analysis can be considered challenging due to the high costs, substantial time required [20], and marker placement errors [55], [PERSON] et al. demonstrated it can be practical, inexpensive, and markerless with good-excellent prognostic ability [51].