The Vertical Drop Jump Is a Poor Screening Test for ACL Injuries: Letter to the Editor

Dear Editor: We read with interest the recent study published in AJSM entitled ‘‘The Vertical Drop Jump Is a Poor Screening Test for ACL Injuries in Female Elite Soccer and Handball Players: A Prospective Cohort Study of 710 Athletes,’’ by Krosshaug et al. The stated purpose of the study was to test whether 5 selected kinematic and kinetic variables were associated with future anterior cruciate ligament (ACL) injuries in a large cohort of Norwegian female elite soccer and handball players. Furthermore, the authors wanted to assess whether the vertical drop jump (VDJ) test could be recommended as a screening test to identify players with increased risk, which could confirm or refute prior studies that have demonstrated just such an effect. We congratulate the authors for performing this highly labor-intensive and technically challenging study. However, the study design and execution entail multiple clear and potentially major flaws. One major flaw in the study is that a large percentage of the subjects had previously been exposed to a neuromuscular training program specifically designed to alter these same 5 kinematic and kinetic variables. Previous studies in the literature have demonstrated that similar neuromuscular training programs are successful in effectively changing these high-risk movement strategies. Their subjects had undergone neuromuscular training designed to decrease dynamic valgus at the hip and knee for various numbers of years prior to their screenings. The authors themselves stated, ‘‘From questionnaire data, 38% of the players (in both soccer and handball) reported to have implemented preventive training as part of their routine during the season. ’’ Krosshaug et al stated that the athletes had been trained with these programs over the prior decade, but the authors did not discuss the potential effect of training over multiple previous seasons. If the authors knew which specific players were involved in training prior to the beginning of the study, why not exclude those players from the analysis or at least use prior training as a covariate in their logistic regression analysis? A second potential major flaw to this study is that the authors treated the 5 variables they examined as parametric data. The authors’ Figure 3 using Gaussian regression lines shows a misunderstanding of nonparametric data and their statistical analysis. These data do not appear to be Gaussian in nature. The authors should reassess their data, determine whether it is indeed parametric or nonparametric, and perform logistic regression analyses with potential confounding variables included as covariates. A third potential major flaw in this study is the clustering together of handball and soccer player data. The authors stated, ‘‘One could speculate that the efforts made to reduce the ACL injury incidence in Norwegian handball for more than a decade have improved landing and cutting technique among all players, thereby reducing inherent variability. Still, 39% of the players were evaluated as having poor landing technique in VDJs. Furthermore, there were no differences in medial knee displacement between the handball and soccer cohorts, indicating that injury prevention training does not seem to have influenced landing technique substantially. ’’ Contrary to this argument, in the absence of contamination with prior neuromuscular training, these variables may have been inherently different in handball and soccer players; as such, the data from both sports should not have been pooled for these analyses. We challenge these authors (1) to reanalyze the data to determine whether it is indeed parametric, as they assume, or nonparametric, as it appears; (2) to reanalyze the data with exclusion criteria in place that remove players who have had prior neuromuscular training or to include prior neuromuscular training as a covariate in the analyses; and (3) to add sport as a covariate for more appropriate, less biased analyses. The authors’ findings did indeed validate that medial knee collapse during the VDJ can determine risk for ACL injury in athletes. Their study managed this although their cohort of athletes was highly biased against knee and hip valgus as a risk indicator because a large percentage of them had undergone repeated bouts of neuromuscular training designed to correct it. As such, this study is the strongest evidence yet published in the literature to confirm that medial knee collapse strongly correlates to ACL injury risk. However, the authors subsequently interpreted their findings to be clinically insignificant. This flawed interpretation was likely due to multiple flaws in their study design.