Letter to the Editor regarding ‘Increased contralateral dynamic valgus in female athletes following ACL reconstruction’

We read with great interest the recent article by Gaugg et al. titled “Increased contralateral dynamic valgus in female athletes following ACL reconstruction” [2]. The authors address a critical gap in our understanding of secondary anterior cruciate ligament (ACL) injury mechanisms by highlighting sex-specific neuromuscular patterns in the contralateral limb. While we commend the authors for this insightful analysis, we wish to discuss several methodological nuances and clinical implications that may further contextualize these findings for the readership of Knee Surgery, Sports Traumatology, Arthroscopy. First, the issue of statistical power warrants careful consideration. As the authors transparently noted, the study included only nine female participants, falling short of the pre-specified sample size of 11 required to detect a clinically relevant 8° difference in valgus angle with 80% power. This underpowering increases the risk of Type II errors. This is particularly relevant given the high risk of contralateral injury in female athletes reported in epidemiological studies, such as Paterno et al. [7], who noted a 6-fold higher risk. We echo the recent calls by the EU–US Meniscus Rehabilitation 2024 Consensus for robust, multi-centre biomechanical designs to overcome such sample limitations in future research [8]. Second, the findings regarding the hamstrings-to-quadriceps (H/Q) ratio present a compelling statistical paradox. The study reported no significant sex difference in H/Q ratios, yet the p value was borderline (p = 0.051). Given the small sample size, it is highly plausible that a true physiological deficit in hamstring strength was masked by the lack of statistical power. However, we agree with the authors' interpretation that dynamic valgus likely stems from broader neuromuscular deficits beyond isolated thigh muscle strength. Research by Hewett et al. [4] and the recent meta-analysis by Ramachandran et al. [9] suggest that deficits in hip and trunk control are dominant drivers of valgus collapse. Therefore, the observed valgus is likely a composite result of both the borderline H/Q deficit and proximal neuromuscular control issues. Third, regarding clinical translation, the timing of assessment (8 months post-ACLR) aligns with the period of highest re-injury vulnerability. However, standard return-to-sport (RTS) criteria often overlook contralateral biomechanics. Asaeda et al. [1] identified persistent rotational deficits in the uninjured limb, and Herrington and Munro [3] have established thresholds for valgus angles that warrant intervention. We advocate that clinicians should incorporate 3D or 2D video analysis of the contralateral limb into RTS protocols, as suggested by the authors. This aligns with the ‘ecological validity’ concerns raised in recent literature; lab-based jumps may not fully reflect the fatigue-induced kinematic changes seen in real-world scenarios [5, 6]. In conclusion, Gaugg et al. [2] provide valuable evidence reinforcing the need for sex-specific rehabilitation. We support the move towards individualized prevention strategies that prioritize neuromuscular reprogramming of the hip and trunk, alongside traditional strength metrics, to mitigate the risk of secondary ACL injuries. The authors have nothing to report. This study was supported by the Medical Research Project of Foshan Health Bureau Program of Guangzhou (202636010101). The authors declare no conflicts of interest. Medical Research Project of Foshan Health Bureau Program of Guangzhou, Grant/Award Number: 202636010101. The authors have nothing to report. The authors have nothing to report.