Abstract
<title>Abstract</title> <p>Background Current anterior cruciate ligament (ACL) injury screening approaches primarily focus on biomechanical risk factors such as dynamic knee valgus (DKV), yet often lack ecological validity by failing to incorporate cognitive demands commonly experienced during sport. Neurocognitive fatigue may impair motor control and expose underlying neuromuscular deficits associated with high-risk landing mechanics. Objective To identify baseline predictors of post-fatigue dynamic knee valgus during the single-leg drop jump in male soccer players and investigate the effects of neurocognitive fatigue and lower-limb stiffness on landing biomechanics. Methods In this longitudinal observational study, male soccer players aged 18–30 years (Tegner activity score > 5) underwent assessment of maximal isometric knee strength at 30° of knee flexion using fixed dynamometry. DKV, reactive strength index (RSI), and lower-limb stiffness were quantified during the single-leg drop jump (SLDJ) and single-leg hop using video-based motion analysis (Kinovea) and jump performance assessment (My Jump2). Outcomes were evaluated before and after a neurocognitive fatigue protocol. Multiple regression analyses were performed to identify predictors of post-fatigue landing mechanics. Results Neurocognitive fatigue did not significantly increase DKV at the group level (p = 0.111). However, weight-bearing ankle dorsiflexion and pre-fatigue SLDJ stiffness independently predicted 14.5% of the variance in post-fatigue kinematic collapse. Participants classified as having safer landing mechanics (DKV ≤ 12°) demonstrated greater baseline stiffness (8.18 vs. 6.61 kN/m, p = 0.005), ankle dorsiflexion (11.64 vs. 10.58 cm, p = 0.013), and hamstring isometric strength (1.48 vs. 1.34 Nm·kg⁻¹, p = 0.045) than those classified as high risk. Additionally, 52.8% of asymptomatic participants exhibited clinically relevant inter-limb asymmetries in RSI (Limb Symmetry Index < 90%). Conclusions Neurocognitive fatigue does not appear to uniformly impair landing control in physically active male soccer players. Instead, altered landing mechanics under fatigue are associated with pre-existing neuromuscular deficits, particularly reduced lower-limb stiffness, limited ankle dorsiflexion, and lower hamstring strength. These findings support the development of a low-cost multidimensional screening approach integrating mechanical, neuromuscular, and reactive performance measures to identify individuals potentially at greater risk of ACL injury-related movement patterns.</p>